Question

Composition of the nuclei of two atomic species A and B is given as under :

	A	B
Protons :	18	20
Neutrons :	22	20

Give the mass numbers of A and B. What is the relation between the two species and which element or elements they represent ?

Answer 1

Mass number is the sum of the number of neutrons and the number of protons. So, mass number of A and B is 40. As the mass number of A and B is same, the two are isobars.
Therefore, A represents Argon $\left({}_{18}{}^{40}\mathrm{Ar}\right)$ while B represents $\left({}_{20}{}^{40}\mathrm{Ca}\right)$ Calcium.