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 energy 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