Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.

Solution:

(i) The number of positively charged protons present in the nucleus of an atom is defined as the atomic number and is denoted by Z. Example: Hydrogen has one proton in its nucleus; hence, its atomic number is one.

(ii) The total number of protons and neutrons present in the nucleus of an atom is known as the mass number. It is denoted by A. ₂₀Ca⁴⁰ . The mass number is 40. The atomic number is 20.

(iii) The atoms which have the same number of protons but a different number of neutrons are referred to as isotopes. Hence, the mass number varies.

Example: The most simple example is the Carbon molecule which exists as ₆C¹² and ₆C¹⁴

(iv) Isobars: Isobars are atoms which have the same mass number but differ in atomic number.

Examples are, ₂₀Ca⁴⁰and ₁₈Ar⁴⁰

Uses of isotopes

• The isotope of the Iodine atom is used to treat goitre, an iodine-deficient disease.
• In the treatment of cancer, an isotope of cobalt is used.
• Fuel for nuclear reactors is derived from the isotopes of the Uranium atom.