Question

Among the following the third ionization energy is highest for

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

Answer 1

Answer: (C)

beryllium

Answer 2

The electronic configuration of the given elements is as
${ }_{12} Mg =[ Ne ] 3 s^{2}$
${ }_{5} B =[ He ] 2 s^{2}, 2 p^{1}$
${ }_{4} Be =[ He ] 2 s^{2}$
${ }_{13} Al =[ Ne ] 3 s^{2} 3 p^{1}$
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.