The elements ${}_4\mathrm{Be}$, ${}_{12}\mathrm{Mg}$ and ${}_{20}\mathrm{Ca}$, each having two valence electrons in their valence shells, are in periods 2,3 and 4 respectively of the modern periodic table. Answer the following questions associated with these elements, giving reason in each case :
(a) In which group should they be ?
(b) Which one of them is least reactive?
(c) Which one of them has the largest atomic size?