• # question_answer A molecule of ${{H}_{2}}$ exists while that of the$H{{e}_{2}}$  does not. Explain.

Hydrogen atom (Z = 1) has only one electron present in the s-orbital$(1{{s}^{1}})$. The two such half filled atomic orbitals combine to form a molecular orbital which contains both these electrons. But helium (Z = 2) has already a filled orbital $(1{{s}^{2}})$. Therefore, the atomic orbitals of the helium atoms do not combine. Thus, a molecule of${{H}_{2}}$ exists while that of  $H{{e}_{2}}$ does not.