Chemical Bonding Chemical Bonding VI:VI:

Valence Bond Valence Bond TheoryTheory

& Orbital & Orbital HybridizationHybridization

Coordinate Covalent Bonds• A co-ordinate bond (also called a dative

covalent bond) is a covalent bond in which both electrons come from the same atom.

N H••

H

H

F B

F

F

+ F B

F

F

N H

H

H

Coordination Compounds

A coordination compound typically consists of a complex ion and a counter ion.

A complex ion contains a central metal cation bonded to one or more molecules or ions.

The molecules or ions that surround the metal in a complex ion are called ligands.

A ligand has at least one unshared pair of valence electrons

H

O

H

• • •• • •

H

N

HH• •

•• Cl

• •

••- ••C O••

Bonding Theories• Valence Bond (VB) Theory

• Molecular Orbital (MO) Theory

- MO theory no longer covered on AP Chem exam

Valence Bond Theory

• e- in a molecule occupy orbitals of the individual atoms

• Stable molecule form when P.E. of a system is minimized

• Atomic orbitals overlap to form bonds

• To accommodate bonds, valence orbitals of central atom combine or hybridize

H 1s H1s

H 1s H

A sigma bond (σ) forms as orbitals overlap

Sigma bond () – electron density between the 2 atoms

Hybridization – mixing of two or more atomic orbitals to form a new set of hybrid orbitals.

7

1. Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals.

2. Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process.

3. Covalent bonds are formed by:

a. Overlap of hybrid orbitals with atomic orbitals

b. Overlap of hybrid orbitals with other hybrid orbitals

C + H

Carbon has only 2 unpaired electrons, it should form only 2 bonds to hydrogen.

Carbon actually forms 4 bonds to

hydrogen.

How??

Hybrid orbitals are energetically more favorable

Carbon Hybridization

Ene

rgy

2s

2p

2sp3

Carbon Hybridizati

on

Ene

rgy

Four sigma bonds forms + p + p + p = sp3

Formation of Covalent Bonds in CH4

Valence Bond Theory and NH3

N – 1s22s22p3

3 H – 1s1

If the bonds form from overlap of 3 2p orbitals on nitrogen with the 1s orbital on each hydrogen atom, what would the molecular geometry of NH3 be?

If use the3 2p orbitalspredict 90o

Actual H-N-Hbond angle is

107.3o

Predict correctbond angle

sp3-Hybridized N Atom in NH3

Need 2 bonding orbitals on S to bond with H.

H2S

sp3 Hybridization in H2S

σ bonds

Lone e- pairs

Predicting Hybridization of the Central Atom

1. Draw the Lewis structure of the molecule.

2. Count the number of lone pairs AND the number of atoms bonded to the central atom

( = STERIC NUMBER)

Steric # Hybridization Examples

2 sp BeCl2

3 sp2 BF3

4 sp3 CH4, NH3, H2O

5 sp3d PCl5

6 sp3d2 SF6

22

Formation of sp Hybrid Orbitals

24

Formation of sp2 Hybrid Orbitals

Hybridization for PCl5

1. Draw the Lewis structure of the molecule.

2. Determine Steric Number ( = # of hybrid orbitals)

Steric Number = 5So 5 orbitals must hybridize

s + p + p + p + d =

sp3d

Carbon Hybridizationsp3d Formation (in Phosphorus)

5 σ bonds can form

Carbon Hybridizationsp3d2 Formation (in Sulfur)

6 σ bonds can form

Pi Bonds ()• e- in unhybridized p orbitals overlap to form

bonds• e- density above and below plane of nuclei of the

bonding atoms

Ozone (O3)

35

Sigma bond () – electron density between the 2 atoms

Pi bond () – electron density above and below plane of nuclei of the bonding atoms

Bonding in Ethylene, C2H4

36

Sigma () and Pi Bonds ()

Single bond 1 sigma bond

Double bond 1 sigma bond and 1 pi bond

Triple bond 1 sigma bond and 2 pi bonds

How many and bonds are in the acetic acid (vinegar) molecule CH3COOH?

C

H

H

CH

O

O H bonds = 6 + 1 = 7

bonds = 1

O

HC

H

How can we explain the bonding in formaldehyde?

Steric Number ?

3 sp2

sp2 Hybridization of Carbon

bond

How can we describe the bonding In acetylene?

45

sp Hybridization of Carbon

2 bonds

Valence Bond Model for Acetylene

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HW p. 442 # 20, 22, 25 -29