Bonding Concepts Worksheet - Unit 8/9, Michalek (Page 26 of 30) in pdf

AP Chemistry - Michalek

Unit 08 Bonding Concepts

9.2 Notes

The molecular orbital model is used to describe bonding.

Just as atomic orbitals are solutions to the quantum mechanical treatment of atoms, molecular orbitals (MOs) are

solutions to the molecular problem. They can hold two electrons with opposite spin and that the square of the

molecular orbital wave function indicates electron probability.

1. The electron probability of both molecular orbitals is centered along the line passing through the two nuclei.

For MO1 the greatest electron probability if between the nuclei, and for MO2 it is on either side of the nuclei.

This type of electron distribution is described as sigma, as in the LEM. Accordingly, we refer to MO1 and MO2

as sigma molecular orbitals.

2. In the molecule only the molecular orbitals are available for occupation by electrons. The 1s atomic orbitals of

the hydrogen atoms no longer exist, because the H

molecule –a new entity__ has its own set of new orbitals.

3. MO1 is lower in energy than the 1s orbital of free hydrogen atoms, while MO2 is higher in energy. Since H

has

a lower energy in the MO1, they will have a lower energy than they do as two separate atoms. This situation

favors molecule formation, because nature tends to seek the lowest energy state. That is, the driving force for

molecule formation is the molecular orbital available to the two electrons has lower energy than the atomic

orbitals. This situation is favorable to bonding. A bonding molecular orbital is lower in energy than the atomic

orbitals of which it is composed. An antibonding orbital is higher in energy than the atomic orbitals of which it

is composed. Electrons in this type of orbital will favor the separated atoms.

4. Since the greatest probability of electrons are between the orbitals this bonding force is greater than the

antibonding force.

5. MO1 =

MO2 = σ

σ1s

6. Each molecular orbital can hold two electrons, but the spins must be opposite

7. Orbitals are conserved. The number of molecular orbitals will always be the same as the number of atomic

orbitals used to construct them.

When using H

the electrons use the antibonding orbital. Two electrons are lowered in energy and one is raised,

producing a new lowering of the energy of only one electron. Thus the model predicts that H

is twice as stable as H

With respect to their separated components H

is twice as strong as H

Bond Order

Bond order is the difference between the number of bonding electrons and the number of antibonding electrons divided

by two.

Bond order = number of bonding electrons – number of antibonding electrons

Bonding Concepts Worksheet - Unit 8/9, Michalek Page 26