How would you justify the presence of 18 elements in the fifth period of the periodic table ?

Explanation

Fifth period corresponds to n = 5 which containis 5s, 5p, 5d, 5f orbitals, but before filling 5d orbital, the electrons enter in 6s orbital (according to Aufbau principle) thus, 5d and 5f orbials are not available. Thus available orbitals are one 5s orbital five 4d.orbitals and three 5p. orbitals i.e., total 9 orbitals which can accommodate maximum of 18 electrons. Hence, this period contains 18 elements.