Two elements are denoted as follows:
$_{20}^{40}A$ and $_{18}^{40}B$
$(a)$ Find the number of electrons present in $A$ and $B$.
$(b)$ Find the number of nucleons present in $A$ and $B$.
$(c)$ Explain the special 'term' used to represent $A$ and $B$ and define the same.

(C) The number of electrons in a neutral atom is equal to its atomic number $(Z)$. For element $A$,$Z = 20$,so it has $20$ electrons. For element $B$,$Z = 18$,so it has $18$ electrons.
$(b)$ The number of nucleons is equal to the mass number $(A)$. Both elements have a mass number of $40$,so both have $40$ nucleons.
$(c)$ The special term used to represent $A$ and $B$ is 'Isobars'.
Definition: Isobars are atoms of different chemical elements that have the same mass number but different atomic numbers.
Example: $_{18}^{40}Ar$ and $_{20}^{40}Ca$.