Composition of the nuclei of two atomic species, X and Y are given as under X Y Protons = 6 6 Neutrons = 6 8Give the mass numbers of X and Y. What is the relation between the two species? Solution : Mass number of X = Number of protons + Number of neutrons = 6 + 6 = 12.Mass number of Y = Number of protons + Number of neutrons = 6 + 8 = 14.
These two atomic species X and Y have the same atomic number, but different mass numbers. Hence, they are isotopes.