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

Explanation

(i) Atomic number. The number of protons in one 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. Similary, 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, so 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. Isotopes are atoms of the same element having the same atomic number but different mass numbers. The isotopes of an element have the same atomic number because they contain the same number of protons (and electrons). The isotopes of an element of isotopes. All the chlorine atoms contain 17 protons, so the atomic number of all the chlorine atoms is 17. Now, some chlorine atoms have 18 neutrons whereas other chlorine atoms contain 20 neutrons. Chlorine atoms can, therefore, have mass numbers of `17 + 18 =35 or 17 + 20 = 37`. Thus, chlorine has two isotopes of mass numbers 35 and 37 respectively. The two isotopes of chlorine can be written as `._(17)^(35)Cl and ._(17)^(37)Cl`.
(iv) Isobars. Isobars are the atoms of different elements having different atomic numbers but the same mass number. Isobars have different number of protons in their nuclei but the total number of nucleons (protons + neutrons) in them is the same. An example of isobars is argon, `._(18)^(40)Ar`, and calcium, `._(20)^(40)Ca` (The is because argon and calcium are atoms of different elements having different atomic numbers of 18 and 20 respectively but they have the same mass number of 40.
Uses of Isotopes (i) Radioactive isotopes (such as Uranium-235) are used as a fuel in nuclear reactors of nuclear power plants for generating electricity (ii) Radioactive isotopes (such as Cobalt-60) are used in the treatment of cancer.