Question

we know that 18 grp elements are He,Ne,Ar,Kr,Xe, Rn and there valency is 8 except He itś valency is 2 then why He is kept in 18 th grp elements?

Answer 1

their valency is not 8. it is zero. these elements do not or can not gain or lose electrons. they are very stable. valency is the number of electrons that take place in a chemical reaction. since no electrons take place here, their valency becomes zero. valency is NOT the number of outer most electrons but the electrons that take place in a chemical reaction. therefore even helium is in group 18

Answer 2

Helium is an inert gas similar to other group 18 elements that is why it is placed in group 18 instead of group 2

Which has valance electron configuration ns^2

Thanks