Question

elements of group 1and elements of group 17 both have valency 1 .explain

Answer 1

Hey,
It's because elements of group 1 have only one valence electron so it will lose that electron and elements of group 17 also have one valence electron so it will gain only one electron.

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Answer 2

elements of group 1 and elements of group 17 both have valency 1 because group 1 contains all metals and have 1 valence electron which they donate to attain stability of nearest noble gas, whereas elements of group 17 are non metals and they also have 1 valence electron which they gain electrons for getting stable electronic configuration
hope it helps you