Question

Which of the following orders regarding ionization energy is correct ?

• A. $N > O > F$
• B. $N < O < F$
• C. $N > O < F$
• D. $N < O > F$

Answer 1

Answer: (C)

$N > O < F$

Answer 2

Ionisation energy is the energy required to remove an electron from outer shell of an atom. Since, on moving from left to right in a period, size decreases, thus, larger amount of energy is required to remove an electron, ie, ionisation energy increases.
Although the size of $O$ atom is smaller as compared to $N$ but the ionisation energy of $N$ is higher. This is because in case of $N$, the electron is to be removed from half-filled $2 p^{3}$ orbital. Thus, the order of ionisation energy is
$O < N < F$