Amadeo Avogadro first proposed that that volume of a gas at a given pressure and temperature is proportional to the number of atoms or molecules, regardless of the type of gas. Although he did not determine the exact proportion, he credited for the idea, that the number of entities in a substance is proportional to its physical mass. Avogadro’s number is a proportion that relates molar mass on atomic scale to physical mass on a human scale. Avogadro’s number is defined as the number of elementary particles per mole of a substance. It is equal to 6.203 x 10²³ mole⁻¹ and is expressed as the symbol NA.

The definition of Avogadro’s number is that it tells us the number of particles in 1 mole (or mol) of a substance. These particles could be electrons or molecules or atoms. So, if you wanted to know the number of particles in 3 moles of a substance will be:

= 3 x 6.203 x 10²³

= 1.18 x 10²⁴ Particles.

Example: One mole of oxygen atoms contains 6.203 x 10²³ oxygen atoms. Also, one mole of nitrogen atoms contains 6.203 x 10²³ nitrogen atoms. The number 6.203 x 10²³ is called Avogadro’s Number (NA) of Avogador’s constant, after 19th century scientist Amedeo Avogadro.

Each carbon – 12 atoms weighs about 1.99265 x 10⁻²³ g.

∴ (1.99265 x 10⁻²³ g) x (6.023 x 10²³) = 12g of carbon – 12.

Avogadro’s number is a similar concept to that of a dozen or a gross. A dozen molecule is 12 molecules. A gross of molecules is 144 molecules, with Avogadro’s number, scientists can discuss and compare very large numbers, which is useful because substances in every quantity contain very large numbers of atoms and molecules.