Li has a configuration: 1s2, 2s1
There are two s orbitals (1s and 2s) on each atom.
These combine to give four MO's. These are
(1s) + (1s) → σ1s + σ*1s
(2s) + (2s) → σ2s + σ*2s
Thus all the six electrons are accommodated in these four orbitals. The electronic configuration for
Li2 is (σ1s) 2(σ*1s)2 (σ2s)2
Nb = 4, Na = 2
Bond order in Li2 = {4 - 2}/{2} = 1
Therefore, Li2 should be a stable species. Its bond dissociation energy is 105 kJ mol-1, as compared to 431 kJ mol-1 for H2. Thus, the bond in Li2 is much weaker than that on H2. This is because 2s orbital of Li (involved in the bonding) is much larger than 1s orbital of H.
