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?

(D) Mass number of $X = \text{Number of protons} + \text{Number of neutrons} = 6 + 6 = 12$.
Mass number of $Y = \text{Number of protons} + \text{Number of neutrons} = 6 + 8 = 14$.
These two atomic species $X$ and $Y$ have the same atomic number (number of protons $= 6$),but different mass numbers ($12$ and $14$). Hence,they are isotopes.