When we move down a group a shell is added inspite of that the fourth and fifth period both have 18 elements each why?
The number of elements in each period depends on the number of subshells that is s , p, d and f .
In fourth and fifth period subshells involved are s, p and d .
Number of orbitals of s subshell = 1
Number of orbitals of p subshell =3
Number of orbitals of d subshell =5
Total number of orbitals = 9
Total number of elements = 18 which is twice the total number of orbitals.