The correct order in which the first ionisation potential increases is

Options

A. K, Be, Na
B. Be, Na, K
C. Na, K, Be
D. K, Na, Be

Answer

D

Explanation

The electronic configuration of the elements are
`""_4Be - 1s^2 2s^2`
`""_11 Na - 1s^2 2s^2 2p^6 3s^1`
`""_K - 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1`
The first ionisation energy of Be is maximum because electron is to be drawn from stable (fully filled) orbital. The Ist ionisation energy of Na is greater than K because size of the K is bigger than Na which facilitates easy removal of electron from its outermost shell.
So, the sequence is K < Na < Be.