Question

the valency of helium is 2(duplet).The valency of calcium is 2,8,8,2.but calcium also has 2 electrons in its valency shell but why it is not called as duplet

Answer 1

helium is called duplet coz it's outer orbit according to aafbau principle contains only 2 electrons ... while this isn't in case of calcium

Answer 2

because in calcium the 2 Valence electrons are in N shell which required 8 electrons to complete it's octate while in helium the 2 electrons are in K shell which required only 2 electrons to complete duplet and become stable