Avogadro's number: definition in chemistry
Avogadro's number: The number of particles in one mole of a substance: exactly 6.02214076 × 10²³, usually rounded to 6.022 × 10²³.
Avogadro's number connects the atomic scale to amounts you can weigh: one mole of carbon-12 atoms has a mass of 12 grams. Since 2019 the value has been fixed exactly, and the mole is defined from it. It is named after the Italian scientist Amedeo Avogadro, although he never measured it; reliable measurements came only in the early 1900s.
Examples
- 18 g of water contains about 6.022 × 10²³ molecules
- A teaspoon of water (about 5 g) holds about 1.7 × 10²³ molecules
- 56 g of iron contains about 6.0 × 10²³ atoms
- A stack of 6.022 × 10²³ pennies would stretch roughly across the Milky Way
On the periodic table
An element's atomic mass in grams always contains Avogadro's number of atoms, so 4.00 g of helium and 207.2 g of lead hold the same number.