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

Explanation

When n = 5, l = 0, 1, 2, 3. The order in which the en.ergy of the available orbitals 4d, 5s and 5p increases is `5s lt 4d lt 5p`. The total number of orbitals available are 9. The maximum number of electrons that can be accommodated is 18, and therefore 18 elements are there in the `5^("th")` period.