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# The elements with atomic numbers 21 to 30 belong to

$\begin{array}{1 1}\text{halogens}\\\text{noble gas}\\\text{first transition elements}\\\text{lanthanides}\end{array}$

$21=2+8+8+3$.Thus,the element of atomic number 21 belongs to $4^{th}$ period and $3^{rd}$ group.
Its electronic configuration will be $(4s)^2(3d)^1$.Hence,the elements from 21 to 30 belongs to the first transition elements