(i) The Avogadro’s law states that “equal volumes of all gases under similar conditions of temperature and pressure contain the equal number of molecules”.
(ii) Let us consider the reaction between hydrogen and chlorine to form hydrogen chloride gas.
H2(g) + Cl2(g) → 2HCl(g)
⇒ 1 volume + 1 volume → 2 volumes.
(iii) According to Avogadro’s law, 1 volume of any gas is occupied by “n” number of molecules,
“n” molecules + “n” molecules → “2n” molecules
If “n” = 1, then
1molecule + 1 molecule → 2 molecules.
\(\frac{1}{2}\) molecule + \(\frac{1}{2}\)molecule → 1 molecule
(iv) 1 molecule of hydrogen chloride gas is made up\(\frac{1}{2}\) of molecule of hydrogen and \(\frac{1}{2}\)molecule of chlorine.
(v) \(\frac{1}{2}\)molecule of hydrogen contains 1 atom.
So, 1 molecule of hydrogen contains 2 atoms.
So, hydrogen atomicity is 2. Similarly, chlorine atomicity is also 2.
So, H2 and Cl2 are diatomic molecules.
