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.



Electronic configuration of - 

Na: 2, 8, 1             Mg: 2, 8, 2             Al: 2, 8, 3

(a) Valency

Al has the highest valency i.e. 3 because, among Na,Mg and Al, Al has the highest number of electrons in its outermost shell.

(b) Atomic Radius

As we move from left to right across a period, atomic radius decreases. Na, Mg and Al all belong to the same period . Na is the leftmost element among the three elements.

Therefore, Na has the largest atomic radius.

(c) Maximum Chemical Reactivity

All three are metals. Metals react by donation of electrons. So, reactivity depends on how easily a metal can donate electrons. As we move across a period from left to right, the ability of an element to lose electrons decreases.

Among the three elements, since Na is the leftmost element, it can easily lose electrons and hence it has maximum chemical reactivity.

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CA Maninder Singh is a Chartered Accountant for the past 13 years and a teacher from the past 17 years. He teaches Science, Economics, Accounting and English at Teachoo