H2 exists but he2 cannot. Explain on the basis of valence bond theory
The fact that the anti-bonding molecular orbitals (MO) energy difference is larger than the bonding MO energy difference is the true reason that He2 doesn't exist. Out of the four valence electrons in He 2, two will fill the bonding MO, and the other two will fill the anti-bonding MO. The two electrons in the bonding MO will achieve some stabilization relative to their position in the atomic orbitals (AOs), but the two electrons in the anti-bonding MO will achieve greater destabilization relative to their position in the AOs. The net result is a less stable molecule than if the electrons remained in their respective AOs.
Compare this to H 2. The same AOs are contributed as in this He 2 example, and the same MOs are made, but in the case of H 2 there are only two valence electrons. The two electrons fill the bonding MO, while the anti-bonding MO remains empty. The net result for H 2 is a stabilisation of the electrons relative to their position in the AOs, and H 2 is preferable over isolated hydrogen atoms.
Also, the bond order of He2 comes out to be zero which shows He2 molecule does not exist.
Bond order of He2 = (number of electrons in bonding MO - number of electrons in anitbonding MO) / 2
Bond order = (2-2)/2 = 0
This implies that He2 is unknown molecule.