$\mathrm{Na},\mathrm{Mg}$ and $\mathrm{Al}$ are the elements of the $3\mathrm{rd}$ period of the Modern Periodic Table having group number 1, 2 and 13 respectively. Which one of these elements has the (a) highest valency, (b) largest atomic radius, and (c) maximum chemical reactivity? Justify your answer stating the reason for each.