Question

why 2nd period has only 8 element​

Answer 1

there are only 8 elements in 2nd period because all of those elements have electrons in the second shell and no electrons in third shell..

if still doubt ask again

Answer 2

You mean group 2, RIGHT?

This is because this group is called as alkaline earth metals group which have 2 valence electrons in their outermost shell. and only 8 metals follow this property.

So 2nd grp contains 8 elements