• # question_answer Out of H and ${{H}_{2}},$ the latter has higher first ionization energy while out of $O$ and ${{O}_{2}},$ the former has higher first ionization energy. Explain why.

In ${{H}_{2}},$ first electron has to be removed from $\sigma (1s)$ which has lower energy (more stable) than H-atom. In ${{O}_{2}},$ first electron has to be removed from ${{\pi }^{*}}\,(2{{p}_{x}})\,or\,{{\pi }^{*}}\,(2{{p}_{y}})$ orbital which has higher energy than 2 p-orbital of O-atom.