Question

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

Answer 1

To answer this question we should about halogen, representative, transition, and alkali elements. We can find the block or group by which the elements belong by the outermost electronic configuration of the atom. We will write the electronic configuration for the given atomic number.

Complete step-by-step answer: The atomic number is the number of protons present in the nucleus of an atom. Every atom has a unique atomic number. The group eighteen elements are halogen. s and p-block elements are known as a representative element. The d-block elements are known as transition elements and the group-first element is known as alkali metals.For an element the proton number is equal to the electron number. So, the element with atomic number $21$ has $21$ electron, so the electronic configuration of the element is as follows:For an element, the proton number is equal to the number of electrons. So, the element with atomic number $21$ has $21$ electrons so, the electronic configuration of the element is as follows:
${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{d}}^1}{\text{4}}{{\text{s}}^2}$
The outermost orbital of having valence electrons of an atom tells the block of that element. The valence electrons of the element having atomic number $21$are present in d-orbital so, the element having atomic number $21$ belongs to d-block. The d-block elements are known as transition elements, so the element having atomic number $21$ is a transition element.The outermost orbital of having valence electrons of an atom tells the block of that element. The valence electrons of the element having atomic number $21$ are present in d-orbital so, the element having atomic number $21$ belongs to d-block. The d-block elements are known as transition elements so, the element having an atomic number $21$ is a transition element.

Therefore, option (C) transition element is correct.

Note: The atomic representation is as follows:
$_{\text{Z}}^{\text{A}}{\text{X}}$
Where ${\text{X}}$is the symbol of the atom.
${\text{Z}}$ is the atomic number of proton number.
${\text{A}}$ is the atomic mass.
The outermost electronic configuration of halogen is ${\text{n}}{{\text{s}}^{\text{2}}}{\text{n}}{{\text{p}}^{\text{6}}}$ . The outermost electronic configuration of alkali metals is ${\text{n}}{{\text{s}}^1}$.