Helium molecule (He2 ): The electronic configuration of helium atom is 1s2. Each helium atom contains 2 electrons, therefore, in He2 molecule there would be 4 electrons. These electrons will be accommodated in σ1s and σ*1s molecular orbitals leading to electronic configuration: He2 : (σ1s)2 (σ*1s)2 .Bond order of He2 is ½(2 – 2) = 0. He2 molecule is therefore unstable and does not exist.
