Question

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

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

Answer 1

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

The expected order is $Na<Mg<Al<Si$
But the actual/experimental order turns out to be $Na<Al<Mg<Si$ because of the fully filled s-subshell of magnesium and the $s^2{p}^1$ configuration of Al which makes it relatively easy for $Al$ to lose its outermost electron.