2.2 The mole and the Avogadro constant

A mole is an amount of substance containing the Avogadro constant, N_A, of the specified entities. Use N_A = 6.02 × 10²³ mol⁻¹ in the supported convention; one mole can count atoms, molecules, ions, electrons or the appropriate formula units of an ionic substance.

Choose the entity before counting: 1 mol of sodium contains N_A sodium atoms, 1 mol of hydrogen molecules contains N_A H₂ molecules, and each formula subscript determines how many atoms are present per entity. Therefore the total number of a particular atom can be a multiple of N_A.

The mole is an amount, not a mass or a volume. Relative atomic, molecular or formula mass provides the numerical molar-mass bridge: one mole of a substance has a mass numerically corresponding to its relative mass in grams per mole, while the particle count remains N_A for one mole.

Do not say that a mole always means molecules, or that all one-mole samples have the same mass. State the particle type and formula, keep particle number separate from mass, and leave reacting-mass, concentration, gas-volume and limiting-reagent calculations to later topics.