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?