**HISTORY:**

An Italian scientist Amedeo Avogadro, in the year 1811, suggested that at a particular pressure or temperature, the volume of gas is proportional to the number of moles or molecules present on the gas irrespective of the nature of gas. To honor and glorify it, a French scientist, Jean Perrin, proposed to name this constant in the name of Avogadro. This happened in 1909. Later on, he won a Nobel Prize in Physics in the year of 1926, for figuring out the exact value of Avogadro number.

**DEFINITION**

In the field of physics and chemistry, Avogadro number is an important constant. Every element is constituted by constituent particles atoms or molecules. Avogadro constant is the number of atoms or molecules in one mole of an element or compound. The value of this constant in SI unit system is 6.022140857 * 10^{23 }which is generally used as 6.023 * 10^{23}. The symbol for this is N_{A }or L.

**AVAGADRO’S LAW **

This law states that, when the temperature and pressure are kept same, equal volumes of different gases contain same number of atoms or molecules. This is derived from ideal gas equation which is,

PV = nRT

Where P is the pressure on gas, V is the volume of gas, n is the number of particles (Avogadro number), R is the gas constant and T is the temperature.

EXAMPLE –

One mole of an oxygen atom contains 6.023 * 10^{23 }atoms. Therefore, Avogadro’s constant is 6.023 * 10^{23 }.

Each carbon-12 atom weighs about 1.99265 * 10^{-23} grams. Therefore, weight of one mole of carbon-12 is,

(1.99265 * 10^{-23}) * (6.023 * 10^{23}) = 12 grams of carbon-12

**ROLE IN SCIENCE **

It is used in the scaling of values in the atomic scale (like building relationships between macroscopic and microscopic values). It provides the relationships between various values. For example, it provides the relationship between Faraday constant and the charge of the element.

F = N_{A} * e = 96485.33289(59) C mol^{-1}

Where F is Faraday constant and e is the charge of element.

It also provides the relationship between gas constant R and the Boltzmann constant k_{B},

R = k_{B} * N_{A} = 8.3144598 (48) J mol^{-1} K^{-1}

It also establishes relationship between unified atomic mass unit, u,

1 u = M_{u }/ N_{A} = 1.660539040 (20) * 10^{-27} (kg)

where M_{u} is molar mass constant.