(a) What are isotopes? Explain by giving an example.
(b) Give one similarity and one difference between a pair of isotopes.

(a) Isotopes are defined as atoms having the same atomic number, but different mass numbers. These atoms contain an equal number of protons and electrons, but different number of neutrons.
For eg. In nature hydrogen is found in three forms with different mass numbers, namely $\mathrm{protium}\left({\mathrm{H}}_1^1\right),\mathrm{deuterium}\left({\mathrm{H}}_1^2\right),\mathrm{and}\mathrm{tritium}\left({\mathrm{H}}_1^3\right)$.

(b) Similarity: Isotopes have similar atomic number
Difference: Isotopes have different mass number.