Dr. S. M. Condren

Chapter 9

Bonding and Molecular Structure:

Fundamental Concepts

Dr. S. M. Condren

Chemical BondingProblemsProblems and questions — and questions —

How is a molecule or How is a molecule or polyatomic ion held polyatomic ion held together?together?

Why are atoms distributed at Why are atoms distributed at strange angles?strange angles?

Why are molecules not flat?Why are molecules not flat?

Can we predict the structure?Can we predict the structure?

How is structure related to How is structure related to chemical and physical chemical and physical properties?properties?

Dr. S. M. Condren

Structure & Bonding

NN triple bond. Molecule is NN triple bond. Molecule is unreactiveunreactive

White phosphorus White phosphorus is a tetrahedron of is a tetrahedron of P atoms. P atoms. Very reactive!

Red phosphorus, Red phosphorus, a polymer. Used a polymer. Used in matches. in matches. Less reactive!

Dr. S. M. Condren

Forms of Chemical Bonds

There are 2 extreme forms of connecting or bonding atoms:There are 2 extreme forms of connecting or bonding atoms:

Ionic—complete transfer of —complete transfer of 1 or more electrons from 1 or more electrons from one atom to anotherone atom to another

Covalent——some valence some valence electrons shared between electrons shared between atomsatoms

Most bonds are somewhere in between.

Dr. S. M. Condren

Ionic Compounds

Metal of low IEMetal of low IE

Nonmetal Nonmetal of high EAof high EA

2 Na(s) + Cl2 Na(s) + Cl22(g) ---> (g) ---> 2 Na2 Na++ + 2 Cl + 2 Cl--

Dr. S. M. Condren

Covalent BondingThe bond arises from the mutual The bond arises from the mutual

attraction of 2 nuclei for the same attraction of 2 nuclei for the same

electrons.electrons. Electron sharing results.results.

Bond is a balance of attractive and repulsive forces.Bond is a balance of attractive and repulsive forces.

Dr. S. M. Condren

A bond can result from a “head-to-head”A bond can result from a “head-to-head”

overlap of atomic orbitals on of atomic orbitals on

neighboring atoms.neighboring atoms.

ClH H Cl••

••

••

••

••

••

+

Overlap of H (1s) and Cl (3p)

Note that each atom has a single, unpaired electron.

Bond Formation

Dr. S. M. Condren

Chemical Bonding: Objectives

Objectives are to understand:are to understand:

1. 1. valence e- distribution in in molecules and ions.molecules and ions.

2. 2. molecular structures

3. 3. bond properties and their and their effect on molecular properties.effect on molecular properties.

Dr. S. M. Condren

• Electron distribution is Electron distribution is depicted withdepicted with

Lewis electron dot structures

• Valence electrons are Valence electrons are distributed as shared ordistributed as shared or BOND PAIRS

and unshared and unshared oror LONE PAIRS.

G. N. Lewis G. N. Lewis 1875 - 19461875 - 1946

Electron Distribution in Molecules

Dr. S. M. Condren

• Valence electrons are distributed as Valence electrons are distributed as

shared orshared or BOND PAIRS and and

unshared orunshared or LONE PAIRS.

•

••

•

••

H Cl

shared orbond pair

This is called a This is called a LEWIS ELECTRON DOT structure.structure.

Bond and Lone Pairs

lone pairs

Dr. S. M. Condren

Electrons are divided between Electrons are divided between core and and valence electrons

B 1sB 1s22 2s 2s22 2p 2p11

Core = [He]Core = [He] , , valence = 2s2 2p1

Br [Ar] 3dBr [Ar] 3d1010 4s 4s22 4p 4p55

Core = [Ar] 3dCore = [Ar] 3d1010 , , valence = 4s2 4p5

Valence Electrons

Dr. S. M. Condren

No. of valence electrons of a main group atom = Group number

••For Groups 1A-4A, no. of bond pairs = For Groups 1A-4A, no. of bond pairs = group number.group number.

• For Groups 5A -7A, BP’s = 8 - Grp. No.Group 3A Group 5A

Rules of the Game

Dr. S. M. Condren

Rules of the Game

No. of valence electrons of an atom = Group numberNo. of valence electrons of an atom = Group number

For Groups 1A-4A, no. of bond pairs = group numberFor Groups 1A-4A, no. of bond pairs = group number

For Groups 5A -7A, BP’s = 8 - Grp. No. For Groups 5A -7A, BP’s = 8 - Grp. No.

••Except for H (and sometimes atoms of 3rd and higher periods),

BP’s + LP’s = 4This observation is called theThis observation is called the

OCTET RULE

Dr. S. M. Condren

Hydrophobic vs. Hydrophilic

• Hydrophobic - translation of Greek – water fear

• Hydrophilic – translation of Greek – water friendship

Dr. S. M. Condren

Ammonia, NH3

1. Decide on the central atom; never H.1. Decide on the central atom; never H.

Central atom is atom of lowest affinity Central atom is atom of lowest affinity for electrons.for electrons.

Therefore, N is centralTherefore, N is central

2. Count valence electrons2. Count valence electrons

H = 1 and N = 5H = 1 and N = 5

Total = (3 x 1) + 5 Total = (3 x 1) + 5

= 8 electrons / 4 pairs= 8 electrons / 4 pairs

Building a Dot Structure

Dr. S. M. Condren

3.3. Form a single bond between Form a single bond between the central atom and each the central atom and each surrounding atomsurrounding atom

H H

H

N

Building a Dot Structure

H••

H

H

N4. 4. Remaining electrons form Remaining electrons form LONE PAIRS to complete octet as needed. to complete octet as needed.

3 BOND PAIRS and 1 LONE PAIR.

Note that N has a share in 4 pairs (8 Note that N has a share in 4 pairs (8 electrons), while H shares 1 pair.electrons), while H shares 1 pair.

Dr. S. M. Condren

Lewis StructuresCH4 methane

Step 1. Central atom = C

Step 2. Count valence electronsC = 44 x H = 4 x 1 = 4

TOTAL = 8 e- or 4 pairsStep 3. Form bonds

Dr. S. M. Condren

Lewis Structures

C2H6 ethane

Dr. S. M. Condren

Multiple Covalent Bonds

double bond => 2 pairs shared

triple bond => 3 pairs shared

normally occurs between:

C atoms; N atoms; O atoms;

a C atom and a N, O or S atom

a N atom and a O or S atom

a S atom and an O atom

Dr. S. M. Condren

Double and even triple bonds are commonly observed for C, N, P, O, and S

HH22COCO

SOSO33

CC22FF44

Dr. S. M. Condren

1. Central atom = _______1. Central atom = _______

O OC This leaves 6 pairs.

Carbon Dioxide, CO2

2. Valence electrons = __ or __ pairs2. Valence electrons = __ or __ pairs

3. Form bonds.3. Form bonds.

C4 2

Dr. S. M. Condren

4. Place lone pairs on outer atoms.4. Place lone pairs on outer atoms.

••O OC

•• ••

••••••

••O OC

•• ••

••••••

••O OC

•• ••

••

••O OC

•• ••

••

The second bonding pair forms aThe second bonding pair forms a pi (π) bond.bond.

5. So that C has an octet, we shall form 5. So that C has an octet, we shall form DOUBLE BONDS between C and O.DOUBLE BONDS between C and O.

Carbon Dioxide, CO2

••O OC

•• ••

••••••

Dr. S. M. Condren

Steps to form Lewis Electron Dot Structure

1. Central atom = _______1. Central atom = _______2. Valence electrons = __ or __ pairs2. Valence electrons = __ or __ pairs3.3. Form bonds.Form bonds.4.4. Place lone pairs on outer atoms.Place lone pairs on outer atoms.5.5. Form multiple bonds as necessary to Form multiple bonds as necessary to

obey Lewis’ obey Lewis’ “Octet Rule”.6.6. Remember that there are MANY Remember that there are MANY

compounds that do not obey the Octet compounds that do not obey the Octet Rule.Rule.

Dr. S. M. Condren

Lewis Structures

CO carbon monoxide

Dr. S. M. Condren

Exceptions to Octet Rule

NO nitric oxide

Dr. S. M. Condren

Exceptions to Octet Rule

NO2 nitrogen dioxide

resonance

Dr. S. M. Condren

Exceptions to Octet Rule

PF5

expanded octet

Dr. S. M. Condren

Exceptions to Octet Rule

expanded octet

SF4 F

F

F

F

•• ••

••••

••

••

••

••

••

••

••••

••

S

F

F

F

F

•• ••

••••

••

••

••

••

••

••

••••

••

S

Dr. S. M. Condren

Exceptions to Octet Rule

SF6 expanded octet

Dr. S. M. Condren

• Atoms in molecules often bear a charge (+ or -).

• The predominant resonance structure of a molecule is the one with charges as close to 0 as possible.

• Formal charge = Group number – 1/2 (no. of bonding electrons)

- (no. of LP electrons)

Formal Atom Charges

Dr. S. M. Condren

••

••O OC

••••

+4 - (1/2)(8) - 0 = 0

+6 - (1/2)(4) - 4 = 0

Carbon Dioxide, CO2

Dr. S. M. Condren

6 - (1/2)(2) - 6 = -1 5 - (1/2)(6) - 2 = 0

4 - (1/2)(8) - 0 = 0

••

•

••S NC

•••

Thiocyanate Ion, SCN-

Dr. S. M. Condren

••

•

••S NC

•••

••

•

••S NC

•••

•••

••S NC

•••

Which is the most important resonance form?

Thiocyanate Ion, SCN-

Dr. S. M. Condren

MOLECULAR GEOMETRY

Dr. S. M. Condren

VSEPR Valence alence Shell hell Electron lectron Pair air Repulsion theory.epulsion theory.

Most important factor in determining geometry is Most important factor in determining geometry is relative repulsion between electron pairs.relative repulsion between electron pairs.

MOLECULAR GEOMETRY

Molecule adopts the shape that minimizes the electron pair repulsions.

Dr. S. M. Condren

Electron Pair Geometries

Dr. S. M. Condren

1. Central atom = S1. Central atom = S

2. Valence electrons = 18 or 9 pairs2. Valence electrons = 18 or 9 pairs

••O OS

••

••

••

••••••

bring inleft pair

OR bring inright pair

3. Form double bond so that S has an 3. Form double bond so that S has an octet — but note that there are two ways of octet — but note that there are two ways of doing this.doing this.

•• OS

••

••

••

••••••O

Sulfur Dioxide, SO2

Dr. S. M. Condren

This leads to the following structures.This leads to the following structures.

••O OS

••

••

••

••••••

bring inleft pair

OR bring inright pair

••O OS

••

••

••

••••••

bring inleft pair

OR bring inright pair

••O OS

••

••

••

••••

••O OS••

••

••

••

••

••O OS

••

••

••

••••

••O OS••

••

••

••

••

These equivalent structures are calledThese equivalent structures are called

RESONANCE STRUCTURES. The true . The true

electronic structure is aelectronic structure is a HYBRID of the two.of the two.

Sulfur Dioxide, SO2

Dr. S. M. Condren

Geometries for Four Electron Pairs

Dr. S. M. Condren

Structure Determination by VSEPR

Ammonia, NHAmmonia, NH33

1. Draw electron dot structure

2. Count BP’s and LP’s = 4

H

H

H

lone pair of electronsin tetrahedral position

N

H••

H

H

N

3. The 4 electron pairs are at the 3. The 4 electron pairs are at the corners of a corners of a tetrahedron..

Dr. S. M. Condren

Ammonia, NHAmmonia, NH33

The electron pair geometry is tetrahedral.The electron pair geometry is tetrahedral.

Structure Determination by VSEPR

H

H

H

lone pair of electronsin tetrahedral position

N

The MOLECULAR GEOMETRY, the positions of the atoms, is PYRAMIDAL.

Dr. S. M. Condren

Structure Determination by VSEPRWater, HWater, H22OO1. Draw electron dot structure1. Draw electron dot structure

H

HO

H

HO

The electron pair geometry is The electron pair geometry is TETRAHEDRAL..

The electron pair geometry is The electron pair geometry is TETRAHEDRAL..

H O H••

••

H O H••

••

2. Count BP’s and LP’s = 42. Count BP’s and LP’s = 42. Count BP’s and LP’s = 42. Count BP’s and LP’s = 43. The 4 electron pairs are at the 3. The 4 electron pairs are at the corners of a tetrahedron.corners of a tetrahedron.3. The 4 electron pairs are at the 3. The 4 electron pairs are at the corners of a tetrahedron.corners of a tetrahedron.

The molecular The molecular geometry is geometry is BENT.

The molecular The molecular geometry is geometry is BENT.

Dr. S. M. Condren

Consequences of H2O Polarity

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Structures with Central Atoms

with More Than or less Than

4 Electron Pairs

Often occurs with Group 3A elements and with those of 3rd period and higher.

Dr. S. M. Condren

Molecular Geometries for

Five Electron PairsAll based on trigonal bipyramid

Dr. S. M. Condren

All are based on the 8-sided octahedron

Molecular Geometries for Six

Electron Pairs

Dr. S. M. Condren

• What is the effect of bonding and structure on molecular properties?

Free rotation Free rotation around C–C single around C–C single bondbond

No rotation No rotation around C=C around C=C double bonddouble bond

Bond Properties

Dr. S. M. Condren

Double bondDouble bondDouble bondDouble bondSingle bondSingle bondSingle bondSingle bond

Triple Triple bondbondTriple Triple bondbond

AcrylonitrileAcrylonitrileAcrylonitrileAcrylonitrile

Bond Order # of bonds between a pair of atoms

Dr. S. M. Condren

Fractional bond orders occur in molecules with resonance structures.

Consider NO2-

Bond order = 3 e - pairs in N— O bonds

2 N — O bondsBond order =

3 e - pairs in N— O bonds2 N — O bonds

Bond order = Total # of e - pairs used for a type of bond

Total # of bonds of that typeBond order =

Total # of e - pairs used for a type of bondTotal # of bonds of that type

O O O O

N••

••••

••••

••••••••••

••••

••N

Bond Order

1 1

The N—O bond order = 1.5

Dr. S. M. Condren

Bond order is proportional to two important bond properties:

(a) bond strength

(b) bond length

745 kJ745 kJ

414 kJ414 kJ

110 pm110 pm

123 pm123 pm

Bond Order

Dr. S. M. Condren

Dr. S. M. Condren

Compare O-O and O=O. Is O=O expected to be

stronger, weaker, or the same strength?

Dr. S. M. Condren

Is O=O expected to be

longer, shorter, or the same length?

Dr. S. M. Condren

Estimate the energy of the reaction

H—H + Cl—Cl ----> 2 H—Cl H—H = 436 kJ/mol H—H = 436 kJ/mol Cl—Cl = 242 kJ/molCl—Cl = 242 kJ/molH—Cl = 432 kJ/molH—Cl = 432 kJ/mol

H—H = 436 kJ/mol H—H = 436 kJ/mol Cl—Cl = 242 kJ/molCl—Cl = 242 kJ/molH—Cl = 432 kJ/molH—Cl = 432 kJ/mol

Sum of H-H + Cl-Cl bond energies = Sum of H-H + Cl-Cl bond energies = 436 kJ + 242 kJ = +678 kJ436 kJ + 242 kJ = +678 kJ

Using Bond Energies

Net = ∆H = [Net = ∆H = [bondsbondsbrokenbroken] – [] – [ bonds bondsformedformed]]

2 mol H-Cl bond energies = 864 kJNet = ∆H = +678 kJ - 864 kJ = -186 kJNet = ∆H = +678 kJ - 864 kJ = -186 kJ∆Hf

o (HCl(g)) = -92.31 kJ/mol or -184 kJ

Dr. S. M. Condren

Molecular Polarity

Why do ionic compounds dissolve in Why do ionic compounds dissolve in water?water?

WaterWaterBoiling point Boiling point

= 100 ˚C= 100 ˚C

MethaneMethaneBoiling point Boiling point

= -161 ˚C= -161 ˚C

Why do water and methane Why do water and methane differ so much in their differ so much in their

boiling points?boiling points?

Dr. S. M. Condren

HCl is HCl is POLAR because it has a because it has a positive end and a positive end and a negative end.negative end.

Cl has a greater share Cl has a greater share in bonding electrons in bonding electrons than does H.than does H.

Cl has a greater share Cl has a greater share in bonding electrons in bonding electrons than does H.than does H.

Cl has slight negative charge Cl has slight negative charge (-(-)) and H and H has slight positive charge has slight positive charge (+ (+ ))

H Cl••

••

+ -••H Cl

••

••

+ -••

Bond Polarity

Dr. S. M. Condren

• Three molecules with Three molecules with polar, covalent bonds.polar, covalent bonds.

• Each bond has one Each bond has one atom with a slight atom with a slight negative charge negative charge (-(-)) and another with a and another with a slight slight positivepositive charge charge (+ (+ ))

Bond Polarity

Dr. S. M. Condren

Linus Pauling, 1901-1994

The only person to receive two unshared The only person to receive two unshared Nobel prizes (for Peace and Chemistry). Nobel prizes (for Peace and Chemistry).

Chemistry areas: bonding, electronegativity, Chemistry areas: bonding, electronegativity, protein structureprotein structure

Dr. S. M. Condren

Electronegativity,

is a measure of the ability of an atom in a is a measure of the ability of an atom in a molecule to attract electrons to itself.molecule to attract electrons to itself.

Dr. S. M. Condren

Due to the bond polarity, the H—Cl Due to the bond polarity, the H—Cl bond energy is GREATER than bond energy is GREATER than expected for a “pure” covalent expected for a “pure” covalent bond.bond.

BOND ENERGY

““pure” bondpure” bond 339 kJ/mol calc’d339 kJ/mol calc’d

real bondreal bond 432 kJ/mol measured432 kJ/mol measured

BOND ENERGY

““pure” bondpure” bond 339 kJ/mol calc’d339 kJ/mol calc’d

real bondreal bond 432 kJ/mol measured432 kJ/mol measured

Difference = 92 kJ. This difference is Difference = 92 kJ. This difference is proportional to the difference in proportional to the difference in

ELECTRONEGATIVITY, , ..

Difference = 92 kJ. This difference is Difference = 92 kJ. This difference is proportional to the difference in proportional to the difference in

ELECTRONEGATIVITY, , ..

H Cl••

••

+ -••

Bond Polarity

Dr. S. M. Condren

Electronegativity

Pauling Scale

• relative attraction of an atom for electrons, its own and those of other atoms

• same trends as ionization energy, increases from lower left corner to the upper right corner

• fluorine: E.N. = 4.0

Dr. S. M. Condren

Electronegativities of the Elements

Dr. S. M. Condren

Which bond is more polar (or DIPOLAR)?Which bond is more polar (or DIPOLAR)?

O—HO—H O—FO—F

3.5 - 2.13.5 - 2.1 3.5 - 4.03.5 - 4.0

1.41.4 0.50.5

OH is more polar than OFOH is more polar than OF

and polarity is “reversed.”and polarity is “reversed.”and polarity is “reversed.”and polarity is “reversed.”

O H+-

O F+ -

O H+-

O F+ -

Bond Polarity

Dr. S. M. Condren

Molecules will be polar ifMolecules will be polar ifa)a) bonds are polarbonds are polar ANDANDb)b) the molecule is NOT “symmetric”the molecule is NOT “symmetric”

All above are All above are NOTNOT polar polar

Molecular Polarity

Dr. S. M. Condren

Polar or Nonpolar?Compare CO2 and H2O. Which one is polar?

Dr. S. M. Condren

Covalent Bond Properties

electronegativity

nonpolar bonds => diff. EN = 0

polar bonds => diff. EN > 0

ionic bonds => diff. EN > 1.5

Dr. S. M. Condren

CH4 … CCl4 Polar or Not?

• Only CH4 and CCl4 are NOT polar. These are the only two molecules that are “symmetrical.”