(a) What are isotopes? Explain by giving an example.
(b) Give one similarity and one difference between a pair of isotopes.
(c) Give the number of protons, neutrons, and electrons per atom in the two isotopes of chlorine ${}_{17}{}^{35}\mathrm{Cl}$ and ${}_{17}{}^{37}\mathrm{Cl}$.

(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 the 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 a similar atomic number
Difference: Isotopes have a different mass number.

(c) For chlorine ${}_{17}{}^{35}\mathrm{Cl}$,
Atomic number = 17
Number of protons = Atomic number
Number of protons = 17
Number of protons = number of electrons
Number of electrons = 17
Number of neutrons = Mass number - Number of protons
Number of neutrons = 35$-$17
Number of neutrons = 18

For chlorine ${}_{17}{}^{37}\mathrm{Cl}$,
Atomic number = 17
Number of protons = Atomic number
Number of protons = 17
Number of protons = number of electrons
Number of electrons = 17
Number of neutrons = Mass number - Number of protons
Number of neutrons = 37$-$17
​Number of neutrons = 20