Question

The antibonding molecular orbital of Hydrogen molecule which is empty has higher energy than its bonding molecular orbital containing two electrons. Why?

Answer 1

Dear Student, When two atomic orbitals in same phase overlaps, increase in electron density between two atoms occur leading to bonding molecular orbitals which are low in energy. When two atomic orbitals which are out of phase overlaps, it removes electron density between the nuclei leading to anti bonding molecular orbitals which are high in energy. When we fill the orbitals, then using the building up principle the orbitals of lower energy are filled first and so in H2, the two electrons(one from each H) occupy the lowest orbital i.e. bonding molecular orbital. Regards,