Question

What are Elements 21 through 30 known as ?

Answer 1

Periodic table is the arrangement of the known elements according to their properties in a tabular form.
In the periodic classification of elements, the elements have been broadly divided into four blocks namely $s,p,d$ and $f$ –blocks. The division is based on the type of atomic orbital which receives the last electron or valence electron in the atom.

Complete answer: The elements $21$ through $30$ known as transition metals.
The d-block elements are also known as transition elements. $d$ -block consists of elements lying between $s -$ and $p$ -blocks that are between groups $2$ and $13$ , starting from fourth period onwards. In transition elements, the outermost shell contains one or two electrons in their $s$-orbital (ns) but the last electron enters the d-subshell $(n - 1)d$.
The transition elements are those elements which have incompletely filled $d -$ subshells in their ground state or in any one of their oxidation states. The general electronic configuration of $d$ -block elements is $(n - 1){d^{1 - 10}}n{s^{0 - 2}}$ . The transition elements consist of four rows of ten elements and the rows are called first, second, third and fourth transition series and this involves the filling of $3d,4d,5d$ and $6d$ orbitals respectively.
The name “transition” is given to the $d$ -block elements because they represent change in properties from most electropositive $s$ -block elements to least electropositive $p$ -block elements.

Note:
Nearly all transition elements (except mercury which is liquid at room temperature) have typical metallic properties such as high tensile strength, ductility, malleability, high thermal and electrical conductivity and metallic lustre. Most of the d-block elements form coloured compounds.