Question

Give reason for why in hcl the valency of the elements chlorine is considered as 1

Answer 1

hydrogen has 1 electron & chlorine has 7 electrons in its valence shell 
hydrogen removes 1 electron & gets stabilised whereas chlorine accepts the ejected electron from hydrogen atom & achieves the octet to get stabilised in hydrogen chloride molecule . as chlorine accepts 1 electron so valency of it is 1

Answer 2

hey your ans is here
if we go through the configuration of HCl
so
for h it is (1)
for cl it is (2,8,7)
now since cl is deficient of 1 electron it takes electron from hydrogen I.e.(1)
and therefore combines forming HCl
and since it takes only 1 electron its valency is 1