Question

Consider the elements $Mg,Al,S,P$ and $Si,$ the correct increasing order of their first ionization enthalpy is :

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• A. $Mg<Al<Si<P<S$
• B. $Al<Mg<S<Si<P$
• C. $Al<Mg<Si<S<P$
• D. $Mg<Al<Si<S<P$

Answer 1

The correct option is C $Al<Mg<Si<S<P$

Across the period, generally ionization enthalpy increases.
IP is expected to increase in the same elemental order of third row as $Mg,Al,Si,PandS$.
But half-filled and fully-filled configuration are stable and may change the regular trend.
$P$ having half-filled p-orbital has more $I{E}_1$ than $S$.
Similarly, $Mg$ has compltely filled s-orbital and more $I{E}_1$ than $Al$, where the 3p electron is effectively shielded by $3s$ electrons.
Finally order should be $Al<Mg<Si<S<P$
Hence, (d) is the correct option.