Which of the following species does not have the same number of electrons as the other three species?

The spectrum of Helium is expected to be similar to that of . . . . . .

$Be^{2+}$ is isoelectronic with:

The mass number of an anion,$X^{3-}$,is $14$. If there are $10$ electrons in the anion,the number of neutrons in the nucleus of an atom of the element $X$ is:

Which of the following species are isoelectronic (i.e.,have the same number of electrons)?
$Na^{+}, K^{+}, Mg^{2+}, Ca^{2+}, S^{2-}, Ar$

Give the relation between the following pairs of species:
$(i)$ $_{20}^{40}Ca$ and $_{19}^{40}K$
$(ii)$ $_{17}^{35}Cl$ and $_{17}^{37}Cl$
$(iii)$ $_{18}^{40}Ar$,$_{20}^{40}Ca$ and $_{19}^{40}K$
$(iv)$ $_{38}^{87}Sr$ and $_{38}^{90}Sr$