A molecule of `H_(2)` exist while that of the `He_(2)` does not. Explain.

Explanation

Hydrogen atom (Z=1) has only one electron present in the s-orbital `(1s^(1)).` The two such half field atomic orbitals compbine to from a molecular orbital which contains both these electrons. But helium (Z=2) has already a filled orbital `(1s^(2)).` Therefore, the atomic orbitals of the helium atoms donot combine. Thus, a molecule of `H_(2)` exists while that of `He_(2)` does not.