Question 13
Composition of the nuclei of two atomic species, X and Y are given as under
X Y
Protons = 6 6
Neutrons = 6 8
Give the mass numbers of X and Y. What is the relation between the two species?

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.