(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 ³⁵₁₇Cl and ³⁷₁₇Cl.

(a) Isotopes are the atoms of the same element with the same atomic number but different mass numbers. It means that the isotopes have the same number of electrons and protons but a different number of neutrons. For example, hydrogen has three isotopes- protium, deuterium, and tritium.

Hence, all three of them have the same atomic number i.e. 1 but different mass numbers.

(b)

Similarity –A pair of isotopes have the same atomic number.

Difference – A pair of isotopes have different mass numbers.

(c) In  3517Cl, there are 17 protons, 17 electrons, and 18 neutrons.

In  3717Cl, there are 17 protons, 17 electrons, and 20 neutrons.