Compare the radii of two species $X$ and $Y$. Give reasons for your answer.
$(a)$ $X$ has $12$ protons and $12$ electrons.
$(b)$ $Y$ has $12$ protons and $10$ electrons.

(B) The radius of species $Y$ is smaller than that of species $X$.
Reasoning:
$1$. Species $X$ is a neutral magnesium atom $(Mg)$ with $12$ protons and $12$ electrons.
$2$. Species $Y$ is a magnesium cation $(Mg^{2+})$ formed by the loss of $2$ electrons,resulting in $12$ protons and $10$ electrons.
$3$. In $Y$,the nuclear charge ($12$ protons) acts on fewer electrons ($10$ electrons) compared to $X$. This increases the effective nuclear charge per electron,pulling the remaining electron shells closer to the nucleus.
$4$. Additionally,the loss of electrons may result in the loss of an entire outermost shell,further decreasing the ionic radius compared to the atomic radius.