Chapter sevenFURTHER ASPECTS OF COVALENTBONDING

The previous chapter concentrated on the octet rule and Lewis diagrams for simple covalent molecules. There are numerous examples, however, of molecules which are quite stable but contain one or more atoms which do not have a noble-gas electron configuration. Furthermore, structural formulas like those in Chap. 6 only show which atoms are connected to which. They do not tell us how the atoms are arranged in three-dimensional space. In other words a Lewis diagram does not show the shape of a molecule.In this chapter we will develop a more detailed picture of molecules—including some which do not obey the octet rule. You will learn how both the shapes and bonding of molecules may be described in terms of orbitals. In addition it will become apparent that the distinction between covalent and ionic bonding is not so sharp as it may have seemed in Chap. 6. You will find that many covalent molecules are electrically unbalanced, causing their properties to tend toward those of ion pairs. Rules will be developed so that you can predict which combinations of atoms will exhibit this kind of behavior.

7.1 EXCEPTIONS TO THE OCTET RULE

Considering the tremendous variety in properties of elements and compounds in the periodic system, it is asking a great deal to expect a rule as

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simple as Lewis’ octet theory to be able to predict all formulas or to account for all molecular structures involving covalent bonds. Lewis’ theory concentrates on resemblances to noble-gas ns2np6 valence octets. Therefore it is most successful in accounting for formulas of compounds of the representative elements, whose distinguishing electrons are also s and p electrons. The octet rule is much less useful in dealing with compounds of the transition elements or inner transition elements, most of which involve some participation of d or f orbitals in bonding.Even among the representative elements there are some exceptions to the Lewis theory. These fall mainly into two categories: (1) Some stable molecules simply do not have enough electrons to achieve octets around all atoms. This usually occurs in compounds containing Be or B. (2) Elements in the third period and below can accommodate more than an octet of electrons. Although elements such as Si, P, S, Cl, Br, and I obey the octet rule in many cases, under other circumstances they form more bonds than therule allows.Good examples of the first type of exception are provided by BeCl2 and BCl3. Beryllium dichloride, BeCl2, is a covalent rather than an ionic substance. Solid BeCl2 has a relatively Complex structure at room temperature, but when it is heated to 750°C, a vapor which consists of separate BeCl2 molecules is obtained. Since Cl atoms do not readily form multiple bonds, we expect the Be atom to be joined to each Cl atom by a single bond. The structure is

Instead of an octet the valence shell of Be contains only two electron pairs. Similar arguments can be applied to boron trichloride, BCl3, which is a stable gas at room temperature. We are forced to write its structure as

in which the valence shell of boron has only three pairs of electrons. Molecules such as BeCl2 and BCl3 are referred to as electron deficient because some atoms do not have complete octets. Electron-deficient molecules typically react with species containing lone pairs, acquiring octets by formation of coordinate covalent bonds. Thus BeCl 2 reacts with Cl– ions to form BeCl4

–;

BCl3 reacts with NH3 in the following way:

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Examples of molecules with more than an octet of electrons are phosphorus pentafluoride (PF5) and sulfur hexafluoride (SF6). Phosphorus pentafluoride is a gas at room temperature. It consists of PF5 molecules in which each fluorine atom is bonded to the phosphorus atom. Since each bond corresponds to a shared pair of electrons, the Lewis structure is

Instead of an octet the phosphorus atom has 10 electrons in its valence shell. Sulfur hexafluoride (also a gas) consists of SF6 molecules. Its structure is

Here the sulfur atom has six electron pairs in its valence shell.An atom like phosphorus or sulfur which has more than an octet is said to have expanded its valence shell. This can only occur when the valence shell has enough orbitals to accommodate the extra electrons. For example, in the case of phosphorus, the valence shell has a principal quantum number n = 3. An octet would be 3s23p6. However, the 3d subshell is also available, and some of the 3d orbitals may also be involved in bonding. This permits the extra pair of electrons to occupy the valence (n = 3) shell of phosphorus in PF5.Expansion of the valence shell is impossible for an atom in the second period because there is no such thing as a 2d orbital. The valence (n = 2) shell of nitrogen, for example, consists of the 2s and 2d subshells only. Thus nitrogen can form NF3 (in which nitrogen has an octet) but not NF5. Phosphorus, on the other hand, forms both PF3 and PF5, the latter involving expansion of the valence shell to include part of the 3d subshell.

7.2 THE SHAPES OF MOLECULES

The location in three-dimensional space of the nucleus of each atom in a molecule defines the molecular shape or molecular geometry. Molecular shapes are important in determining macroscopic properties such as melting and boiling points, and in predicting the ways in which one molecule can react with another. A number of experimental methods are avail- able for finding molecular geometries, but we will not describe them here. Instead we will concentrate on several rules based on Lewis diagrams which will allow you to predict molecular shapes.To provide specific cases which illustrate these rules, “ball-and stick” models for the four molecules discussed in the previous section are shown in Fig. 7.1. In addition to BeCl2, BCl3, PF5, and SF6, we have included CCl4, a molecule which does obey the octet rule. The atoms (spheres) in each

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Figure 7.1 The shapes and geometries of molecules which contain no lone pairs. In each ease the shape adopted is the one in which the electron-pair bonds are as far apart as possible. Parts (b) and (f) are computer generated. (Copyright © 1976 by W. G. Davies and J. W. Moore.)

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ball-and-stick model are held together by bonds (sticks). These electron-pair bonds determine the positions of the atoms and hence the molecular geometry.In each of the molecules shown in Fig. 7.1 the electron-pair bonds are arranged so that they avoid each other in space to the maximum possible extent. This may be understood in terms of the repulsion between electron clouds due to their like charges. During the 1950s the Australian R. S. Nyholm (1917 to 1971) and the Canadian R. J. Gillespie (1924 to ) summed up this behavior in terms of the valence-shell-electron-pair repulsion (VSEPR) theory. The VSEPR theory states that, because of their mutual repulsions, valence electron pairs surrounding an atom stay as far as possible from one another.A simple model for demonstrating the behavior of electron pairs under the influence of their mutual repulsion is provided by a set of spherical balloons of equal size. It is a model that you can easily make for yourself. If, say, four balloons are tied together so that they squeeze each other fairly tightly, they inevitably adopt the tetrahedral arrangement shown for CCl4

in col. f of Fig. 7.1 Although it is possible to flatten the balloons on a table until they are all in the same plane, they invariably spring back to the tetrahedral configuration as soon as the pressure is removed. A similar behavior is found if two, three, five, or six balloons are tightly tied together, except that in each case a different stable shape is adopted once the balloons are left to themselves. The overall appearance of the balloons is shown in,col. f of Fig. 7.1 In col. e a sketch of the geometrical figure to which these shapes correspond is also drawn.Since all the shapes described in Fig. 7.1 constantly recur in chemical discussions, it is worth being able to recall them and their names without hesitation. To this end we will discuss the geometry of each of the five molecules.In BeCl2 central Be atom has only two electron pairs in its valence shell. These are arranged on opposite sides of the Be atom in a straight line, and they bond the two atoms to the Be atom. Thus the three nuclei are all in a straight line, and the Cl―Be―Cl angle is 180°. A molecule such as BeCl2, whose atoms all lie on the same straight line, is said to be linear. In BCl3 the three valence electron pairs, and hence the three Cl nuclei, are arranged in an equilateral triangle around the B atom. Each Cl―B―Cl angle is 120° and all four nuclei (B included) lie in the same plane. The three Cl atoms are said to be trigonally arranged around B.In CCl4 the four Cl nuclei are at the four corners of a geometric figure called a tetrahedron. A tetrahedron has six equal edges, four equilateral triangular faces, and four identical corners (apices). The C nucleus lies in the exact center of the tetrahedron, equidistant from each corner. All the Cl—C—Cl angles are the same, namely, 109.5°. This important angle is called the tetrahedral angle. The four Cl atoms are said to be tetrahedrally arranged around the C atom. This tetrahedral arrangement is the most important of the five described in Fig. 7.1.In PF5 the five F nuclei are arranged at the corners of a trigonal bi-pyramid. As drawn in the figure, one F atom lies directly above the P atom

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and one directly below. The remaining three F atoms are arranged in a triangle around the middle of the P. Some of the F―P―F angles are 90°, while others are 120°.In SF6 the six F atoms are arranged at the corners of an octahedron. An octahedron has twelve edges, eight equilateral triangular faces, and six identical corners. The name octahedron is derived from the eight faces, but it is usually the six corners of this figure which are of interest to chemists. Thus you will have to remember that an octahedral arrangement involves six atoms, not the eight that the name seems to imply.In SF6 the six F atoms are octahedrally arranged around the S. All the F―S―F angles are 90°. Octahedral arrangements are quite common in chemistry. In crystals of LiH and NaCl, for instance (see Fig. 6.3), six anions are arranged octahedrally around each cation while six cations are arranged octahedrally around each anion.

Molecules with Lone PairsThe VSEPR theory is also able to explain and predict the shapes of molecules which contain lone pairs. In such a case the lone pairs as well as the bonding pairs are considered to repel and avoid each other. For example, since there are two bonds in the SnCl 2

molecule, one might expect it to be linear like BeCl2. If we draw the Lewis diagram, though, we find a lone pair as well as two bonding pairs in the valence shell of the Sn atom:

Ideally the three pairs of electrons should arrange themselves trigonally around the Sn atom, giving an angle of 120° between electron pairs, and hence between the two Cl atoms. Experimental measurements on SnCl2 reveal that the molecule is angular or V-shaped, as shown, but the Cl—Sn—Cl angle is significantly less than the predicted 120°.This smaller angle occurs because a lone pair of electrons is always “fatter” than a bonding pair. That is, a lone pair is like a bigger balloon which takes up more room and squeezes the bonding pairs closer together. This decreases the angle between bonding pairs in SnCl2, and hence between the bonded Cl atoms, from the expected value of 120°. The “fatness” of a lone pair compared with a bonding pair is shown in Fig. 7.2.A lone pair also affects the structure of ammonia, NH3. Since this molecule obeys the octet rule, the N atom is surrounded by four electron pairs:

If these pairs were all equivalent, we would expect the angle between them to be the regular tetrahedral angle of 109.5°. Experimentally, the angle is found to he somewhat less, namely, 107°. Again this is because the lone pair is “fatter” than the bonding pairs and able to squeeze them closer together.

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Figure 7.2 Comparison of the electron clouds of a lone pair and a bonding pair. (a) The lone pair of electrons on the nitrogen in an ammonia molecule, . (b) One of the three bonding pairs of electrons in the ammonia molecule. Boundary lines which enclose equal percentages of each electron cloud have been drawn. Note that the lone pair (a) takes up more space (is “fatter”) near the nitrogen nucleus than the bonding pair (b). (Computer generated.) (Copyright © 1975 by W. G. Davies and J. W. Moore.)

The electronic structure of the H2O molecule is similar to that of NH3 except that one

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bonding pair has been replaced by a lone pair:

Here there are two “fat” lone pairs, and so the bonding pairs are squeezed even closer together than in NH3. The H―O―H angle is found to be 104.5°. The structures of BeCl2, BCl3, SnCl2, CCl4, NH3 and H2O include all the combinations of lone pairs and bonding pairs and all molecular shapes which are possible for two, three, and four pairs of electrons. These shapes, together with details of their geometries, are summarized in Fig. 7.3. Again, because of their frequent occurrence, it is wise to commit these to memory. Note in particular that the shape of a molecule is described in terms of the geometry of the nuclei and not of the electron clouds. For example, the shape of the NH 3 molecule is described as a trigonal pyramid since the N nucleus forms the apex of a pyramid, slightly above an equilateral triangle of H nuclei. Although the electron-pair clouds are arranged in an approximate tetrahedron around the N nucleus, it is incorrect to describe the molecular shape as tetrahedral. The atomic nuclei are not at the corners of a tetrahedron.

EXAMPLE 7.1 Sketch and describe the geometry of the following molecules: (a) GaCl 3, (b) AsCl3, and (c) AsOCl3.

Solution

a) Since the element gallium belongs to group III, it has three valence electrons. The Lewis diagram for GaCl3 is thus

Since there are three bonding pairs and no lone pairs around the Ga atom, we conclude that the three Cl atoms are arranged trigonally and that all four atoms are in the same plane.

b) Arsenic belongs to group V and therefore has five valence electrons. The Lewis structure for AsCl3 is thus

Since a lone pair is present, the shape of this molecule is a trigonal pyramid, with the As nucleus a little above an equilateral triangle of Cl nuclei.

c) The Lewis diagram for AsOCl3 is similar to that of AsCl3.

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Figure 7.3 The arrangement of electron pairs and the shapes of molecules which contain lone pairs. Bonding pairs are indicated in color and have purposely been made very thin for diagrammatic effect. Lone pairs are indicated in gray. Note that the geometry of these molecules is described in terms of the nuclei and not of the electron pairs; it is described in terms of the ball-and-stick diagrams shown in the figure.

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Since there are four bonding pairs, the molecule is tetrahedral. Sketches of each of these molecules are

The VSEPR theory can also be applied to molecules which contain five and six pairs of valence electrons, some of which are lone pairs. We have not included such species here because the majority of compounds fall into the categories we have described.

Multiple Bonds and Molecular ShapesIn a double bond, two electron pairs are shared between a pair of atomic nuclei. Despite the fact that the two electron pairs repel each other, they must remain between the nuclei, and so they cannot avoid each other. Therefore, for purposes of predicting molecular geometry, the two electron pairs in a double bond behave as one. They will, however, be somewhat “fatter” than a single electron-pair bond. For the same reason the three electron pairs in a triple bond behave as an “extra-fat” bond.As an example of the multiple-bond rules, consider hydrogen cyanide, HCN. The Lewis structure is

H—C≡≡N:

Treating the triple bond as if it were a single “fat” electron pair, we predict a linear molecule with an H―C―H angle of 180°. This is confirmed experimentally. Another example is formaldehyde, CH2O, whose Lewis structure is

Since no lone pairs are present on C, the two H’s and the O should be arranged trigonally, with all four atoms in the same plane. Also, because of the “fatness” of the double bond, squeezing the C—H bond pairs together, we expect the H―C―H angle to be slightly less than 120°. Experimentally it is found to have the value of 117°.

EXAMPLE 7.2 Predict the shape of the two molecules (a) nitrosyl chloride, NOCl, and (b) carbon dioxide, CO2.

Solution

a) We must first construct a skeleton structure and then a Lewis diagram. Since N has a valence of 3, O a valence of 2, and Cl is monovalent, a

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Cl

O

probable structure for NOCl is

O==N—Cl

Completing the Lewis diagram, we find

Since N has two bonds and one lone pair, the molecule must be angular. The O—N—Cl angle should be about 120°. Since the “fat” lone pair would act to reduce this angle while the “fat” double bond would tend to increase it, it is impossible to predict at this level of argument whether the angle will be slightly larger or smaller than 120°.b) The Lewis structure of CO2 was considered in the previous chapter and found to be

Since C has no lone pairs in its valence shell and each double bond acts as a fat bond pair, we conclude that the two O atoms are separated by 180° and that the molecule is linear.

7.3 ORBITALS CONSISTENT WITH MOLECULAR SHAPES

In Chap. 6 we showed that a covalent bond results from an overlap of atomic orbitals—usually one orbital from each of two bonded atoms. Maximum bond strength is achieved when maximum overlap occurs. When we try to integrate this idea of orbital overlap with VSEPR theory, however, a problem arises. For example, the 2s and 2p atomic orbitals shown in Plate 5 for a C atom are either spherically symmetrical (2s) or dumbbell shaped at angles of 90° to each other (2px2py). The VSEPR theory predicts that the C―Cl bonds in CCl4 are oriented at angles of 109.5° from one another and that all four C―Cl bonds are equivalent. If each C―Cl bond is formed by overlap of Cl orbitals with C 2s and 2p orbitals, it is hard to understand how four equivalent bonds can be formed. It is also difficult to see why the angles between bonds are 109.5° rather than the 90° angle between p orbitals

sp Hybrid OrbitalsTo answer these questions we need to consider a simpler case, that of beryllium chloride. As we have already seen in Fig. 7.1, VSEPR theory predicts a linear BeCl2 molecule with a Cl—Be—Cl angle of 180°, in agreement with experiments. However, if we associate two valence electron pairs with a beryllium atom and place them in the lowest energy orbitals, we obtain the configuration 2s22x

2 . (Note that since the electrons must be paired in order to form bonds, they do not obey Hund’s rule.) The electron density distribution of such a configuration is shown in Fig. 7.4a. The 2s cloud is indicated in red, and the 2p cloud in gray.

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(b)Figure 7.4 Electron-density distribution for the valence electron configuration 2s22x

2 . (a) Color coded to shows s (red) and px (gray) electron densities. (b) Color coded to show left-pointing and right-pointing sp hybrid orbital electron densities. (Computer generated.) (Copyright © 1975 by W. G. Davies and J. W. Moore.)

An important aspect of Fig. 7.4a the fact that the density of electron probability is greater along the x axis than in any other direction. This can be seen more clearly in Fig. 7.4b where the dots have been color coded to indicate one electron pair to the left and one to the right of the nucleus. In this case we should certainly predict that chlorine atoms bonded to the beryllium through these electron pairs would lie on a straight line, the x axis. Each of the two orbitals whose electron densities are shown in Fig. 7.4b is called an sp hybrid. The word hybrid indicates that each orbital is

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derived from two or more of the atomic orbitals discussed in Chap. 5, and the designation sp indicates that a single s and a single p orbital contributed each sp hybrid.Careful comparison of the s and p electron densities with those of the two sp hybrid orbitals will reveal another important fact. For every dot in Fig. 7.4a, there is a corresponding dot (in the same location) in Fig. 4.7b. That is, the overall electron density (due to the four electrons occupying two orbitals) is exactly the same in both cases. We have not created something new with the two sp hybrids. Rather, we are looking at the same electron density, but we have color coded it to emphasize its concentration along the x axis. In actual fact all electrons are identical—we cannot distinguish one from another experimentally. Labeling one electron cloud as s and another as p is an aid to our thinking, just as color-coding one sp-hybrid electron density red and the other gray is, but it is the overall electron density which determines the experimentally observable molecular geometry. In other words, it does not matter to the molecule whether we think of the total electron cloud as being formed from s and p orbitals or from sp hybrids, but it does matter to us. It is much easier to think of two Be—Cl bonds separated by 180° in terms of sp hybrids than in terms of separate s and p orbitals. By contrast it is much easier to explain the periodic table by using s and p orbitals rather than hybrids. Since both correspond to the same physical reality, we can use whichever approach suits us best.When beryllium forms a linear molecule such as beryllium chloride, it is not the sp hybrids themselves that form the two bonds but rather an overlap between each of these orbitals and some orbital on each other atom. The situation is shown schematically in Fig. 7.5. As a result of each orbital overlap, there is a concentration of electron density between two nuclei. This pulls the nuclei together and forms a covalent bond.

Hybrid Orbitals

Somewhat more complex hybrid orbitals are found in BCl3, where the boron is surrounded by three electron pairs in a trigonal arrangement. If we place these three electron pairs in the valence shell of boron, one 2s and two 2p orbitals (2px and 2px, for example) will be filled, giving the electron configuration 2s22x

22y2 . This configuration is illustrated in Plate

6a as a dot-density diagram in which each electron pair is represented in a different color. Immediately below this diagram is another (Plate 6b) which is dot-for-dot the same as the upper diagram but with a different color-coding. The total electron density distribution is the same in both diagrams, but in the bottom diagram the three electron pairs are distributed in a trigonal arrangement. All three electron clouds are identical in shape, and they are

Figure 7.5 Schematic representation of the bonding in BeCl2. Each of the two sp hybrids around the Be overlaps with an orbital from a chlorine atom, The result is a concentration of negative charge between the beryllium and each of the chlorine nuclei, holding them together.

oriented at angles of 120° with respect to each other. Again we have a set of hybrid orbitals,

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but this time they are derived from one s orbital and two p orbitals. Accordingly these orbitals are known as sp2 hybrids.Because electrons are indistinguishable, and because both the upper and the lower diagrams in Plate 6 correspond to an identical total electron density, we are entitled to use either formulation when it suits our purposes. To explain the trigonal geometry of BCl 3 and similar molecules, the hybrid picture is obviously more suitable than the s and p picture. The three electron-pair bonds usually formed by boron result from an overlap of each of these three sp2 hybrids with a suitable orbital in each other atom.

Hybrid OrbitalsIn addition to the two hybrids just considered, a third combination of s and p orbitals, called sp3 hybrids, is possible. As the name suggests, sp3 hybrids are obtained by combining an s orbital with three p orbitals (px, py and pz). Suppose we have two s electrons, two px

electrons, two py electrons, and two pz electrons, as shown by the boundary-surface diagrams in Fig. 7.6. When these are all arranged around the nucleus, the total electron cloud is essentially spherical. The boundary-surface diagrams in Fig. 7.6 show a slightly “bumpy” surface, but we must remember that electron clouds are fuzzy and do not stop suddenly at the boundary surface shown. When this fuzziness is taken into account, the four atomic orbitals blend into each other perfectly to form an exactly spherical shape. This blending of s and p orbitals is much the same as that discussed in the previous cases of sp and sp2 hybrids. It can be clearly seen in two dimensions in Fig. 7.4 or Plate 6a. The total electron probability cloud in Fig. 7.6 can be subdivided in a different way―into four equivalent sp3 hybrid orbitals, each occupied by two electrons. Each of these four hybrid orbitals has a similar appearance to each of the sp and sp2 hybrids encountered previously, but the sp3 hybrids are arranged tetrahedrally around the nucleus. The four sp3 orbitals, each occupied by two electrons, also appear bumpy in a boundary-surface diagram, hut when the fuzziness of the electron clouds is taken into account, the result is a spherical electron cloud equivalent in every way to an ns2np6 configuration. We are thus equally entitled to look at an octet of electrons in terms of four sp3 hybrids, each doubly occupied, or in terms of one s and three p orbitals, each doubly occupied. Certainly, nature cannot tell the difference!According to the VSEPR theory, if an atom has an octet of electrons in its valence shell, these electrons are arranged tetrahedrally in pairs around it. Since sp3 hybrids also correspond to a tetrahedral geometry, they are the obvious choice for a wave-mechanical description of the octets we find in Lewis structures. Consider, for example, a molecule of methane, CH4, whose Lewis structure is

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Figure 7.7 The use of sp3 hybrid orbitals in explaining the bonding in molecules which obey the octet rule. Each octet is represented by four sp3 hybrids. These hybrids can either overlap with the 1s orbital of H or with another sp3 hybrid as in F―F and H3C—CH3. Alternatively they can form lone pairs (shown in gray), in which case no overlap occurs.

Here the eight electrons of the octet can be thought of as occupying four orbitals, each formed by the overlap of an sp3 hybrid centered on the C atom and a 1s orbital belonging to an H atom. A schematic diagram of this bonding scheme is shown in Fig. 7.7.Also shown in this figure are four other simple molecules whose Lewis structures are

In each case the octets in the Lewis structure can be translated into sp3 hybrids. These sp3

hybrids can either overlap with the 1s orbitals of H or with sp3 hybrids in other atoms. Alternatively they can form lone pairs, in which case no bond is formed and no overlap is necessary.

EXAMPLE 7.3 Suggest which hybrid orbitals should be used to describe the bonding of the central atom in the following molecules: (a) BeF2, (b) PbF2, (c) SbCl3, and (d) InCl3.

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4 3 2CH , methane NH , ammonia H O, water

2 2 6F , fluorine C H , ethane

Solution In making decisions on which hybrids need to be used, we must first decide on the shape of the molecule from VSEPR theory. This in turn is best derived from a Lewis structure.The Lewis structures of these four molecules are

a) In BeF2 there are only two electron pairs in the valence shell of Be. These must be arranged linearly according to VSEPR theory thus necessitating an sp hybrid.

b) In PbF2 there are three electron pairs in the valence shell of Pb. According to VSEPR theory these will be arranged trigonally and will thus utilize sp2 hybrids.

c) In SbCl3 there is an octet of electrons—four pairs arranged tetrahedrally. Accordingly hybrids are needed to describe the bonding.

d) The atom of In has only three pairs of electrons in its valence shell. As in case b, this produces a trigonal arrangement of electron pairs and is described by sp2 hybridization.

Other Hybrid OrbitalsHybrid orbitals can also be used to describe those shapes which occur when there is more than an octet of electrons in an atom’s valence shell. The combination of a d orbital with an s orbital and three p orbitals yields a set of five dsp3 hybrids. These dsp3 hybrids are directed toward the corners of a trigonal bipyramid. An octahedral arrangement is possible if one more d orbital is included. Then dsp3 hybrids, all at 90° to one another, are formed. Note that as soon as we get beyond an octet (whose eight electrons fill all the s and p orbitals), the inclusion of d orbitals is mandatory. This can only occur for atoms in the third row of the periodic table and below. Thus bonding in PCl5 involves dsp3 hybrid orbitals which include a P 3d orbital. Bonding in SF6 uses two S 3d orbitals in d2sp3 hybrids.

7.4 ORBITAL DESCRIPTIONS OF MULTIPLE BONDS

The association of four sp3 hybrid orbitals with an octet can be applied to multiple bonds as well as single bonds. A simple example is ethene (ethylene), C2H4. The Lewis structure for this molecule is

As shown in Fig. 7.8, we can look at the double bond as being formed by two overlaps of sp3 hybrid orbitals, one above the plane of the molecule and one

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Figure 7.8 The bent-bond or "banana-bond" model of double bond. In (a) two sp3 hybrid orbitals overlap without pointing directly at each other. The resulting bond, shown in (b), is somewhat curved and in this representation resembles two bananas. Note that the tetrahedral arrangement of bonds around each carbon atom requires that all four hydrogens and both carbons lie in the same plane.

below. Since the orbitals which overlap are not pointing directly at each other, each of these bonds is referred to as a bent bond or (more frivolously) as a banana bond.In ball-and-stick models of molecules, a double bond is usually represented by two springs or by curved sticks (shown in Fig. 7.9) joining the two atoms together. In making such a model, it is necessary to bend the springs a fair amount in order to fit them into the appropriate holes in the balls. The ability to bend or stretch is characteristic of all chemical bonds—not just those between doubly bonded atoms. Thus each atom can vibrate about its most stable position. Perhaps ball-and-spring models would be more appropriate than ball-and-stick models in all cases.The bent-bond picture makes it easy to explain several characteristics of double bonds. As we have noted in the previous chapter, the distance between two atomic nuclei connected by a double bond is shorter than if they were connected by a single bond. In the case of carbon-carbon bonds, for example, the C==C distance is 133 pm, while the C—C distance is 156 pm. This makes sense when we realize that each bent bond extends along a curved path. The distance between the ends of such a path (the C nuclei) is necessarily shorter than the path itself.

Figure 7.9 Ball-and-stick models for the three isomeric forms of C 2H2F2. Carbon atoms are dark gray, hydrogen atoms are light red, and fluorine atoms are light gray. Note that all three structures are different, although the molecular formula is always C2H2F2. Structures (b) and (c) differ only because of the barrier to rotation around the carbon-carbon double bond. (Computer-generated.) (Copyright © 1977 by W. G. Davies and J. W. Moore.)

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Another characteristic of double bonds is that they make it difficult to twist one end of a molecule relative to the other. This phenomenon usually is called a barrier to rotation. Such a barrier accounts for the fact that it is possible to prepare three different compounds with the formula C2H2F2. Their structures are shown in Fig. 7.9. Structure (a) is unique because both F atoms are attached to the same C atom, but (b) and (c) differ only by a 180° flip of the right-hand ==CHF group. If there were no barrier to rotation around the double bond, structures (b) and (c) could interconvert very rapidly whenever they collided with other molecules. It would then be impossible to prepare a sample containing only type (b) molecules or only type (c) molecules.Since they have the same molecular formula, (a), (b) and (c) are isomers. Structure (b) in which the two F atoms are on opposite sides of the double bond is called the trans isomer, while structure (c) in which two like atoms are on the same side is called the cis isomer. It is easy to explain why there is a barrier to rotation preventing the interconversion of these cis and trans isomers in terms of our bent-bond model. Rotation of one part of the molecule about the line through the C atoms will cause one of the bent-bond electron clouds to twist around the other. Unless one-half of the double bond breaks, it is impossible to twist the molecule through a very large angle.

Sigma and Pi BondsAn alternative, somewhat more complex, description of the double bond is the sigma-pi model shown in Fig. 7.10. In this case only two of the p orbitals on each C atom are involved in the formation of hybrids. Consequently sp2 hybrids are formed, separated by an angle of 120°. Two of these hybrids from each C atom overlap with H 1s orbitals, while the third overlaps with an sp2 hybrid on the other C atom. This overlap directly between the two C atoms is called a sigma bond, and is abbreviated by the Greek letter σ. A second carbon-carbon bond is formed by the overlap of the

Figure 7.10 The sigma-pi model of a double bond. Three sp2 hybrids around each carbon atom are indicated in color. Two of these overlap directly between the carbon atoms to form the σ bond. Two p orbitals, one on each C atom, are shown in gray. These overlap sideways to form a π bond, also shown in gray.

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remaining p orbital on one C atom with that on the other. This is called a pi bond, Greek letter π. The pi bond (π bond) has two halves—one above the plane of the molecule, and the other below it. Each electron in the pi bond (π bond) spends half its time in the top part and the other half its time in the bottom part. Overall this sigma-pi picture of the double bond is reminiscent of a hot dog in a bun. The sigma bond (σ bond) corresponds to the wiener, while the pi bond corresponds to the bun on either side of it.Although the sigma-pi picture is more complex than the bent-bond picture of the double bond, it is much used by organic chemists (those chemists interested in carbon compounds). The sigma-pi model is especially helpful in understanding what happens when visible light or other radiation is absorbed by a molecule. We will discuss this subject in some detail in Chap. 21.In actual fact the difference between the two models of the double bond is more apparent than real. They are related to each other in much the same way as s and p orbitals are related to sp hybrids. Figure 7.11 shows two dot-density diagrams for a carbon-carbon double bond in a plane through both carbon nuclei but at right angles to the plane of the molecule. Figure 7.11a corresponds to a sigma-pi model with the sigma bond (σ bond) in color and the pi bond in gray. Figure 7.11b shows two bent bonds. Careful inspection reveals that both diagrams are dot-for-dot the same. Only the color coding of the dots is different. Thus the bent-bond and sigma-pi models of the double bond are just two different ways of dividing up the same overall electron density.A similar situation applies to triple bonds, such as that found in a molecule of ethyne (acetylene), H—C≡≡C—H. As shown in Fig. 7.12a, we can regard this triple bond as being the result of three overlaps of sp3 hybrids on different carbon atoms forming three bent bonds. Alternatively we can regard it as being composed of one sigma bond and two pi bonds, the sigma bond being due to the overlap of an sp hybrid from each carbon atom. Again both pictures of the bond correspond to the same overall electron density, and hence both are describing the same physical reality. We can use whichever one seems more convenient for the problem under consideration.

7.5 POLARITY OF BONDS: ELECTRONEGATIVITY

In the previous chapter we divided chemical bonds into two classes: covalent bonds, in which electrons are shared between atomic nuclei, and ionic bonds, in which electrons are transferred from one atom to the other. However, a sharp distinction between these two classes cannot be made. Unless both nuclei are the same (as in H2), an electron pair is never shared equally by both nuclei. There is thus some degree of electron transfer as well as electron sharing in most covalent bonds. On the other hand there is never a complete transfer of an electron from one nucleus to another. The first nucleus always maintains some slight residual control over the transferred electron.

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Figure 7.11 Dot-density diagrams comparing the sigma-pi and bent-bond models of the double bond. (a) The sigma-pi model. The σ bond is in color and the π bond is in gray. (b) The bent-bond model. One lies above the two nuclei and the other below. Since both diagrams are dot-for-dot the same, they are both describing the same physical reality. (Computer generated.) (Copyright © 1975 by W. G. Davies and J. W. Moore.)

Figure 7.12 Two alternative models for the triple bond in ethyne, H—C≡≡C—H. (a) Three sp3 hybrids from each carbon atom overlap to form three bent bonds. (b) Two sp hybrids overlap to form the sigma bond. Two p orbitals on one carbon overlap with two on the other to form two pi bonds (one in light gray, the other in dark gray). Though these two models appear to be different, the indistinguishability of electrons makes them exactly equivalent.

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PolarizabilityThe latter phenomenon can be observed by studying carefully the lithium hydride ion pair discussed in the previous chapter. Figure 7.13 shows a dot-density diagram of the 1s2

electron cloud of the hydride ion, H–, as well as the two nuclei. For the sake of clarity the two electrons around the nucleus have been omitted. Were it not for the presence of the Li+

ion on the right-hand side of the diagram, we could expect a spherical (or, in the two dimensions shown, a circular) distribution of electron density around the H nucleus. As can be seen by comparing the density of dots to the left of the colored circle in Fig. 7.13 with that to the right, the actual distribution is not exactly circular. Instead the electron cloud is distorted by the attraction of the Li+ ion so that some of the H– 1s2 electron density is pulled into the bonding region between the Li and H nuclei. This contributes partial covalent character to the bond.Distortion of an electron cloud, as described in the previous paragraph, is called polarization. The tendency of an electron cloud to be distorted from its normal shape is referred to as its polarizability. The polarizability of an ion (or an atom) depends largely on how diffuse or spread out its

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Figure 7.13 Electron dot-density diagram for valence electrons in the LiH ion pair. Note that more dots are found to the right than to the left of the colored circle centered on the H nucleus. This is the result of distortion (polarization) of the electron cloud of H– toward Li+ (Computer generated.) (Copyright © 1975 by W. G. Davies and J. W. Moore.)

electron cloud is. For example, most positive ions have relatively small radii, and their electrons are held rather tightly by the excess of protons in the nucleus. Thus their polarizabilities are usually small. Only quite large positive ions such as Cs+ are significantly polarizable. On the other hand, negative ions have excess electrons, large radii, and diffuse electron clouds which can be polarized easily. Thus negative ions, especially large ones, have high polarizabilities. Small, highly charged positive ions can distort them quite extensively.The slight shift in the electron cloud of H– shown in Fig. 7.13 can be confirmed experimentally. An ion pair like LiH has a negative end (H–) and a positive end (Li+). That is, it has two electrical “poles,” like the north and south magnetic poles of a magnet. The ion pair is therefore an electrical dipole (literally “two poles“), and a quantity known as its dipole moment may be determined from experimental measurements. The dipole moment μ is proportional to the size of the separated electrical charges Q and to the distance r between them:

μ = Qr (7.1)

In the LiH ion pair the two nuclei are known to be separated by a distance of 159.5 pm. If the bond were completely ionic, there would be a net charge of –1.6021 × 10–19 C (the electronic charge) centered on the H nucleus and a charge of +1.6021 × 10–19 C centered on the Li nucleus:

The dipole moment would then be given by

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+

19 19

H Li

1.6 10 C +1.6 10 C

159.5 pm

The measured value of the dipole moment for the LiH ion pair is only about 77 percent of this value, namely, 1.963 × 10–29 C m. This can only be because the negative charge is not centered on the H nucleus but shifted somewhat toward the Li+ nucleus. This shift brings the opposite charges closer together, and the experimental dipole moment is smaller than would be expected.If we increase the degree of polarization of an ionic bond, a bond which is more covalent than ionic is eventually obtained. This is illustrated in Fig. 7.14. Three bonds involving hydrogen are shown, and the diagrams are arranged so that the midpoint of each bond lies on the same vertical (dashed) line. We have already discussed the bond between hydrogen and lithium, in which most of the electron density is associated with hydrogen. By comparison, electron density in the bond between hydrogen and carbon is much more evenly distributed—the bond certainly appears to be covalent. The third bond involves fluorine, which is three places farther to the right along the second row of the periodic table than carbon. In the H―F bond, electron density has been distorted away from hydrogen even more. Thus as we move from lithium with a nuclear charge of +3, through carbon witha nuclear charge of +6, to fluorine with a nuclear charge of +9, there is a continual shift in electron density away from hydrogen. The original H– ion is polarized to the point where much of its electron density has been removed, and it begins to look more like an H+ ion.

Polar Covalent BondsSo far we have looked at polarization from the standpoint of ions, but we could consider the same three bonds shown in Fig. 7.14 by assuming that each had been distorted from a purely covalent, electron-pair sharing situation. In that case the H―C bond is closest to pure covalency—in it the H has roughly the same electron density it would have in an H 2

molecule. In the H—Li bond, however, the H has almost complete control over both electrons and hence has a negative charge. This situation is often indicated as follows:

The Greek letter δ (delta) is used here to indicate that electron transfer is not complete and that some sharing takes place. The dipole moment of LiH shows that in effect only 77 percent of a full electronic charge has been transferred to H, and so δ = 0.77. If the transfer had been complete, δ would have been 1.0. Because the Li—H bond is only partially negative at the one end and partially positive at the other, we often say that the bond is polar or polar covalent, rather than 100 percent ionic.The H―F bond is also a polar covalent bond, but in this case F rather than H has the partial negative charge, and we can write

Again the value of δ can be found from a dipole-moment measurement, as the following example illustrates.

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Figure 7.14 Comparison of bonding electron densities in (a) LiH band; (b) CH bond; and (c) HF bond. Note the considerable shift of electron density away from hydrogen as the electronegativity of its bonding partner increases. (Computer generated.) (Copyright © 1975 by W. G. Davies and J. W. Moore.)

EXAMPLE 7.4 The dipole moment of the HF molecule is found to be 6.37 × 10–30 C m, while the H―F distance is 91.68 pm. Find the partial charge on the H and F atoms.

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Solution Rearranging Eq. (7.1), we have

Thus the apparent charge on each end of the molecule is given by

Since the charge on a single electron is , we have

It is worth noting in the above example that the dipole moment measures the electrical imbalance of the whole molecule and not just that of the H―F bonding pair. In the HF molecule there are four valence electron pairs:

The three lone pairs sticking out on the right of the F atom also contribute to the overall negative charge of F.

ElectronegativityThe ability of an atom in a molecule to attract a shared electron pair to itself, forming a polar covalent bond, is called its electronegativity. The negative side of a polar covalent bond corresponds to the more electronegative element. Furthermore the more polar a bond, the larger the difference in electronegativity of the two atoms forming it.Unfortunately there is no direct way of measuring electronegativity. Dipole-moment measurements tell us about the electrical behavior of all electron pairs in the molecule, not just the bonding pair in which we are interested. Also, the polarity of a bond depends on whether the bond is a single, double, or triple bond and on what the other atoms and electron pairs in a molecule are. Therefore the dipole moment cannot tell us quantitatively the difference between the electronegativities of two bonded atoms. Various attempts have been made over the years to derive a scale of eledronegativities for the elements, none of which is entirely satisfactory. Nevertheless most of these attempts agree in large measure in telling us which elements are more electronegative than others. The best-known of these scales was devised by the Nobel prize-winning California chemist

Linus Pauling (1901 to 1994) and is shown in Table 7.l. In this scale a value of 4.0 is

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arbitrarily given to the most electronegative element, fluorine, and the other electronegativities are scaled relative to this value.As can be seen from this table, elements with electronegativities of 2.5 or more are all nonmetals in the top right-hand comer of the periodic table. These have been color-coded dark red. By contrast, elements with negativities of l.3 or less are all metals on the lower left of the table. These elements have been coded in dark gray. They are often referred to as the most electropositive elements, and they are the metals which invariably form binary ionic compounds (Sec. 6.3). Between these two extremes we notice that most of the remaining metals (largely transition metals) have electronegativities between l.4 and l.9 (light gray), while most of the remaining nonmetals have electronegativities between 2.0and 2.4 (light red). Another feature worth noting is the very large differences in electronegativities in the top right-hand comer of the table. Fluorine, with an electronegativity of 4, is by far the most electronegative element. At 3.5 oxygen is a distant second, while chlorine and nitrogen are tied for third place at 3.0.If the electronegativity values of two atoms are very different, the bond between those atoms is largely ionic. In most of the typical ionic compounds discussed in the previous chapter, the difference is greater than l.5, although it is dangerous to attach too much significance to this figure since electronegativity is only a semiquantitative concept. As the electronegativity difference becomes smaller, the bond becomes more covalent. An important example of an almost completely covalent bond between two dif-

TABLE 7.1 Relative Electronegativities of the Elements.*

* Data from Linus Pauling. “The Nature of the Chemical Bond,” 3d ed., Cornell University Press, Ithaca, N.Y., 1960.

ferent atoms is that between carbon (2.5) and hydrogen (2.l). We will describe the

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properties of numerous compounds of hydrogen and carbon (hydrocarbons) in the next chapter. These properties indicate that the C―H bond has almost no polar character.

EXAMPLE 7.5 Without consulting Table 7.l, arrange the following bonds in order of decreasing polarity: B—Cl, Ba—Cl, Be—Cl, Br—Cl, Cl—Cl.

Solution We first need to arrange the elements in order of increasing electronegativity. Since the electronegativity increases in going up a column of the periodic table, we have the following relationships:

Ba < Be and Br < Cl

Also since the electronegativity increases across the periodic table, we have

Be < B

Since B is a group III element on the borderline between metals and non-metals, we easily guess that

B < Br

which gives us the complete order

Ba < Be < B < Br < Cl

Among the bonds listed, therefore, the Ba—Cl bond corresponds to the largest difference in electronegativity, i.e., to the most nearly ionic bond. The order of bond polarity is thus

Ba—Cl > Br—Cl > B—Cl > Br—Cl > Cl—Cl

where the final bond, Cl—Cl, is, of course, purely covalent.

Polarity in Polyatomic MoleculesWhen more than one polar bond is present in the same molecule, the polarity of one bond may cancel that of another. Thus the presence of polar bonds in a polyatomic molecule does not guarantee that the molecule as a whole will have a dipole moment. In such a case it is necessary to treat each polar bond mathematically as a vector and represent it with an arrow. The length of such an arrow shows how large the bond dipole moment is, while the direction of the arrow is a line drawn from the positive to the negative end of the bond. Adding the individual bond dipole moments as vectors will give the overall molecular dipole moment.As an example of this vector addition, consider the BF3 molecule in Fig. 7.l5. The dipole moments of the three B―F bonds are represented by the arrows BF′ (pointing straight left), BF′′ (pointing down to the right), and

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.

Figure 7.15 Although all three BF bonds in BF3are very polar, they are arranged at 120° to each other. The net result is that the three dipole moments (indicated by colored arrows) cancel each other and there is zero dipole moment.

BF′′′ (pointing up to the right). The sum of vectors BF′′ and BF′′′ may be obtained by the parallelogram law—a line from F′′′ drawn parallel to BF′′ intersects a line from F′′ drawn parallel to BF′′′ at point E. Thus the resultant vector BE (the diagonal of the parallelogram BF′′′EF′′) is the sum of BF′′ and BF′′′. The resultant BE is exactly equal in length and exactly opposite in direction to bond dipole BF′′. Therefore the net result is zero dipole moment.Those arrangements of equivalent bonds that give zero dipole moment in this way are shown in Fig. 7.16. In addition to the trigonal arrangement just discussed, there is the obvious case of two equal bonds 180° apart. The other is much less obvious, namely, a tetrahedral arrangement of equal bonds. Any combination of these arrangements will also be nonpolar. The molecule PF5 for example, is nonpolar since the bonds are arranged in a trigonal bipyramid, as shown in Fig. 7.1. Since three of the five bonds constitute a trigonal arrangement, they will have no resultant dipole moment. The remaining two bonds have equal but opposite dipoles which will likewise cancel.If we replace any of the bonds shown in Fig. 7.16 with a different bond, or with a lone pair, the vectors will no longer cancel and the molecule will

Figure 7.16 The simplest arrangements of equivalent bonds around a central atom which produce a resultant dipole moment of zero: (a) linear; (b) trigonal. The two right-hand bonds (black resultant) cancel the left-hand bond. (c) Tetrahedral. The three right-hand bonds (black resultant) cancel the left-hand bond.

have a resultant dipole moment. Using this rule together with VSEPR theory, you can

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predict whether a molecule is polar or not. You can also make a rough estimate of how polar it will be.

EXAMPLE 7.6 Which of the following molecules are polar? About how large a dipole moment would you expect for each? (a) CF4; (b) CHF3; (c) H2O; (d) NF3.

Solution

a) VSEPR theory predicts a tetrahedral geometry for CF4. Since all four bonds are the same, this molecule corresponds to Fig. 7.16c. It has zero dipole moment.

b) For CHF3, VSEPR theory again predicts a tetrahedral geometry. However, all the bonds are not the same, and so there must be a resultant dipole moment. The C―H bond is essentially nonpolar, but the three C―F bonds are very polar and negative on the F side. Thus the molecule should have quite a large dipole moment:

The resultant dipole is shown in color.

c) The O atom in H2O is surrounded by four electron pairs, two bonded to H atoms and two lone pairs. All four pairs are not equivalent, and so there is a resultant dipole. Since O is much more electronegative than H, the two O―H bonds will produce a partial negative charge on the O. The two lone pairs will only add to this effect.

d) In NF3 the N atom is surrounded by four electron pairs in an approximately tetrahedral arrangement. Since all four pairs are not equivalent, the molecule is polar. The dipole moment, though, is surprisingly small because the lone pair cancels much of the polarity of the N―F bonds:

7.6 OXIDATION NUMBERS

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A useful concept, closely related to electronegativity, is the oxidation number of an element or atom. The oxidation number is a somewhat artificial device—invented by chemists, not by molecules—which enables us to keep track of electrons in complicated reactions. We can obtain oxidation numbers by arbitrarily assigning the electrons of each covalent bond to the more electronegative atom in the bond. When this has been done for all bonds, the charge remaining on each atom is said to be its oxidation number. If two like atoms are joined, each atom is assigned half the bonding electrons.

EXAMPLE 7.7 Determine the oxidation number of each atom in each of the following formulas: (a) Cl2; (b) CH4; (c) NaCl; (d) OF2; (e) H2O2.

Solution In each case we begin by drawing a Lewis diagram:

In each Lewis diagram, electrons have been color coded to indicate the atom from which they came originally. The boxes enclose electrons assigned to a given atom by the rules for determining oxidation number.

a) Since the bond in Cl2 is purely covalent and the electrons are shared equally, one electron from the bond is assigned to each Cl, giving the same number of valence electrons (7) as a neutral Cl atom. Thus neither atom has lost any electrons, and the oxidation number is 0. This is indicated by writing a 0 above the symbol for chlorine in the formula

b) Since C is more electronegative than H, the pair of electrons in each C―H bond is assigned to C. Therefore each H has lost the one valence electron it originally had, giving an oxidation number of +1. The C atom has gained four electrons, giving it a negative charge and hence an oxidation number of – 4:

c) In NaCl each Na atom has lost an electron to form an Na+ ion, and each Cl atom has gained an electron to form Cl–. The oxidation numbers therefore correspond to the ionic charges:

+1 1

Na Cl

d) Since F is more electronegative than O, the bonding pairs are assigned to F in oxygen

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difluoride (OF2). The O is left with four valence electrons, and each F has eight. The oxidation numbers are

e) In Hydrogen peroxide (H2O2) the O—H bond pairs are assigned to the more electronegative O’s, but the O―O bond is purely covalent, and the electron pair is divided equally. This gives each O seven electrons, a gain of 1 over the neutral atom. The oxidation numbers are

Although one could always work out Lewis diagrams to obtain oxidation numbers as shown in Example 7.7, it is often easier to use a few simple rules to obtain them. The rules summarize the properties of oxidation numbers illustrated in the example.

1 The oxidation number of an atom in an uncombined element is 0. Since atoms of the same element always form pure covalent bonds, they share electrons equally, neither losing nor gaining, e.g., Cl2 in Example 7.7a.

2 The oxidation number of a monatomic ion equals the charge on that ion, e.g., Na+

and Cl–in Example 7.7c.

3 Some elements have the same oxidation number in nearly all their compounds.a Elements in periodic group IA have oxidation numbers of +1, and elements in periodic group IIA have oxidation numbers of +2, e.g., Na+ in Example 7.7c.b The most electronegative element, fluorine, is always assigned both electrons from any bond in which it participates. This gives fluorine an oxidation number of –1 in all its compounds, e.g., OF2 in Example 7.7d.c Oxygen usually exhibits an oxidation number of –2, but exceptions occur in peroxides (Example 7.7e), superoxides, and when oxygen combines with fluorine (Example 7.7d).d Hydrogen exhibits an oxidation number of +1 unless it is combined with an element more electropositive than itself, e.g., with lithium, in which case its oxidation number is –1.

4 The sum of the oxidation numbers of all atoms in a complete formula must be 0 ; that is, when an electron is lost by one atom (+1 contribution to oxidation number), the same electron must be gained by another atom (–1 contribution to oxidation number).

5 If a polyatomic ion is considered by itself, the sum of the oxidation numbers of its constituent atoms must equal the charge on the ion.

As an illustration of these rules, let us consider a few more examples.

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EXAMPLE 7.8 Determine the oxidation number of each element in each of the following formulas: (a) NaClO; (b) ClO4

–; and (c) MgH2.

Solution

a) Since Na is a group IA element, its oxidation number is +1 (rule 3a). The oxidation number of O is usually –2 (rule 3c). Therefore (rule 4), +1 + oxidation number of Cl + (–2) = 0.

Oxidation number of Cl = 2 – 1 = +1

Thus we write the formula

if oxidation numbers are to be included.

b) In this case the oxidation numbers must add to –1, the charge on the polyatomic ion. Since O is usually –2, we have

Oxidation number of Cl + 4(–2) = –1Oxidation number of Cl = –1 + 8 = +7

c) In MgH2, H is combined with an element more electropositive than itself, and so its oxidation number is –1. Mg is in group IIA, and so its oxidation number is +2:

As a check on these assignments, it is wise to make sure that the oxidation numbers sum to 0:

+2 + 2(–1) = 0 OK

Oxidation numbers are mainly used by chemists to identify and handle a type of chemical reaction called a redox reaction, or an oxidation-reduction reaction. This type of reaction can be recognized because it involves a change in oxidation number of at least one element. We will deal with redox reactions in Chap. 11. Oxidation numbers are also used in the names of compounds. The internationally recommended rules of nomenclature involve roman numerals which represent oxidation numbers. For example, the two bromides of mercury, Hg2Br2 and HgBr2, are called mercury(I) bromide and mercury(II) bromide, respectively. Here the numeral I refers to an oxidation number of +1 for mercury, and II to an oxidation number of +2.Oxidation numbers can sometimes also be useful in writing Lewis structures, particularly for oxyanions. In the sulfite ion, SO3

2– for example, the oxidation number of sulfur is +4, suggesting that only four

sulfur electrons are involved in the bonding. Since sulfur has six valence electrons, we

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conclude that two electrons are not involved in the bonding, i.e., that there is a lone pair. With this clue, a plausible Lewis structure is much easier to draw:

7.7 SOME DIFFICULTIES WITH ELECTRON-PAIR BONDS

Free RadicalsAll the molecules we have discussed up to this point have involved pairs of electrons. However, there are a few stable molecules which contain an odd number of electrons. In such cases it is impossible to arrange the electrons in pairs, let alone into octets. Three well-known examples of such molecules are nitrogen (II) oxide, nitrogen(IV) oxide, and chlorine dioxide. The most plausible Lewis structures for these molecules are

Molecules like this are called odd-electron molecules or free radicals. Free radicals are usually more reactive than the average molecule in which all electrons are paired. In particular they tend to combine with other molecules so that their unpaired electron finds a partner of opposite spin. Since most molecules have all electrons paired, such reactions usually produce a new free radical. This is one reason why automobile emissions which cause even small concentrations of NO and NO2 to be present in the air can be a serious pollution problem. When one of these free radicals reacts with other automobile emissions, the problem does not go away. Instead a different free radical is produced which is just as reactive as the one which was consumed. To make matters worse, when sunlight interacts with NO2, it produces two free radicals for each one destroyed:

In this way a bad problem is made very much worse.

Resonance

In addition to molecules containing odd numbers of electrons, there is another class of molecules which does not fit easily into the electron-pair theory of the covalent bond that we have developed in the last two chapters. For these molecules it is possible to draw more than one Lewis structure which obeys the octet rule but which is unsatisfactory in other ways. A

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simple example of such a molecule is ozone, an unusual form of oxygen, whose molecular formula is O3. Like the oxides of nitrogen, ozone is important in a discussion of atmospheric-pollution problems, but for the moment we will confine ourselves to its structure.We can draw two Lewis diagrams for O3, both of which obey the octet rule:

Structure 1 suggests that there is an O―O single bond on the left and an O==O double bond on the right side of the molecule. Structure 2 suggests the opposite placement of the double bond. However, it seems unlikely that electrons should be able to distinguish left from right in this way, and experimental evidence confirms this suspicion. Both bonds are found to have the same length, namely, 128 pm. This is intermediate between the O==O double-bond length of 121 pm in O2 and the O―O single-bond length of 147 pm in H2O2. In other words the structure of O2 is somehow intermediate in character between the two structures shown.On a mathematical level we can satisfactorily account for the properties of ozone by regarding its structure as a hybrid of the two structures shown above, the term hybrid having exactly the same sense as for hybrids. We then obtain an electron probability distribution in which both bonds receive equal treatment and are intermediate in character between double and single bonds. Such a structure is called a resonance hybrid and is indicated in one of the following ways:

The term resonance and the use of a double-headed arrow, are both unfortunate since they suggest that the structure is continually oscillating between the two alternatives, so that if only you were fast enough, you could “catch” the double bond on one side or the other. One can no more do this than “catch” an sp hybrid orbital instantaneously in the form of an s or a p orbital.The most important example of resonance is undoubtedly the compound benzene, C6H6, which has the structure

The circle within the second formula indicates that all C—C bonds in the hexagon are equivalent. This hexagonal ring of six carbon atoms is called a benzene ring. Each carbon-carbon bond is 139 pm long, intermediate

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or

between the length of a C―C single bond (154 pm) and a C==C double bond (135 pm). Whereas a double bond between two carbon atoms is normally quite reactive, the bonding between the carbons in the benzene ring is difficult to alter. In virtually all its chemical reactions, the ring structure of benzene remains intact.Even when a molecule exhibits resonance, it is still possible to predict its shape. Any bonds which are intermediate in character can be treated as though they were single bonds, though perhaps a bit fatter. On this basis one would predict that the ozone molecule is angular rather than linear because of the lone pair on the central oxygen atom, with an angle slightly less than 120°. Experimentally the angle is 117°. In the same way each carbon atom in benzene can be expected to be surrounded by three atoms in a plane around it, separated by angles of approximately 120°. Again this agrees with experiment. All the atoms in C 6H6 lie in the same plane, and all bond angles are 120°.

EXAMPLE 7.9 Write resonance structures to indicate the bonding in the carbonate ion, CO3

2–. Predict the O—C—O angle and the carbon-oxygen bond length.

Solution We must first write a plausible Lewis structure for the ion. Counting valence electrons, we have a total of

4(from C) + 3 × 6(from O) + 2(from charge) = 24 electrons

There are 4 octets to be filled, making a total of 4 × 8 = 32 electrons. We must thus count 32 – 24 = 8 electrons twice, and so there are 4 shared pairs. Since there are only three oxygen atoms, one must be double bonded to the carbon atom:

Two other equivalent structures can also be drawn, and so the carbonate ion corresponds to the following resonance hybrid:

Since the carbon has no lone pairs in its valence shell, the three oxygens should be arranged trigonally around the carbon and all four atoms should lie in a plane. As we saw in the previous chapter, the C—O single-bond length is 143 pm, while the C==O double-bond length is 122 pm. We can expect the carbon-oxygen distance in the carbonate ion to lie somewhere in between these values. Experimentally it is found to be 129 pm.

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SUMMARYIn this chapter we have seen that Lewis diagrams may be used to predict the structures for covalently bonded molecules. This applies to some which do not obey the octet rule and to those which contain multiple bonds, as well as to simple molecules. The structure of a molecule is determined by the positions of atomic nuclei in three-dimensional space, hut it is repulsions among electron pairs—bonding pairs and lone pairs—which determine molecular geometry.When dealing with molecular shapes, it is often convenient to think in terms of sp, sp2, sp3, or other hybrid orbitals. These correspond to the same overall electron density and to the same physical reality as do the s and p atomic orbitals considered earlier, but hybrid orbitals emphasize the directions in which electron density is concentrated. Hybrid orbitals may also be used to describe multiple bonding, in which case bonds must he bent so that two or three of them can link the same pair of atoms. A second equivalent approach to multiple bonding involves sigma and pi bonds. Electron density in a sigma bond is concentrated directly between the bonded nuclei, while that of the pi bond is divided in two—half on one side and half on the other side of the sigma bond.No chemical bond can be 100 percent ionic, and, except for those between identical atoms, 100 percent covalent bonds do not exist either. Electron clouds—especially large, diffuse ones—are easily polarized, affecting the electrical balance of atoms, ions, or molecules. Large negative ions are readily polarized by small positive ions, increasing the covalent character of the bond between them. Electron density in covalent bonds shifts toward the more electronegative atom, producing partial charges on each atom and hence a dipole. In a polyatomic molecule, bond dipoles must be added as vectors to obtain a resultant which indicates molecular polarity. In the case of symmetric molecules, the effects of individual bond dipoles cancel and a nonpolar molecule results.Oxidation numbers are used by chemists to keep track of electrons during the course of a chemical reaction. They may be obtained by arbitrarily assigning valence electrons to the more electronegative of two bonded atoms and calculating the resulting charge as if the bond were 100 percent ionic. Alternatively, some simple rules are available to predict the oxidation number of each atom in a formula. Oxidation numbers are used in the names of compounds and are often helpful in predicting formulas and writing Lewis diagrams.In addition to deficiency of electrons and expansion of the valence shell, the octet rule is violated by species which have one or more unpaired electrons. Such free radicals are usually quite reactive. A difficulty of another sort occurs in benzene and other molecules for which more than one Lewis diagram can be drawn. Rearranging electrons (but not atomic nuclei) results in several structures which are referred to collectively as a resonance hybrid. Like an sp hybrid, a resonance hybrid is a combination of the contributing structures and has properties intermediate between them.

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