Question

Justify the presence of 18 elements in 4th period of periodic table?

Answer 1

When we consider —
n = 4,
the orbitals 3d, 4s and 3p present in the (form of) outermost shells show the order of their increasing energies — i.e. 3p<4s<3d.
Since the total number of orbitals available are 9 (1 from 4s, 3 from 3p & 5 from 3d orbital), the  maximum number of electrons that can occupy the three orbitals is 18.


Henceforth, 18 elements should be present in the 4th period.

Answer 2

When n = 4, we find that in the orbitals 3d, 4s and 3p which are present in the outermost shells have energies in increasing order — i.e. 3p<4s<3d.

Total number of orbitals = 9 (1 from 4s, 3 from 3p & 5 from 3d orbital),
So, maximum number of electrons found in the three orbitals is 9X2 = 18.


Therefore,  4th period should have 18 elements.