Question

When the $O_2$ molecule becomes the $O_2^{-}$ ion, the electron adds on to which one of the following orbitals?

• A. ${\sigma }^{\ast }2p_{z}orbital$
• B. $\sigma 2p_{z}orbital$
• C. ${\pi }^{\ast }2p_x$ or ${\pi }^{\ast }2p_{y}orbital$
• D. $\pi 2p_x$ or $\pi 2p_{y}orbital$

Answer 1

The correct option is C ${\pi }^{\ast }2p_x$ or ${\pi }^{\ast }2p_{y}orbital$

The electronic configuration of $O_2$,
$O_2\to \sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\sigma 2p_2^2\pi 2p_n^2\pi 2p_y^2{\pi }^{\ast }2p_x^1{\pi }^{\ast }2p_y^1$

So electronic configuration of $O_2^{-}$
$O_2^{-}\to \sigma 1s^2{\sigma }^{\ast }1s^2\sigma 2s^2{\sigma }^{\ast }2s^2\sigma 2p_z^2\pi 2p_x^2\pi 2p_y^2{\pi }^{\ast }2p_x^2{\pi }^{\ast }2p_y^1$

As in $O_2$ HOMO is ${\pi }^{\ast }2p_x$ and ${\pi }^{\ast }2p_y$ and they have one electron each so next electron can be added to any of these orbitals.