Question

During the change of ${{O}_{2}}$ to ${{O}_{2}}^{-}$, the incoming electrons goes to the orbital:
A. ${{\sigma }^{*}}^{2}{{P}_{z}}$
B. ${{\pi }^{2}}{{P}_{y}}$
C. ${{\pi }^{*}}^{2}{{P}_{x}}$
D. ${{\pi }^{2}}{{P}_{x}}$

Answer 1

Hint:. The molecular orbital theory tells us about the arrangement of electrons. Filling of electrons is based on the Aufbau Principle which states that atoms first fill the lower energy state then move into an excited state.

Complete step by step answer:
The total number of molecular orbitals formed will always be equal to the total number of atomic orbitals. There are different types of molecular orbitals like bonding molecular orbitals, anti-bonding molecular orbitals and non-bonding molecular orbitals. Out of three, anti-bonding molecular orbitals will always have higher energy than the parent orbitals whereas bonding molecular orbitals will always have lower energy than the parent orbitals. The electrons are filled from the orbital of lowest energy to the orbital of highest energy.

Let’s discuss the electronic configuration of
${{O}_{2}}$= 16 ; $\sigma 1{{s}^{2}}{{\sigma }^{*}}1{{s}^{2}}\sigma 2{{s}^{2}}{{\sigma }^{*}}2{{s}^{2}}\sigma 2{{p}_{z}}^{2}\pi {{P}_{x}}^{2}\pi {{P}_{y}}^{2}{{\pi }^{*}}{{P}_{x}}^{1}{{\pi }^{*}}{{P}_{y}}^{1}$
${{O}_{2}}^{-}$= 17; $\sigma 1{{s}^{2}}{{\sigma }^{*}}1{{s}^{2}}\sigma 2{{s}^{2}}{{\sigma }^{*}}2{{s}^{2}}\sigma 2{{p}_{z}}^{2}\pi {{P}_{x}}^{2}\pi {{P}_{y}}^{2}{{\pi }^{*}}{{P}_{x}}^{2}{{\pi }^{*}}{{P}_{y}}^{1}$

Hence from the above calculation we see that electron will transfer in ${{\pi }^{*}}^{2}{{P}_{x}}$ shell as in ${{O}_{2}}$ there is only one electron is filled in ${{\pi }^{*}}^{2}{{P}_{x}}$ and in ${{O}_{2}}^{-}$ two electrons are present in ${{\pi }^{*}}^{2}{{P}_{x}}$,
So, the correct answer is “Option C”.

Note: Molecular orbitals which are formed by the additive effect of the atomic orbitals are known as bonding molecular orbitals and probability of finding the electrons is more in this case. Molecular orbitals that are formed by the subtractive effect of atomic orbitals are known by anti-bonding molecular orbitals and probability of finding electrons is less here.