Question

The element having greatest difference between its first and second ionisation energy is:

• A. $K$
• B. $Ca$
• C. $Mg$
• D. $Sc$

Answer 1

The correct option is A $K$

$K\left(19\right)=1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^1$
$Mg\left(12\right)=1s^{2}2s^{2}2p^{6}3s^2$
$Ca\left(20\right)=1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^2$
$Sc\left(21\right)=1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{1}4s^2$
Here, $K$ has the least first ionisation energy among the given options since it is easier to remove an electron from $4s^1$.
Also, $K$ has the highest second ionization among the given options since it is very difficult to remove an electron from
$2p^6$
So, here $K$ has the highest difference between first and second ionization energy.