How would you justify the presence of $18$ elements in the $5^{\text{th}}$ period of the Periodic Table?

(N/A) For the $5^{\text{th}}$ period,the principal quantum number is $n=5$.
The orbitals available for filling are $5s$,$4d$,and $5p$.
The number of orbitals in each subshell are: $5s$ ($1$ orbital),$4d$ ($5$ orbitals),and $5p$ ($3$ orbitals).
The total number of orbitals is $1 + 5 + 3 = 9$.
According to the Pauli Exclusion Principle,each orbital can hold a maximum of $2$ electrons.
Therefore,the total number of electrons that can be accommodated is $9 \times 2 = 18$.
Since each element corresponds to the addition of one electron,there are $18$ elements in the $5^{\text{th}}$ period.