Definition · Plain-language
Avogadro’s number
Avogadro’s number is the number of elementary particles in one mole of a substance — about 6.022 × 10²³ — the bridge between counting atoms and weighing grams.
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Counting the uncountable
Atoms and molecules are so small that any visible sample contains an almost unimaginable number of them. Avogadro’s number, about 6.022 × 10²³, is the agreed count that makes this workable: it is the number of particles in one mole of a substance. The figure is staggering — written out it is a 6 followed by twenty-three digits — but its role is practical. It provides a standard "package size" so that chemists can talk about a definite number of atoms or molecules by referring to moles, rather than juggling impossible quantities of individual particles.
The mole and the bridge to grams
Avogadro’s number defines the mole, the SI base unit for amount of substance. One mole is exactly 6.02214076 × 10²³ elementary entities. Its great usefulness is that it ties the atomic scale to the laboratory scale: the mass of one mole of a substance in grams equals its relative atomic or molecular mass. One mole of carbon-12 weighs 12 grams; one mole of water weighs about 18 grams. This is why the mole and Avogadro’s number are the workhorses of chemistry — they convert between counts of particles and weighable masses.
A fixed value since 2019
Until 2019 the mole was tied to a mass of carbon-12, and Avogadro’s number was a measured quantity carrying experimental uncertainty. The redefinition of the SI reversed the logic: the Avogadro constant was given an exact, fixed value of 6.02214076 × 10²³ per mole, and the mole is now defined as exactly that many entities. The number is therefore no longer measured but defined, by international agreement under the BIPM. This change made the mole independent of any physical artefact, in keeping with the broader 2019 reform of the base units.
Key facts
At a glance
- Definition: the number of particles in one mole of a substance
- Value: approximately 6.022 × 10²³ per mole
- Exact since 2019: 6.02214076 × 10²³ per mole, by definition
- Defines: the mole, the SI base unit for amount of substance
- Bridge: links counts of atoms to weighable masses in grams
- Authority: fixed by the BIPM in the 2019 SI redefinition
Common misconceptions
What people often get wrong
Often heard: Avogadro’s number is a measured value with experimental uncertainty.
Actually: Not since 2019. The Avogadro constant is now fixed by definition at exactly 6.02214076 × 10²³ per mole, so it carries no uncertainty.
Often heard: A mole is a unit of mass, like the gram.
Actually: A mole is a count of particles, the SI unit for amount of substance. It links to mass only because one mole of a substance weighs its relative atomic or molecular mass in grams.
Often heard: Avogadro himself discovered or measured the number.
Actually: The number is named in his honour but was determined long after his death. Avogadro’s contribution was the hypothesis that equal volumes of gases contain equal numbers of particles.
Going deeper
