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LMALaput Chemistry 16 Lecture 14

BONDING THEORIES (part 2)

1. Valence Bond Theory

2. Hybridization of Orbitals

VALENCE BOND THEORY

Covalent bonds are formed by the overlap of atomic

orbitals.

Atomic orbitals on the central atom can mix and

exchange their character with other atoms in a

molecule.

Process is called hybridization.Hybrid Orbitals have the same shapes as predicted by

VSEPR.

The Central Themes of VB Theory

Basic Principle

A covalent bond forms when the orbitals of two atoms

overlap and the overlap region, which is between the

nuclei, is occupied by a pair of electrons.

The two wave functions are in phase so the amplitude

increases between the nuclei.

Themes

A set of overlapping orbitals has a maximum of twoelectrons that must have opposite spins.

The greater the orbital overlap, the stronger (more stable)

the bond.

The valence atomic orbitals in a molecule are different

from those in isolated atoms. There is a hybridization of

atomic orbitals to form molecular orbitals.

Hybrid Orbitals

Key Points

The number of hybrid orbitals obtained equalsthe number

of atomic orbitals mixed.

Thetypeof hybrid orbitals obtained varieswith the types of

atomic orbitals mixed.

Types of Hybrid Orbitals

e- group EG

Atomic Orbital Mixed

from Valence Shell of

Central Atom

Hybrid-

ization

2 linear 1 s, 1p sp

3 trigonal planar 1s, 2p sp2

4 tetrahedral 1s,3p sp3

5trigonal

bipyramidal1s, 3p, 1d sp3d

6 octahedral 1s, 3p, 2d sp3d2

Linear Electronic Geometry:

AB2Species (No Lone Pairs of Electrons on A)

Some examples of molecules with this geometry are:

BeCl2, BeBr2, BeI2, HgCl2, CdCl2

All of these examples are linear, nonpolar molecules.

Important exceptions occur when the two substituents

are not the same!

BeClBr or BeIBr will be linear and polar!

Trigonal Planar Electronic Geometry:

AB3Species(No Lone Pairs of Electrons on A)Some examples of molecules with this geometry are:

BF3, BCl3

All of these examples are trigonal planar, nonpolar

molecules.

Important exceptions occur when the three substituents

are not the same!

BF2Cl or BCI2Br will be trigonal planar and polar!

Tetrahedral Electronic Geometry:

AB4Species (No Lone Pairs of Electrons on A)

Some examples of molecules with this geometry are:

CH4, CF4, CCl4, SiH4, SiF4

All of these examples are tetrahedral, nonpola

molecules.Important exceptions occur when the four substituents

are not the same!

CF3Cl or CH2CI2will be tetrahedral and polar!

Example of Molecules with More Than One Central Atom

Alkanes are hydrocarbons with the general formula

CnH2n+2.

CH4 -methane

C2H6 or (H3C-CH3) -ethane

C3H8 or (H3C-CH2-CH3) -propane

The C atoms are located at the center of a tetrahedron

Each alkane is a chain of interlocking tetrahedral.

Sufficient H atoms to form a total of four bonds for each

C.

Tetrahedral Electronic Geometry:

AB3U Species (One Lone Pair of Electrons on A)

Some examples of molecules with this geometry are:

NH3, NF3, PH3, PCl3, AsH3

These molecules are our first examples of central atoms

with lone pairs of electrons.

Thus, the electronic and molecular geometries are

different.

All three substituents are the same but molecule is polar.

NH3and NF3 are trigonal pyramidal, polar molecules.

Tetrahedral Electronic Geometry:

AB2U2Species (Two Lone Pair of Electrons on A)

Some examples of molecules with this geometry are:

H2O, OF2, H2S

These molecules are our first examples of central atoms

with two lone pairs of electrons.

Thus, the electronic and molecular geometries are

different.

Both substituents are the same but molecule is polar.

Molecules are angular, bent, or V-shaped and polar.

7/27/2019 Lec14 Hybridization

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LMALaput Chemistry 16 Lecture 14

Trigonal Bipyramidal Electronic Geometry:

AB5, AB4U, AB3U2, and AB2U3

Some examples of molecules with this geometry are:

PF5, AsF5, PCl5, etc.

These molecules are examples of central atoms with five

bonding pairs of electrons.

The electronic and molecular geometries are the same.

Molecules are trigonal bipyramidal and nonpolar when

all five substituents are the same.

If the five substituents are not the same polar molecules

can result, AsF4Cl is an example.AB4U molecules have:

1. trigonal bipyramid electronic geometry

2. seesaw shaped molecular geometry

3. and are polar

One example of an AB4U molecule is SF4

Hybridization of S atom is sp3d.

AB3U2molecules have:

1. trigonal bipyramid electronic geometry

2. T-shaped molecular geometry

3. and are polar

One example of an AB3U2molecule is IF3

Hybridization of I atom is sp3d.

AB2U3molecules have:

1. trigonal bipyramid electronic geometry2. linear molecular geometry

3. and are nonpolar

One example of an AB3U2molecule is XeF2

Hybridization of Xe atom is sp3d.

Octahedral Electronic Geometry:

AB6, AB5U, and AB4U2

Some examples of molecules with this geometry are:

SF6, SeF6, SCl6, etc.

These molecules are examples of central atoms with six

bonding pairs of electrons.

Molecules are octahedral and nonpolar when all six

substituents are the same.

If the six substituents are not the same polar moleculescan result, SF5Cl is an example.

AB5U molecules have:

1. octahedral electronic geometry

2. Square pyramidal molecular geometry

3. and are polar.

One example of an AB5U molecule is IF5

Hybridization of I atom is sp3d2.

AB4U2molecules have:

1. octahedral electronic geometry

2. square planar molecular geometry

3. and are nonpolar.

One example of an AB4U2molecule is XeF4

Hybridization of Xe atom is sp3d2.

Composition and Orientation of Hybrid Orbitals

LinearTrigonal

Planar

Tetra-

hedral

Trigonal

Bipyra-

midal

Octa-

hedra

Atomic

Orbitals

mixed

1s, 1p 1s, 2p 1s, 3p1s, 3p,

1d1s, 3p, 2

Hybrid

orbitals

formed

2 sp 3 sp2 4 sp3 5 sp3d 6 sp3d

Unhybridized orbitals

remaining

2 p 1 p None 4 d 3 d

Orientation