Question

The first ionization energy (in kJ/mol) of $Na,Mg,Al$ and $Si$ respectively, are

• A. $496,577,737,786$
• B. $786,737,577,496$
• C. $496,577,786,737$
• D. $496,737,577,786$

Answer 1

The correct option is D $496,737,577,786$

Generally, as we move from left to right in a period, ionisation energy increases. But there are few exceptions to this rule when there exist a case of half filled and full filled subshells. Here, all the given elements are from third period in the order from left to right as: $Na,Mg,Al,Si$. But as $Mg$ has full filled subshell($n{s}^2$) so it is more stable and it's ionisation energy becomes more than that of $Al$. So the correct order of ionisation energy will be $Na<Al<Mg<Si$

Hence, (d) is the correct option.