Which of the following atoms has the highest first ionisation energy ?

Options

A. Rb
B. Na
C. K
D. Sc

Answer

D

Explanation

Rb. Na and K are alkali metals . After losing oen electron , they achieve stable electronic configuration (E.C) of the nearest inert gas and hence their first ionization energies are low. In contrast E.C. Of Sc is [Ar] `3d^(1) 4s^(2)` .On losing one electron from 4s- orbital it does not attain the stable E.C. of the nearest inert gas. Therefore , its ionization energy is much higher than those of Rb, Na and K. Thus option ( d) is correct.