How would you justify the presence of `18` elements in the `5th` period of the periodic table?

Explanation

In the modern periodic table each period starts with the fillings of a principal energy level . Thus the 5th period begins with the filling of principal quantum number n=5. When n= 5, l = 0, 1,2,3 .But according to aufbau principal , the electrons are added to different orbitals in order of their increasing energies . Now the energies of 5d - and 5f- subshell. are higher than that of 6s- subshell but the filled only in 5s, 4d- and 5p- subshells whose energies increase in the order `: 5 s lt 4 d lt 3 t p.` Now s- subshell has two , p- subshell has three and d-subshell has five -orbitals. Hence in all there are 9 (1+3+5) orbitals that can have 2 electrons therefore 9 orbitals at the maximum can have 18 electrons and hence 5th period has 18 elements