Question

Out of ${H}$ and ${H}_2, {H}$ has higher first ionisation energy while out of ${O}$ and ${O}_2, {O}_2$ has higher first ionisation energy. Explain why?

Answer 1

In ${H}_2$, first electron is removed from $\sigma(1 s)$ which has lower energy than $1 s$-orbital of ${H}$-atom. However, in ${O}_2$, first electron is removed either from $\pi^*\left(2 p_y\right)$ or $\pi^*\left(2 p_x\right)$ orbital which has higher energy than $2 p$-orbital of O-atom.