What are nuclides?

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 $_{38}^{90}Sr$

Which one of the following is not an iso-electronic pair?

Which of the following has an isotopic number (neutron number) equal to that in $_{13}^{27}Al$?

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

The number of electrons and neutrons of an element is $18$ and $20$ respectively. Its mass number is