Discuss the position of $d$-block elements in the periodic table.

(N/A) In a periodic table,the $d$-block elements are situated between the $s$-block and $p$-block elements,occupying the large middle section of the periodic table.
The $d$-block elements are present from group-$3$ to group-$12$ in periods $4$ to $7$. These elements have $d$-orbitals in the penultimate energy level of their atoms that receive electrons,giving rise to four rows of transition elements,namely $3d, 4d, 5d$,and $6d$,as follows:
$(i)$ The first transition series or $3d$-series belongs to the $4^{th}$ period.
$(ii)$ The second transition series or $4d$-series belongs to the $5^{th}$ period.
$(iii)$ The third transition series or $5d$-series belongs to the $6^{th}$ period.
$(iv)$ The fourth transition series or $6d$-series belongs to the $7^{th}$ period.
Thus,there are a total of forty elements with ten elements in each series.