Question

Consider the following molecular orbitals formed from the combination of two hydrogen 1s orbitals:

a. Which is the bonding molecular orbital, and which is the antibonding molecular orbital? Explain how you can tell by looking at their shapes.

b. Which of the two molecular orbitals is lower in energy? Why is this true?

Short answer

1. Bonding molecular orbitals have enhanced electron probability between two nuclei while antibonding molecular orbitals do not have probability between two nuclei.
2. Bonding molecular orbitals will have lower energy because participating in molecule creation is energy favourable process rather than being separate as two independent atoms.

Step-by-step solution

Identifying the bonding and anti-bonding molecular orbital from the shape of the given molecular orbitals (for part a.)

Bonding molecular orbitals have enhanced electron probability between two nuclei while antibonding molecular orbitals do not have probability between two nuclei. According to the left picture, that is antibonding molecular orbitals, the blue colour represents the bonding molecular orbitals. In the picture, the area between the two nuclei is blue, so the electron probability is higher than zero.

Determining which molecular orbital has lower energy. (for part b.)

Bonding molecular orbitals will have lower energy because participating in molecule creation is energy favourable process rather than being separate as two independent atoms.