What is meant by the term 'variable valency'. Give a reason why silver exhibits a valency of +1 and +2.

Certain elements exhibit more than one valency hence it is said that these elements have variable valency.

Reasons for exhibiting variable valency — An atom of an element can sometimes lose more electrons than are present in it's valence shell. This happens when it loses electrons from the penultimate [i.e., last but one] shell and hence exhibit more than one or variable valency.

Variable valency of Silver — Atomic number of Silver (Ag) is 47. Its electronic configuration is [2, 8, 18, 18, 1]. The outermost shell has one electron and the penultimate shell has 18 electrons. However, the penultimate shell has not attained stability and one more electron sometimes jumps to the outermost shell there by increasing valence electrons and the new configuration [2, 8, 18, 17, 2] loses two electrons and has valency [+2]. Therefore, Silver exhibits varibale valency forming Ag¹⁺ and Ag²⁺.