Question

On the basis of quantum numbers, justify that the fifth period contain $18$ elements in periodic table.

Answer 1

In 5th period n = 5, and l = 0, 1, 2, 3. The order in which the energy of the available orbitals 4d, 5s and 5p increases is 5s < 4d < 5p. The total number of orbitals available is 9. The maximum number of electrons that can be accommodated is 18, and therefore 18 elements are there in the 5th period.