Question

how Avogadro hypothesis used to deduce the atomicity of elementary gases

Answer 1

Atomicity
Atomicity is the number of atoms of an element present in one molecule of that element. They can be monoatomic (He), diatomic (H2), triatomic (O3) and polyatomic (S8). Homoatomic molecules have different atomicity.
Atomicity of Hydrogen and Chlorine
Let up take the example of the same reaction between hydrogen and chlorine. We saw earlier that;
atomicity of hydrogen and chlorine
Hence,

But 1 molecule of hydrogen chloride contains 1 atom of hydrogen and 1 atom of chlorine.
molecule of hydrogen = 1 atom of hydrogen
molecule of chlorine = 1 atom of chlorine
1 molecule of hydrogen = 2 atoms
1 molecule of chlorine = 2 atoms
Hence hydrogen and chlorine are both diatomic.
Atomicity of oxygen and nitrogen


One molecule of nitric oxide contains one oxygen atom and one nitrogen atom.
Thus,
molecule of nitrogen = 1 atom
molecule of oxygen = 1 atom
or, 1 molecule of nitrogen =2 atoms
1 molecule of oxygen = 2 atoms
Thus, both nitrogen and oxygen are diatomic.

Answer 2

Avogadro hypothesis states that equal volume of all gases under similar conditions of temperature and pressure contain equal number of molecules.

One of the applications of Avogadro’s law is to calculate atomicity of elementary gases. It is defined as the number of atoms of the elements present in one molecule of the substance. For example oxygen molecule has two oxygen atoms. So, atomicity of O2 is 2.

Similarly, atomicity of sulphur (S8) is 8.