Question

An element with atomic number 21 is a/an
A.Halogen
B.Representative element
C.Transition element
D.Alkali metal

Answer 1

Periodic table consist of 18 groups and 7 periods. The S and P block elements (Group 1 ,2 and 13-18) are together known as Main group elements (Representative elements).

Complete step by step answer:
Periodic table is a table in which elements are arranged in the increasing order of their Atomic number.
-Alkali and Alkaline earth metals
Periodic table consists of 18 groups and 7 periods. The S and P block elements (Group 1 ,2 and 13-18) are together known as Main group elements (Representative elements).
Group one elements are known as Alkali metals have one electron in their outermost s orbital. Group two elements are known as Alkaline earth metals have two electrons in the outermost s orbital. Group 13 to 18 which have one, two, three, four, five and six electrons respectively, in the outermost p orbital. In all these elements the inner orbitals are completely filled. General configuration is $n{s^{1 - 2}}$ for s block and $n{s^2}n{p^{1 - 6}}$ for p block elements.
-Transition elements
Transition elements include groups 3 to 12. They have partly filled d orbitals. Transition elements consist of three rows. First, Second, Third transition series. First transition series starts with scandium $\left( {Z = 21} \right)$ and ends with $Zinc\left( {Z = 30} \right)$ . Electronic configuration for scandium is $1{s^2}2{s^2}2{p^6}3s23{p^6}3{d^1}4{s^2}$ . The ten elements from yttrium to cadmium (Z=39 to 48) constitute the second transition series. The third transition series starts from hafnium $\left( {Z = 72} \right)$ in which the filling of 5d subshell begins
-Halogens include fluorine. Chlorine, Bromine, Iodine and Astatine. It belongs to Group 17.
Halogens do not occur in the free state. They present in the combined state. The general outer electronic configuration is $n{s^2}n{p^5}$ .

Hence, the correct answer is (C) i.e Transition element.

Note:
Elements which have partly filled f orbital are known as f block elements or inner transition elements. This includes the Lanthanide series and Actinide series. In Lanthanides 4f subshell is filled while in actinides 5f subshell is being filled.