One of the excited states of a C2 molecule has the electron configuration: (σ1s) 2 (σ1s*) 2 (σ2s) 2 (σ2s*) 1 (π 2p) 4 (σ2p) 1.? What is the bond order of C2 in this excited state? What is the bond order of C2 in the ground state? How does the bond length in this excited state compare to that in the ground state?

The bond order is bonding - anti bonding/2. Here you have 9 bonding and 3 anti bonding (in the excited state) thus (9-3) / 2 = 6/2 = 3.

The ground state would move the sigma 2p1 back down to the incomplete sigma 2s1*. This takes away one bonding electron and adds one anti bonding electron, therefore the bond order is (8-4) / 2=4/2=2.

Higher bond orders correspond to shorter bonds.

To recap: Bond order of C2 in the excited state = 3

Bond order of C2 in the ground state = 2