Question

The first ionization energy of oxygen is less than of oxygen. Which of the following is / are the incorrect reason (s) for this observation?
 (A) Lesser effective nuclear charge of oxygen than nitrogen.
(B) Lesser atomic size of oxygen than nitrogen.
(C) Greater inter electron repulsion between two electrons in the same p – orbital counter balances the increase in effective nuclear charge on moving from nitrogen to oxygen.
(D) Greater effective nuclear charge of oxygen than nitrogen.

Answer 1

Hint: Ionization energy is the minimum amount of energy required to remove an electron from an isolated gaseous atom to form monovalent cation. Ionization energy depends upon size of atom, nuclear charge, half-filled and fully filled configuration, etc.

Complete step by step answer:
As we move from the left to right in a periodic table, then the first ionization energy of oxygen is less than that of the nitrogen. Due to increase in the nuclear charge, the atomic size decreases and ionization energy increases. The increase in the effective nuclear charge is counterbalanced by the inter electron repulsion between two electrons in the same p-orbital on moving from nitrogen to oxygen. We know that half-filled and fully filled degenerate orbitals are more stable than the partially filled degenerate orbitals. That is why the first ionization energy of oxygen is less than that of the nitrogen.

So, the correct answer is “Option B & D”.

Note:
It also gives an insight to the ease with which an atom can change itself into cation by losing electrons. Ionization energy is expressed in units of kJ/mol. Ionization energy is always positive. The term ionization energy is not appropriate and is taken as the first ionization energy, second ionization energy, third ionization energy for the energy changes required for removal of first, second, third electron respectively.