Isotopes of the same element have the same number of

Which of the following is $NOT$ a hydrogen-like species?

What do you understand by isoelectronic species? Name a species that will be isoelectronic with each of the following atoms or ions.
$(i)$ $F^{-}$
$(ii)$ $Ar$
$(iii)$ $Mg^{2+}$
$(iv)$ $Rb^{+}$

Which of the following is an isobaric pair?

Which of the following is an isotonic pair?

Identify the isoelectronic pair from the following.