Question

Among the following the third ionization energy is highest for

• A. magnesium
• B. boron
• C. beryllium
• D. aluminium

Answer 1

Answer: (C)

The electronic configuration of the given elements is as
${}_{12}Mg=[Ne]3s^2$
${}_{5}B=[He]2s^2,2p^1$
${}_{4}Be=[He]2s^2$
${}_{13}Al=[Ne]3s^{2}3p^1$
From the electronic configuration, it is clear that third ionisation energy of $Mg$ and $Be$ is higher as compared to $Al$ and $B$ as in them the third electron have to be removed from the stable inert gas configuration.
Between $Mg$ and $Be$, third $IE$ of $Be$ is higher because of its smaller size.