1 Why we need a counting unit

• Atoms, molecules and ions are so tiny that we can never count them one by one, and we cannot weigh a single one on a laboratory balance.
• Chemists therefore group them into a very large "package", exactly as a shop sells eggs by the dozen or paper by the ream.
• The package used in chemistry is the mole.

• The amount of a chemical substance is expressed in the International System of Units (SI) in the unit mole (symbol: mol).
• One mole of any substance contains the same fixed number of particles (molecules, atoms, ions, formula units or electrons).
• That fixed number is called Avogadro's number:

Avogadro's number = 6.02 × 10²³ particles per mole

The central relation is therefore:

Number of particles = number of moles × Avogadro's number

or, written with symbols, N = n × 6.02 × 10²³. Rearranged, n = N / (6.02 × 10²³).

Important notes:

• The word "particle" must always be specified. "1 mol of hydrogen" is ambiguous: it may mean 1 mol of H atoms or 1 mol of H₂ molecules. One mole of H₂ molecules contains 6.02 × 10²³ molecules but twice as many atoms, because each molecule has two atoms.
• The mole of a formula unit (an ionic compound such as NaCl) contains 6.02 × 10²³ formula units, and these release a known number of ions when dissolved or melted.