Question

Which of the following elements has highest first ionization potential ?

• A. C
• B. N
• C. B
• D. O

Answer 1

The correct option is B N

The Ionization Energy increases across a period, therefore the I.E of O should be highest, but the first I.E of N is greater than O because in the oxygen case the electron being removed is one of the $2p{x}^2$ pair. The repulsion between the two electrons in the same orbital means that the electron is easier to remove than it would otherwise be. Therefore, the I.E of N is highest.