H2,Li2,B2 are not equally stable. Li atom is much larger in size than H-atom. The bond order in Li2 is much larger than bond length in H2. Moreover, Li2 molecule has two electrons in the antibonding molecular orbital while H2 has no electrons in the antiobinding molecular orbital. Thus, Li2 is less sable than H2( Bond energy of Li2=110kJmol−2+ Bond energy of H2=438k. mol−1)
Boron atom is smaller than lithium atom but larger than hydrogen atom. The B2 is more stable than Li2 but less stable than H2( Bond energy of B2=290kJmol−1)