Draw an MO Energy Diagram and Predict the Bond Order of Li₂⁺ and Li₂

To understand the molecular orbitals (MO) of lithium molecules, we need to first draw the MO energy diagram for both Li₂ and Li₂⁺.

MO Energy Diagram for Li₂

In a typical MO energy diagram for diatomic molecules like lithium, we consider the following orbitals: the 1s orbitals of two lithium atoms combine to form a bonding (σ_1s) and an antibonding (σ*_1s) orbital. Since lithium has 3 electrons, the configuration for Li₂ would be:

• 2 electrons in the bonding σ_1s orbital
• 1 electron in the antibonding σ*_1s orbital

This means that the total number of electrons in bonding orbitals is 2, and in antibonding orbitals, it is 1. Therefore, the bond order can be calculated as follows:

Bond Order = (Number of Electrons in Bonding Orbitals – Number of Electrons in Antibonding Orbitals) / 2

For Li₂:

Bond Order = (2 – 1) / 2 = 0.5

MO Energy Diagram for Li₂⁺

Now, for Li₂⁺, we remove one electron from the molecule. This electron is taken out from the antibonding σ*_1s orbital, adjusting the configuration:

• 2 electrons in the bonding σ_1s orbital
• 0 electrons in the antibonding σ*_1s orbital

This leads to a bond order calculation for Li₂⁺ as:

Bond Order = (2 – 0) / 2 = 1

Conclusion

In summary, the bond order of Li₂ is 0.5, indicating a very weak bond, whereas for Li₂⁺, the bond order is 1, suggesting a stable single bond. The removal of an electron from the antibonding orbital in Li₂⁺ strengthens the overall bond compared to the neutral Li₂.