Final answer:
The bond order for the anion Li2- is calculated using molecular orbital theory to be 0.5, which is derived from having two electrons in a bonding molecular orbital and one in an antibonding molecular orbital, indicating a less stable bond compared to the neutral Li2 molecule.
Step-by-step explanation:
To predict the bond order for the anion Li2- using molecular orbital theory, first consider that each Li atom contributes one valence electron and the anion has an extra electron, totaling three electrons for bonding. In a molecular orbital diagram for homonuclear diatomic molecules of alkali metals, the valence electrons fill the lowest energy orbitals first. For Li2, two electrons would fill the bonding σ2s molecular orbital and one electron would go into the antibonding σ2s* molecular orbital. The bond order, calculated as (number of electrons in bonding orbitals - number of electrons in antibonding orbitals)/2, would be (2 - 1)/2 = 0.5. This suggests a less stable bond compared to Li2 and indicates that Li2- might exist under certain conditions but it is less stable than the neutral Li2 molecule.