Question

Explain the atomicity of a gas molecule with an example.

Answer 1

Hint: As we know that atomicity is defined as the total number of atoms constituted by a molecule. For example, each molecule of oxygen (O2) is composed of two oxygen atoms. So, atomicity of oxygen is equal to 2.

Complete step by step answer:
From atomicity we also know that atomicity is used in the same sense as valence. So on the basis of atomicity, molecules is classified as:
Monatomic-composed of 1 atom e.g. He, Ne, Ar
Diatomic-composed of 2 atoms e.g. ${H_2},{N_2},{O_2}$
Tri atomic-composed of 3 atoms e.g. ${O_3}$
Polyatomic-composed of 3 or more atoms e.g. ${P_4},{S_8}$
All metals and other elements, like carbon, do not have a simple structure but consist of a large and indefinite number of atoms bonded together. So, their atomicity cannot be determined and is usually considered to be 1. But atomicity usually varies in different allotropes of the same element.
Let’s consider water molecules to understand the concept of atomicity. We can define atomicity as the number of atoms of molecules that each atom possesses.
Example: In case of water molecules, the reaction involves hydrogen and oxygen as the reactants.
               $2{H_2} + {O_2} \to 2{H_2}O$
In this reaction two molecules of hydrogen react with one molecule of oxygen forming two molecules of water.

So, the formula used to derive atomicity:

Atomicity is determined as the ratio of molecular mass to atomic mass. For example, in the case of an oxygen molecule, the molecular mass of oxygen is 32 (\[ = {\text{ }}2 \times 16\]) and the atomic mass is 16. So, to derive the value of atomicity we will divide 32 / 16 =2. Thus, the atomicity of oxygen ${O_2}$ is 2.

Note:
 We must know that the atomicity was discovered by a British Physicist named J.J Thompson in the year 1897. J.J Thompson had proposed that an atom constitutes of a negatively charged particle.