Spencer L. SeagerMichael R. Slabaugh

www.cengage.com/chemistry/seager

Jennifer P. Harris

Chapter 4:Forces Between Particles

NOBLE GAS CONFIGURATIONS• An electronic configuration that is characterized by two electrons

in the valence shell of helium and eight electrons in the valence shell of all other group VIIIA noble gases.

LEWIS STRUCTURES• A representation of an atom or ion in which the elemental

symbol represents the atomic nucleus and all but the valence-shell electrons. The valence electrons are represented by dots arranged around the elemental symbol.

VALENCE ELECTRONS• Write an electronic configuration for the atom and identify the

valence electrons as those having the largest n value in the configuration.

• A simpler alternative for representative elements is to refer to the periodic table and note the group to which the element belongs. The number of valence electrons is the same as the Roman numeral group number.

• Examples: Calcium, Ca, is in group IIA. The number of valence electrons is 2. Phosphorus, P, is in group VA. The number of valence electrons is 5.

LEWIS STRUCTURE EXAMPLE• Potassium, K, is in group IA and so it has one valence electron.

The Lewis structure is: K • Aluminum, Al, is in group IIIA and so it has three valence

electrons. The Lewis structure is: Al

THE OCTET RULE• According to the octet rule, atoms will gain or lose sufficient

electrons to achieve an outer electron arrangement identical to that of a noble gas. This arrangement usually consists of eight electrons in the valence shell.

• SIMPLE ION• A simple ion is an atom that has acquired a net positive or

negative charge by losing or gaining one or more electrons.

SIMPLE ION EXAMPLES• Magnesium, Mg, has two valence electrons which it loses to

form a simple ion with a +2 electrical charge. The ion is written as Mg2+.

• Oxygen, O, has six valence electrons. It tends to gain two electrons to form a simple ion with a -2 electrical charge. The ion is written as O2-.

• Bromine, Br, has seven valence electrons. It tends to gain one electron to form a simple ion with a -1 electrical charge. The ion is written as Br -.

DETERMINING IONIC CHARGES FOR REPRESENTATIVE ELEMENTS• Representative metals will form ions having the same positive

charge as the number (Roman numeral) of the group to which they belong.

• Representative nonmetals will form ions with a negative charge equal to 8 minus the number (Roman numeral) of the group to which they belong.

• For example, strontium, Sr, a group IIA metal forms Sr2+ ions and phosphorus, P, a group VA nonmetal forms P3- ions.

IONIC BOND FORMATION• Ions with positive charges are attracted to ions with negative

charges. The attractive force between such ions holds them together and is called an ionic bond.

• Ionic bonds form between simple ions when representative metal atoms lose valence electrons and the electrons are gained by representative nonmetal atoms. Both atoms are changed into ions with noble gas configurations. The resulting ions are then attracted to each other.

ISOELECTRONIC• Isoelectronic is a term that literally means “same electronic,”

used to describe atoms or ions that have identical electronic configurations.

IONIC COMPOUNDS• The substances that result when ionic bonds form between

positive and negative ions are called ionic compounds.• When ionic compounds are formed by the reaction of only two

elements the resulting ionic compound is called a binary ionic compound.

Cu2O CuO

BINARY IONIC COMPOUND FORMULAS

• Binary ionic compounds typically form when a metal and a nonmetal react.

• The metal tends to lose one or more electrons and forms a positive ion.

• The nonmetal tends to gain one or more electrons and forms a negative ion.

• The symbol for the metal is given first in the formula.

• The formula for a binary ionic compound represents the minimum number of each ion that will provide equal numbers of positive and negative electrical charges when combined together.

BINARY IONIC COMPOUND FORMULA EXAMPLES• Sodium and fluorine:

• Sodium, a group IA metal, will form sodium ions with the symbol Na+.

• Fluorine, a group VIIA nonmetal, will form fluoride ions with the symbol F-.

• The minimum number of ions needed to give the same number of positive and negative charges is one of each.

• The one Na+ provides one positive charge and the one F-

provides one negative charge. • The correct formula that results is NaF.

BINARY IONIC COMPOUND FORMULA EXAMPLES (continued)• Sodium and sulfur:

• Sodium is a group IA metal and will form sodium ions with the symbol Na+.

• Sulfur is a group VIA nonmetal and will form sulfide ions with the symbol S2-.

• The minimum number of ions required to give the same number of positive and negative charges is two Na+ ions and one S2- ion.

• The two Na+ ions provide two positive charges and the one S2- ion provides two negative charges.

• The resulting formula is Na2S.

BINARY IONIC COMPOUND FORMULA EXAMPLES (continued)• Aluminum and oxygen:

• Aluminum is a group IIIA metal and will form ions with the symbol Al3+.

• Oxygen is a group VIA nonmetal and will form ions with the symbol O2-.

• The minimum number of ions required to give the same number of positive and negative charges is two Al3+ ions and three O2- ions.

• The resulting formulas is Al2O3.

NAMING BINARY IONIC COMPOUNDS

• Binary ionic compounds are named using the following pattern:

name = metal name + stem of nonmetal name + -ide

• The stem names and ionic symbols for some common nonmetals are given in the following table:

BINARY IONIC COMPOUND NAME EXAMPLES• Name K2O:

• name = metal name + nonmetal stem + -ide• name = potassium + ox- + -ide = potassium oxide

• Name Mg3N2:

• name = metal name + nonmetal stem + -ide• name = magnesium + nitr- + -ide = magnesium nitride

• Name BeS:• name = metal name + nonmetal stem + -ide• name = beryllium + sulf- + -ide = beryllium sulfide

• Name AlBr3:

• name = metal name + nonmetal stem + -ide• name = aluminum + brom- + -ide = aluminum bromide

NAMING BINARY IONIC COMPOUNDS (continued)• Some metal atoms, especially those of transition and inner-

transition elements form more than one type of charged ion. (e.g. Iron forms both Fe2+ and Fe3+ ions.)

• The binary compounds containing such ions are named following the pattern given earlier with one addition, the number of positive charges on the metal ion is indicated by a Roman numeral in parentheses following the metal name.

• For example, the compounds FeCl2 and FeCl3 contain iron ions with 2+ and 3+ charges, respectively. Their names are iron (II) chloride and iron (III) chloride.

FeCl2 FeCl3

IONIC COMPOUND STRUCTURE• The stable form of an ionic compound is not a molecule, but a

crystal in which many ions of opposite charge occupy lattice sites in a rigid three-dimensional arrangement called a crystal lattice.

IONIC COMPOUND FORMULAS & WEIGHTS

• Formulas for ionic compounds represent only the simplest combining ratio of the ions in the compounds, not the precise numbers of atoms of each element found in a crystal lattice.

• Formula weight is the sum of the atomic weights of the atoms shown in the formula of an ionic compound. This is similar to molecular weight.

• One mole of an ionic compound contains Avogadro’s number (6.022 x 1023) of the simplest combining ratio of ions in the compounds.

COVALENT BONDS• A covalent bond is a type of bond in which the octet rule is

satisfied when atoms share valence electrons. The shared electrons are counted in the octet of each atom that shares them as illustrated below for fluorine, F2.

• The atoms sharing one or more pairs of electrons are each attracted to the shared electrons, and thus, are attracted to each other. The attraction to each other is called a covalent bond. The covalent bond may be represented by the shared pair or by a single line between the bonded atoms.

COVALENT BONDS (continued)• The sharing of electrons takes place when electron-containing

orbitals of atoms overlap. This is shown below for the formation of the H2 molecule.

COVALENT BONDS (continued)• Electron sharing resulting in covalent bonding can occur

between identical atoms or between different atoms.

• Molecules such as Cl2, O2 and N2 are formed when electron sharing occurs between identical atoms.

• Molecules such as H2O, and CH4 are formed when electron sharing occurs between different atoms.

COVALENT BONDING EXAMPLES

COVALENT BONDING EXAMPLES (continued)

DRAWING LEWIS STRUCTURES FOR COVALENT MOLECULES• Step 1: • Use the molecular formula to determine how many

atoms of each type are in the molecule.

• Step 2: • Use the provided connecting pattern of atoms to draw an

initial molecular structure with the atoms properly arranged.

• Step 3: • Determine the total number of valence-shell electrons

contained in the atoms of the molecule.

DRAWING LEWIS STRUCTURES FOR COVALENT MOLECULES (continued)• Step 4: • Put one pair of electrons between each bonded pair of

atoms in the initial structure drawn in Step 2.• Subtract the number of electrons used in this step from the

total number determined in Step 3. • Use the remaining electrons to complete the octets of all

other atoms in the structure, beginning with the atoms that are present in greatest number in the molecule.

• Remember, hydrogen atoms only require one pair of electrons to achieve the electronic configuration of helium.

• Step 5:• If all octets cannot be satisfied with the available electrons,

move pairs that are not located between atoms to positions between atoms to complete octets. This will create double or triple bonds between some atoms.

DRAWING LEWIS STRUCTURES FOR COVALENT MOLECULES EXAMPLE

• Draw a Lewis structure for SO3.• Step 1: • The formula indicates one S and three O atoms are in the

molecule.• Step 2: • The connecting pattern is that each O is bonded only to

the S. Thus, the following arrangement is drawn:

O S OO

• Step 3: • Sulfur and oxygen are both in group VIA, and so each

atom has six valence electrons. The total number of electrons is 24 (six from the one S atom and 18 from the three O atoms).

DRAWING LEWIS STRUCTURES FOR COVALENT MOLECULES EXAMPLE (continued)

• Step 4:• One pair of electrons is put between each O atom and the S

atom of the arrangement drawn in step 2.

• This required six of the 24 available electrons. The remaining 18 are used to complete the octets of the atoms, beginning with the O atoms.

DRAWING LEWIS STRUCTURES FOR COVALENT MOLECULES EXAMPLE (continued)• Step 5:• After step 4, it is seen that the octet of S is not completed,

even though all available electrons have been used. • One nonbonding pair from any of the three O atoms will be

moved to a location between the O and the S atoms. This pair will continue to count toward the octet of the O, but will also now count toward the octet of the S.

• The resulting correct Lewis structure contains one double bond (two shared pairs) between the S and one of the O atoms.

POLYATOMIC IONS• Polyatomic ions are covalently-bonded groups of atoms that

carry a net electrical charge. Most common polyatomic ions are negatively charged.

• Lewis structures can be drawn for polyatomic ions using the same steps that were shown earlier for covalent molecules with one change:• In Step 3, one electron is added to the total for each negative

charge found on the polyatomic ion and one electron is subtracted from the total for each positive charge found on the polyatomic ion.

• All other steps are used unchanged.

SHAPES OF MOLECULES & POLYATOMIC IONS• Most molecules and polyatomic ions are not flat two-

dimensional objects. Most have distinct three-dimensional shapes.

• The shapes of molecules or polyatomic ions can be predicted using a theory called the valence-shell electron-pair repulsion theory, or VSEPR theory (sometimes pronounced "vesper" theory).

• According to the VSEPR theory, electron pairs in the valence shell of an atom will repel each other and get as far away from each other as possible.

• When the theory is applied to the valence-shell electron pairs of the central atom in a molecule or ion, the shape of the molecule or ion can be predicted. A central atom is an atom that is bonded to other surrounding atoms, such as the S atom in the SO3 molecule whose Lewis structure was drawn in the earlier example.

VSEPR THEORY• When the VSEPR theory is used, two rules are followed:

• Rule 1:• All valence-shell electron pairs around the central atom

are considered to behave the same regardless of whether they are bonding or nonbonding pairs.

• Rule 2: • Double or triple bonds between surrounding atoms and

the central atom are treated like a single pair of electrons when shapes are predicted.

ELECTRON PAIR ARRANGEMENTS• The VSEPR theory can be applied to molecules or ions that

have up to six pairs of electrons around the central atom. This discussion will be limited to molecules with no more than four pairs.

ELECTRON PAIR ARRANGEMENTS• According to the VSEPR theory, the arrangement of electron

pairs around the central atom (represented by E) depends on the number of electron pairs.• Two pairs locate opposite each other. • Three pairs arrange themselves in a flat triangle around the

central atom. • Four pairs become located at the four corners of a pyramid-

like shape called a tetrahedron.

VSEPR THEORY EXAMPLES• Draw a Lewis structure and determine the shape of the molecule

for CO2.

• Solution: • The Lewis structure drawn according to the rules given earlier

is• The central C atom is surrounded by two double bonds.• Each double bond counts as a single electron pair, thus the

central atom behaves as if it has two pairs of electrons around it.

• They will take up positions on opposite sides of the C. • The O, C, and O atoms are arranged in a line and the

molecule is linear.

VSEPR THEORY EXAMPLES (continued)• Draw a Lewis structure and determine the shape of the molecule

for NH3.• Solution:

• The Lewis structure drawn according to the rules given earlier is:

• The central atom is N.• It has four electron pairs surrounding it. • The four pairs will be located at the corners

of a tetrahedron, with the N in the middle:

VSEPR THEORY EXAMPLES (continued)

• The shape of the molecule is determined only by the positions of the atoms, not by the positions of the unshared electron pair. Thus, the NH3 molecule has the shape of a pyramid with a triangular base. The N atom is at the peak of the pyramid and an H atom is at each corner of the base as shown below:

COVALENT MOLECULE POLARITY• The shared electrons of covalent bonds are not always shared

equally by the bonded atoms. • Electrons of a covalent bond are attracted toward atoms of

highest electronegativity.

COVALENT MOLECULE POLARITY (continued)• Unequal sharing of the bonding electrons of a covalent bond cause

the bond to become a polar covalent bond.

• For atoms bonded by a polar covalent bond, the more electronegative atom acquires a partial negative charge (δ-) and the less electronegative atom acquires a partial positive charge (δ+).

COVALENT MOLECULE POLARITY (continued)• When the resulting partial charges are distributed

symmetrically in a molecule, the molecule is nonpolar. When the partial charges are distributed nonsymmetrically, the molecule is polar.

THE POLARITY OF MOLECULES