Periodic Classification of Elements (C10)

Na, Mg and Al are the elements of the 3rd 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.


Aluminium (Al) has the highest valency. Valency of Na is 1, Mg is 2, Al is 3.

Sodium (Na) has the largest atomic radius. As we move from left to right in a period, the atomic radius decreases as increase in nuclear charge pulls the electrons closer to the nucleus reducing the atomic size.

Sodium (Na) has the maximum chemical reactivity. Reactivity decreases on moving from left to right in a period.