Nucleons are equal to :

Among the following, the isoelectronic species is/are:
$(I)$ $O^{2-}, F^{-}, Na^{+}, Mg^{2+}$
$(II)$ $Na^{+}, Mg^{+}, Al^{3+}, F^{-}$
$(III)$ $N^{3-}, O^{2-}, F^{-}, Ne$

In the nucleus of $_{20}Ca^{40}$,there are:

On comparing the chemical reactivity of $C^{12}$ and $C^{14}$,it is revealed that:

$A$: Atoms with the same mass number but different atomic numbers are called isobars.
$R$: In isobars,the sum of protons and neutrons is always different.

Consider an imaginary ion ${}_{22}^{48}X^{3-}$. The nucleus contains '$a$'$\%$ more neutrons than the number of electrons in the ion. The value of '$a$' is $.....$ [nearest integer].