Question

Which one of the following has the highest first ionization potential?
(A) $C$
(B) $N$
(C) $B$
(D) $O$

Answer 1

We know that ionization potential is the amount of energy required to remove an electron from an atom. So, the first ionization potential can be defined as the energy required to remove one mole of the most loosely held electron from one mole of gaseous atoms to produce one mole of gaseous ions with a charge of $+ 1$

Complete step-by-step answer: We know that the ionization potential increases when we move across a period from left to right and decreases as we move down the group in a period.
Ideally on the basis of the trends of ionization potential oxygen must possess the highest first ionization potential but this is not the case. Nitrogen will have the highest ionization potential because the atomic number of nitrogen is $7$ and the atomic number of oxygen is $8$ therefore, the outermost shell of nitrogen will contain $3$ electrons i.e. $2{p^3}$ and oxygen will have $4$ electrons in its outermost shell i.e. $2{p^4}$
As we know that an atom is highly stable if it has a fully filled or half-filled orbital which in this case is nitrogen with half-filled $p-subshell$ so as nitrogen is very stable it will require high energy to remove an electron from its outermost shell and therefore will possess the highest ionization potential among carbon, boron and oxygen.

So, the correct answer is Option (B).

Note: The ionization potential increases on moving across a period and decreases while moving down the group because as we move across a period the atomic size decreases and the effective nuclear charge on the atom increases whereas on moving down the group the number of shells keeps on increasing which in turn increases the atomic size and because of that shielding effect also increases.