Define valency by taking examples of silicon and oxygen.

The combining capacity of an element is called its valency. The number of electrons present in the outermost shell of an atom gives its valency. But, if the number of valence electrons of the atom of an element is less than or equal to four, then the valency of that element is equal to the number of valence electrons. For example, the atom of silicon has four valence electrons. Thus, the valency of silicon is four. 
On the other hand, if the number of valence electrons of the atom of an element is greater than four, then the valency of that element is obtained by subtracting the number of valence electrons from eight. For example, the atom of oxygen has six valence electrons. Thus, the valency of oxygen is (8 − 6) i.e., two.   

(i) Atomic number is defined as number of protons present in the nucleus of an atom. For example, there are 6 protons in carbon, so the atomic number of carbon is 6. All atoms are characterized by their atomic numbers.

(ii) Mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom. For example there are 6 protons and 6 neutrons in the nucleus of carbon, so its mass number is 12.

(iii) Isotopes are atoms of the same element thus having same atomic number but different mass number. For example, chlorine has two isotopes with atomic number 17 but mass numbers as 35 and 37.

(iv) Isobars are such atoms which have same mass number but different atomic numbers. Thus isobars are different elements. For example, Ne has atomic number as 10 and sodium has atomic number as 11 but both of these have mass numbers as 22.

Uses of Isotopes

(i) Isotope of cobalt, ( Co), is used in the treatment of cancer.

An atom of Na has a total of 11 electrons. Its electronic configuration is 2, 8, 1. But, Na⁺ ion has one electron less than Na atom i.e., it has 10 electrons. Therefore, K-shell contains 2 electrons and L-shell 8 electrons. Thus, Na⁺ has completely filled K and L-shells.

Isotope of bromine with atomic mass 79 u = 49.7%

So, Contribution of 79 Br to atomic mass of bromine =$79 \mathrm{x} \frac{49.7}{100} =39.26\mathrm{u}$

Isotope of bromine with atomic mass 81 u = 50.3%
therefore, Contribution of 81  Br to atomic mass of bromine = $81 \mathrm{x} \frac{50.3}{100 } =40.74\mathrm{u}$

Hence, atomic mass of bromine atom = 39.26 + 40.74 = 80.0 u

If z = 3, i.e., atomic number is 3. Its electronic configuration is 2, 1. Hence, the valency of the element is 1 (since the outermost shell has only one electron). Then element is lithium. It has distribution of electrons as 2, 1.