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Avogadro's Number

General

Avogadro's Number (Avogadro Constant)

The fixed constant 6.02214076 x 10^23, representing the number of elementary entities contained in exactly one mole of a substance.

Definition

Avogadro's Number, also called the Avogadro constant, is 6.02214076 × 10^23. It represents the number of elementary entities — atoms, molecules, ions, or other specified particles — contained in exactly one mole of a substance. This constant is what makes the mole a useful bridge between the invisible atomic world and quantities you can actually weigh on a lab scale.

The number is named after 19th-century Italian scientist Amedeo Avogadro, who proposed that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. The actual numerical value, however, was determined much later through independent experimental methods, and since 2019 it has been fixed by international definition as an exact constant rather than a measured quantity.

Avogadro's Number appears anywhere a chemist needs to convert between a mole-based quantity and an actual particle count. The Avogadro's Number Calculator and Moles to Atoms Calculator both use this constant to move between moles, mass, and the literal number of atoms or molecules in a sample.

Formula

Number of Particles = Moles (n) × 6.02214076 × 10^23

Rearranged to solve for moles when the particle count is known:

Moles (n) = Number of Particles / 6.02214076 × 10^23

Worked Example

Suppose you have 0.25 moles of carbon dioxide gas and want to know how many individual CO2 molecules that represents.

Number of molecules = 0.25 mol × 6.02214076 × 10^23 = 1.5055 × 10^23 molecules

So 0.25 moles of carbon dioxide contains roughly 150 sextillion molecules. Working in reverse, if you were told a sample contains 3.011 × 10^24 molecules, dividing by Avogadro's Number gives 5 moles. Try both directions with the Avogadro's Number Calculator.

Key Things to Know

  • It is the definition of the mole: A mole is defined as containing exactly Avogadro's Number of particles, making the two concepts inseparable in chemistry calculations.
  • The value is now an exact constant: Since the 2019 redefinition of SI units, Avogadro's Number is fixed by definition at 6.02214076 × 10^23, rather than being an experimentally measured approximation that could change with better instruments.
  • It applies to any particle type: The constant works identically whether you are counting atoms, molecules, ions, or even electrons, as long as you specify which particle you mean.
  • It scales calculations from micro to macro: Multiplying tiny mole quantities by this huge constant is what allows chemists to reason about lab-scale masses in terms of countable particles.
  • It underlies stoichiometric ratios: Balanced equations describe mole ratios between reactants and products, and Avogadro's Number is what converts those ratios into actual particle counts when needed.

Related Calculators

Avogadro's Number Calculator →Use Avogadro's number (6.022 × 10²³) to convert moles to particles or particles to moles. Works for atoms, molecules, ions, and formula units with step-by-step working.
Moles to Atoms Calculator →Convert moles to atoms or molecules using Avogadro's number (6.022 × 10²³). Enter the moles and get the exact number of atoms instantly with step-by-step working.

Related Terms

MoleMole (SI unit of amount of substance)

Frequently Asked Questions

Avogadro's Number is 6.02214076 x 10^23, the fixed number of elementary entities, such as atoms or molecules, contained in exactly one mole of any substance. It was named after the Italian scientist Amedeo Avogadro, though the precise value was measured by later scientists using techniques he did not have access to.
Avogadro's Number is enormous because atoms and molecules are extraordinarily tiny, so an everyday, weighable amount of a substance, like a gram, must contain a correspondingly huge number of them. For example, one gram of hydrogen atoms contains roughly 6 x 10^23 individual atoms.
It is used as a conversion factor to move between moles and the actual number of particles, by multiplying moles by Avogadro's Number to get particle count, or dividing particle count by Avogadro's Number to get moles. The Moles to Atoms Calculator applies this conversion directly.
Yes, Avogadro's Number is a universal constant and does not change based on which substance you are measuring. One mole of helium atoms, one mole of water molecules, and one mole of electrons all contain exactly the same number of particles, 6.02214076 x 10^23.
Scientists determined Avogadro's Number using multiple independent methods, including X-ray crystallography measurements of atomic spacing in silicon crystals and electrochemical experiments measuring charge per mole of electrons. Since 2019, the value has been fixed by international definition rather than measured, as part of the redefinition of the mole in the SI system.