Na, Mg and Al are the elements having one, two and three valence electrons respectively. Which of these elements (i) has the largest atomic radius, (ii) is least reactive ? Justify your answer stating reason for each.

Explanation

These three elements belong to third period in the same order. Their electronic configuration are :
Na (Z = 11) 2, 8, 1 , Mg (Z = 12) 2, 8, 2, Al (Z = 13) 2, 8, 3.
(i) Since the atomic radius decreases along a period, the element Na has the largest radius.
(ii) All the three elements are metals. As the reactivity of the metals decreases along a period, the element Al is the least reactive.