Question

Which of the follwing atoms has the highest first ionization energy?

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

Answer 1

The correct option is C $Sc$

Going down the group, ionization energy decreases.
$Na$, $K$ and $Rb$ are first group elements and the first I.E. of $Na$ is highest among them.
$Sc$ is a d-block element with [Ar]$3d^{1}4s^2$ configuration.
First I.E. involves removal of electrons from a filled s-orbital which is higher than that of $Na$.
C is the correct option.