The third period is similar to the second, filling the 3s and 3p sublevels. Notice that the 3 d sublevel does not actually fill until after the 4s sublevel. This results in the fourth period containing 18 elements due to the additional 10 electrons that are contributed by the d sublevel.

Why does the fourth period have 18 elements and not 8 elements?

After the filling of 4s orbital, instead of 4p orbitals, there starts the filling 3d orbitals. This is in keeping with the fact that 3d orbitals have less energies than 4p orbitals. Thus 10 elements are built up by the filling of 3d orbitals. … Hence the fourth period consists up of 18 and not 8 elements.

Why are there 18 elements in the 5th period?

When n=5,l=0,1,2,3 and 4. The order in which the enery of the available orbitals 4d,5s and 5p increases in 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.