Covalent Bonding Sec. 8.3: Molecular Structures. Objectives List the basic steps used in drawing...

Covalent BondingCovalent Bonding

Sec. 8.3: Molecular StructuresSec. 8.3: Molecular Structures

ObjectivesObjectives

List the basic steps used in List the basic steps used in drawing Lewis structures.drawing Lewis structures.

Explain why resonance occurs and Explain why resonance occurs and identify resonance structures.identify resonance structures.

Explain 3 exceptions to the octet Explain 3 exceptions to the octet rule and identify molecules in rule and identify molecules in which these exceptions occur.which these exceptions occur.

Molecular StructuresMolecular Structures Molecular structures are used to predict the Molecular structures are used to predict the

arrangement of atoms in a covalent arrangement of atoms in a covalent compoundcompound

5 models are used - see pg. 253, Fig. 135 models are used - see pg. 253, Fig. 13 The The Lewis structureLewis structure model is the most model is the most

useful; letter symbols and bonds show the useful; letter symbols and bonds show the position of atoms & “dots” show unpaired position of atoms & “dots” show unpaired electrons.electrons.

Electron dot diagramsElectron dot diagrams must be drawn to must be drawn to predict the structural formula.predict the structural formula.

Determining Lewis Determining Lewis Structures of Molecular Structures of Molecular

CompoundsCompounds1. 1. Predict the location of the atomsPredict the location of the atoms - -

• Hydrogen is always a terminal or end Hydrogen is always a terminal or end atom; it only shares 1 electron pair.atom; it only shares 1 electron pair.

• The atom with the least attraction for The atom with the least attraction for shared electrons is the central atom - shared electrons is the central atom - the central atom is usually the one closer to the left in the periodic table.

• All other atoms become terminal atoms.All other atoms become terminal atoms.• For example, N is the central atom in NHFor example, N is the central atom in NH33; C is ; C is

the central atom in COthe central atom in CO22..


CompoundsCompounds2.Find the total number of electrons 2.Find the total number of electrons

available for bonding by available for bonding by adding up adding up the number of valence electronsthe number of valence electrons of of each atom in the compound.each atom in the compound.• In NHIn NH33, N has 5 and each H has 1, so the , N has 5 and each H has 1, so the

total is 8.total is 8.

• In COIn CO22, C has 4 and each O has 6, so the , C has 4 and each O has 6, so the total is 16.total is 16.


CompoundsCompounds3. 3. Determine the number of Determine the number of bonding bonding

pairspairs by dividing the total number of by dividing the total number of electrons available by 2. electrons available by 2. – For NHFor NH33: 8/2 = 4: 8/2 = 4

– For COFor CO22: 16/2 = 8: 16/2 = 8


CompoundsCompounds4. 4. Place one bonding pair between the Place one bonding pair between the

central atom and each terminal central atom and each terminal atomatom. .

H

H

H N

CO O


CompoundsCompounds5A. 5A. Subtract the number of pairs you usedSubtract the number of pairs you used

in step 4 from the total pairs you in step 4 from the total pairs you

determined in step 3.determined in step 3.– For NHFor NH33, there are 4 bonding pairs and 3 , there are 4 bonding pairs and 3

were used in step 4. One bonding pair were used in step 4. One bonding pair remains.remains.

– For COFor CO22, there are 8 bonding pairs and 2 , there are 8 bonding pairs and 2 were used in step 4. 6 bonding pairs remain. were used in step 4. 6 bonding pairs remain.


CompoundsCompounds5B.5B. The remaining pairs include The remaining pairs include

lone pairs and pairs used in lone pairs and pairs used in double and triple bonds.double and triple bonds.• First, place lone pairs around each First, place lone pairs around each

terminal atom (EXCEPT HYDROGEN) to terminal atom (EXCEPT HYDROGEN) to satisfy the octet rule.satisfy the octet rule.

• Next, assign any remaining pairs to the Next, assign any remaining pairs to the central atom.central atom.

H – N – H

HO – C - O

In NH3, 1 bonding pair remained. Lone pairs cannot be placed around H, so it must go on the N.

In CO2, 6 bonding pairs remained, so they are placed around the O’s.


CompoundsCompounds6. Determine whether the central atom 6. Determine whether the central atom

satisfies the octet rule. satisfies the octet rule. If it doesn’tIf it doesn’t, , you MUST convert one or two of the you MUST convert one or two of the lone pairs on the terminal atoms into lone pairs on the terminal atoms into a double or triple bond between the a double or triple bond between the terminal and central atoms. Keep in terminal and central atoms. Keep in mind, mind, carbon, nitrogen, oxygen carbon, nitrogen, oxygen and sulfurand sulfur often form double and often form double and triple bonds. triple bonds.

O – C - OSince the C in CO2 is only surrounded by 2 electron pairs, a lone pair on each O must be converted to a double bond.

O = C = O

Practice ProblemsPractice Problems

NFNF33

CSCS22

HH22SeSe

Polyatomic ionsPolyatomic ions The atoms The atoms withinwithin the ion are covalently the ion are covalently

bonded.bonded. Drawing Lewis structures is similar to the Drawing Lewis structures is similar to the

procedure just practiced. The procedure just practiced. The differencedifference is is in step #2.in step #2.– Determine the number of electrons available in Determine the number of electrons available in

the atoms of the ionthe atoms of the ionThen Then subtractsubtract the ion charge if it is the ion charge if it is positivepositive and and

addadd the ion charge if it is the ion charge if it is negativenegative – Put a bracket around the ion structure with the Put a bracket around the ion structure with the

charge as a superscript charge as a superscript

Example: POExample: PO44-3-3

The number of valence electrons The number of valence electrons available are 29available are 29

33 must be must be addedadded to this to this Total of 32 electrons are available - Total of 32 electrons are available -

this is 16 bonding pairsthis is 16 bonding pairs

O

0 – P – 0

O

3-


ClOClO44--

NHNH44++

AsOAsO44-3-3

TeOTeO44-2-2

Resonance StructuresResonance Structures

Using the same sequence of atoms, Using the same sequence of atoms, it is possible to have more than one it is possible to have more than one correct Lewis structure when a correct Lewis structure when a molecule or polyatomic ion has both molecule or polyatomic ion has both a double bond a double bond andand a single bond. a single bond.


ResonanceResonance is a condition that occurs when more than one Lewis is a condition that occurs when more than one Lewis structure can be written for a molecule or ion.structure can be written for a molecule or ion.

Resonance structuresResonance structures are the 2 or more correct Lewis structures for a are the 2 or more correct Lewis structures for a molecule or ionmolecule or ion– They differ ONLY in position of the lone & bonding pairs, NEVER atom They differ ONLY in position of the lone & bonding pairs, NEVER atom

positionspositions


Each actual molecule or ion that Each actual molecule or ion that undergoes resonance undergoes resonance behavesbehaves as if it as if it has only one structure.has only one structure.

The actual bond lengths are The actual bond lengths are equalequal to to each other - the bond length is an each other - the bond length is an averageaverage of the bonds in the of the bonds in the resonance structure.resonance structure.– All bonds are shorter than single bonds All bonds are shorter than single bonds

but longer than double bonds!but longer than double bonds!


Draw the resonance structures ofDraw the resonance structures of– OO33

– NONO22--

– SOSO44-2-2

Exceptions to the Octet RuleExceptions to the Octet Rule

Some molecules and ions do not Some molecules and ions do not obey the obey the octet ruleoctet rule..

Three (3) reasons exist for these Three (3) reasons exist for these exceptions.exceptions.– First, a small group of molecules First, a small group of molecules

has an odd number of valence has an odd number of valence electrons and electrons and cannotcannot form an octet form an octet around each atom.around each atom.


Second, some compounds form with Second, some compounds form with fewer than 8 electrons present around fewer than 8 electrons present around an atom – a suboctet.an atom – a suboctet.

This group is relatively rare; BHThis group is relatively rare; BH33 is an is an example.example.

Compounds like BHCompounds like BH33 tend to be reactive - they tend to be reactive - they will share an entire pair donated by another will share an entire pair donated by another atom.atom.

If this occurs, the bond formed is called a If this occurs, the bond formed is called a coordinate covalent bondcoordinate covalent bond..– Definition: a bond formed when an atom donates a Definition: a bond formed when an atom donates a

pair of electrons to be shared with an atom/ion pair of electrons to be shared with an atom/ion that needs 2 electrons to become stable.that needs 2 electrons to become stable.

– Atoms/ions with Atoms/ions with lone pairs lone pairs often form coordinate often form coordinate covalent bonds with atoms/ions that need 2 covalent bonds with atoms/ions that need 2 electronselectrons

Exceptions to the Octet RuleExceptions to the Octet Rule Third, some compounds have central Third, some compounds have central

atoms that contain more than 8 valence atoms that contain more than 8 valence electrons.electrons.

This electron arrangement is referred to This electron arrangement is referred to as an as an expanded octet.expanded octet.– Expanded octetsExpanded octets are explained by considering are explained by considering

the d orbitals of certain elementsthe d orbitals of certain elements– Lewis structures are drawn by adding lone Lewis structures are drawn by adding lone

pairs or more than 4 bonding pair to the pairs or more than 4 bonding pair to the central atomcentral atom

Covalent Bonding Sec. 8.3: Molecular Structures. Objectives List the basic steps used in drawing...

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