Chapter 11: Chemical Bonding II: Additional Aspects
• We will look at these topics:
o Covalent bonding and orbital overlap.
– Electrons in overlapping orbitals are shared.
o Hybrid orbitals.
– LCAO used to explain molecular shapes.
– sp, sp2, sp3, sp3d, sp3d2
– Multiple covalent bonds are firmed by overlapping
unhybridized p orbitals.
o Molecular orbital (MO) theory.
– Orbitals encompass the entire molecule.
Covalent bonding and orbital overlap
Valence Bond Theory tells us that covalent bonds are formed by
the overlap of valence atomic orbitals; for example, H2
Let’s consider the possible electrostatic interactions that occur
1.
2.
3.
1
The most likely internuclear distance
(the “bond length”) arises from the
interplay among these interactions.
But we encounter a problem when applying valence bond theory
to even simple molecules, such as water.
Consider the discrepancy between the geometry of water
predicted by VSEPR, and that from valence bond theory.
Lewis structure and VSEPR geometry of water, H2O
Valence bond geometry of water, H2O
2
Note: Bonds occur where orbitals overlap because…
We need another orbital description consistent with quantum
mechanics and the Schrödinger equation: Hybridization!
Linear combinations of hydrogen-like atomic orbitals that are close
in energy create a set of degenerate hybrid orbitals with new
geometries.
These linear combination occurs through an excited electronic
state (‘promotion’) followed by the formation of hybrid orbitals
(‘hybridization’) which are stabilized by the formation of covalent
bonds.
3
Super-important truisms about hybrid orbitals!!
1. The number of atomic orbitals needed is equal to the steric
number from the Lewis structure, that is, the sum of the
number of BONDED ATOMS and lone pairs on the central
atom.
2. The number of degenerate hybrid orbitals formed is equal to
the number of atomic orbitals used; some p and d atomic
orbitals might be leftover.
3. Terminal atoms are never hybridized.
We shall examine these truisms in practice.
Let’s look at five different hybridization schemes:
Number of
BONDED ATOMS
and Lone Pairs
Number of Atomic
Orbitals needed
Atomic Orbitals
Used
Number of Hybrid
Orbitals formed
Name of Hybrid
Orbitals
2
3
4
5
6
2
3
4
5
6
one s + one s + one s + one s +
one s +
one p
two p
three p three p + three p +
one d
two d
2
3
4
5
6
sp
sp2
sp3
sp3d
sp3d2
1. Draw the Lewis structure.
2. Count BONDED ATOMS and Lone Pairs on a central atom.
(Equals the number of atomic = number of hybrid orbitals.)
3. Draw Hybridization Scheme to show orbital formation.
4. Draw Bonding Scheme on Lewis to show which orbitals
overlap to form bonds (σ single bonds, π multiple bonds).
4
sp — BeCl2
5
sp2 — BF3
6
sp3 — CH4
7
sp3d — PCl5
8
sp3d2 — SF6
9
Multiple Bonds Are Formed From a Combination of
Hybridized Atomic Orbitals and Unhybridized p Orbitals
sp2 + p — C2H4
σ bonds formed by end-to-end orbital overlap (internuclear axis)
π bonds formed by side-by-side overlap of unhybridized p orbitals
10
sp + two p — C2H2
11
(some more on the) The Bonding Scheme
• We have seen how to write the “hybridization scheme” for
valence electrons around a central atom:
Ground State

Promotion

Hybridization
• We also want to be able to write a “bonding scheme” for a
molecule which shows the types of bonds to the central atom (σ
or π), and the orbitals which overlap to create these bonds
• Let’s look at the bonding scheme of H2CO as an example
Bonding Schemes in Molecules with Multiple Central Atoms
12
Molecular Orbitals: A Top-of-the-Line Bonding Scheme
A multi-view description and rationalization
13
How we usually show the formation and occupancy of molecular
orbitals
1. Draw the molecular orbital (MO) scheme
2. Fill in the valence electrons using the same rules we learned
for atomic orbitals (Aufbau, Hund, Pauli)
14
As usual, the number of MO formed is equal to the number of
starting atomic orbitals. So two atoms, each with one s and three
p atomic orbitals, will form eight MO orbitals.
The relative energies of the molecular orbitals formed by second
period elements depends on the atomic number.
15
16
17
What about heteronuclear diatomic species?
Benzene: Delocalized Molecular Orbitals
18
19
Metals: Electron Sea Model and Valence Band Theory
20