Justify that the fifth period of the periodic table should have 18 elements on the basis of quantum numbers.

Explanation

Fifth period of the periodic table have 18 elements. 5th period starts from Rb to Xe (18 elements), 5th period starts with principal quantum number n-5 and 1 = 0, 1, 2, 3 and 4
When n=5, the number of orbitals
1 for 5s
5 for 4d
3 for 5pTotal number of orbitals -9. `:.`Total number of electrons that can be accommodated in 9 orbitals = `9xx2` = 18. Hence the number of elements in 5th period is 18.