5 Steps to a 5 AP Chemistry 2019

Bonding ❮ 157

Molecular Orbital Theory

Still another model to represent the bonding that takes place in covalent compounds

is the molecular orbital theory. In the molecular orbital (MO) theory of covalent

bonding, atomic orbitals (AOs) on the individual atoms combine to form orbitals that

encompass the entire molecule. These are called molecular orbitals (MOs). These molecu-

lar orbitals have definite shapes and energies associated with them. When two atomic orbit-

als are added, two molecular orbitals are formed, one bonding and one antibonding. The

bonding MO is of lower energy than the antibonding MO. In the molecular orbital model,

the atomic orbitals are added together to form the molecular orbitals. Then the electrons

are added to the molecular orbitals, following the rules used previously when filling orbit-

als: lowest-energy orbitals get filled first, maximum of two electrons per orbital, and half

fill orbitals of equal energy before pairing electrons (see Chapter 5). When s atomic orbitals

are added, one sigma bonding (s) and one sigma antibonding (s*) molecular orbital are

formed. Figure 11.9 shows the molecular orbital diagram for H 2.

Note that the two electrons (one from each hydrogen) have both gone into the sigma

bonding MO. The bonding situation can be calculated in the molecular orbital theory

by calculating the MO bond order. The MO bond order is the number of electrons in

bonding MOs minus the number of electrons in antibonding MOs, divided by 2. For H 2

in Figure 11.9 the bond order would be (2 – 0)/2 = 1. A stable bonding situation exists

between two atoms when the bond order is greater than zero. The larger the bond order,

the stronger the bond.

When two sets of p orbitals combine, one sigma bonding and one sigma antibond-

ing MO are formed, along with two bonding pi MOs and two pi antibonding (p*)

MOs. Figure 11.10, on the next page, shows the MO diagram for O 2. For the sake of

simplicity, the 1s orbitals of each oxygen and the MOs for these elections are not shown,

just the valence-electron orbitals.

The bond order for O 2 would be (10 – 6)/2 = 2. (Don’t forget to count the bonding

and antibonding electrons at energy level 1.)

Increasing energy

Atomic

orbital

of H

Atomic

orbital

of H

1 s 1 s

Molecular

orbital

of H 2

Figure 11.9 Molecular orbital diagram of H 2.

KEY IDEA

ENRICHMENT