The first ionisation energy of Al is smaller than that of Mg because `:`

Options

A. The atomic number of Al is smaller than that of Mg.
B. the atomic size of Al is less than that of Mg.
C. Penetration of `s-` subshell electrons in case of Mg is greater than that of `p-` subshell in Al
D. Mg has incompletely filled `s-` orbital.

Answer

3

Explanation

Penetration of `p-` subshell electron is less than `s-` subshell electrons. In case of `Mg`, the first electron is to be removed from completely filled `3s^(2)` valence shell configuration as compared to partially filled `3p^(1)` of Al. These two factors collectively accounts for the higher ionisation energy of Mg than that of Al. Therefore, `(C )` option is correct.