(i) Atomic number: The number of protons in an atom of an element is known as atomic number of that element. For example, one atom of sodium element has 11 protons in it, so the atomic number of sodium is 11. Similarly, one atom of carbon element has 6 protons in it, so the atomic number of carbon is 6.

(ii) Mass number: The total number of protons and neutrons present in one atom of an element is known as its mass number. For example, one atom of sodium element contains 11 protons and 12 neutrons. Hence, the mass number of sodium is 11 + 12 = 23. Similarly, a normal carbon atom has 6 protons and 6 neutrons, so the mass number of carbon is 6 + 6 = 12.

(iii) Isotopes: Atoms of same element with same atomic number but different mass number are called isotopes of that element. For example, Hydrogen has three isotopes, protium (H¹₁), deuterium (H²₁ or D²₁) and tritium (H³₁ or T³₁). Many elements consist of a mixture of isotopes. Each isotope of an element is a pure substance. The chemical properties of isotopes are similar but their physical properties are different.

(iv) Isobars: Atoms of different elements which have the same mass number but have different atomic numbers are called isobars. For example, calcium, atomic number-20 and argon, atomic number 18 are isobars. Both these elements have same mass number which is equal to 40. Therefore, the total number of nucleons is the same in both the elements.

Uses of Isotopes: Isotopes of some elements have special properties makes them useful in various fields. For example, (i) An isotope of uranium is used as a fuel in nuclear reactors. (ii) An isotope of cobalt is used in the treatment of cancer. (iii) An isotope of iodine is used in the treatment of goitre.