"While the neutral lithium dimer (Li2) is stable, this dimer in the +2 oxidation state is not stable. Conversely, the neutral beryllium dimer (Be2) is not stable, and the +2 form of this molecule is stable. Explain this result by using full and labelled molecular orbital diagrams for each species. Evaluate the bond order of all four species."

 

This is a question from my pratice midterm, and I am completely stuck!