How would you justify the presence of `18` elements in the `5th` period of the periodic table?

Explanation

When n = 5 , I=0, 1,2,3, . The order of the increasing energy orbitals are 5s `lt 4d lt 5p` . The toatal number of orbitals available is nine . The maximum number of electrons that can accommodate is 18 and therefore 18 elements are there in the 5th period.