Like H atoms, He atoms have their electrons in 1s orbitals, so we can construct an energy level diagram for He₂ in a similar way. But there is one big difference: each helium atom has two electrons so now both the bonding MO and the antibonding MO are full! Any bonding due to the electrons in the bonding orbital is cancelled out by the electrons in the antibonding orbital, and the He₂ molecule falls apart. He₂ does not exist.