Question

How will you justify the presence 18th element in the 5th Period of the periodic table

Answer 1

In the 5th period the 5th energy level has to be filled. The 5th energy level consist of 5s,5p,5d,5f orbitals ie.(1+3+5+7) orbitals. But 5d and 5f have greater energy than 6s and 7s orbitals respectively. So the orbitals that have to be filled are 5s,4f and 5p.(4f because 4f has higher energy than 5s orbital)
Thus no. of electrons 5s=2,4f=12,5p=6
Thus total no of electrons=18
And hence there are 18 elements in 5th period.