Question

Explain why the first period of modern periodic table has two elements whereas the second period has eight elements?

Answer 1

well.. This question has quite a simple answer.. The first shell .. which is K shell can hold upto 2 electrons .. If you have idea about quantum numbers.. then for quantum number 1.. only one azimuthal Q.N is defined.. that is 0 which means.. 1st shell has only S subshell.. and S subshell holds upto 2 electrons at max .. therefore only 2 electrons are required to be filled in the first shell.. so.. only two elements are there in period 1 .. (Since period number corresponds to the Shell number)
but in case of 2nd shell there can be 8 electrons in it .. ( there is a rule for it that is 2*(shell number)^2) So.. eight more elements are there in the Periodic table in 2nd period

Answer 2

no. of elements in a period depends on the electronic config/valence elecrons/no. of shells