Account for the fact that the fourth period has 18 and not 8 elements.
The first element of fourth period is potassium (Z = 19) having electronic configuration 1s2 2s2 2p6 3s2 3p6 4s1. Thus it starts with the filling of 4s orbital., After the filling of 4s orbital, instead of 4p orbitals, there start the filling 3d orbitals. This is in keeping with the fact that 3d orbitals have fewer energies than 4p orbitals. Thus 10 elements are built up by the filling of 3d orbitals. After the filling of 3d orbitals, 4p orbitals are filled up and that process is completed at krypton [Kr, Z = 36]. Hence the fourth period consists up of 18 and not 8 elements.
