The Mole Concept

This lesson introduces the mole — the chemist's unit for counting particles. The mole concept links the mass of a substance (which you can weigh) to the number of particles it contains. It is one of the most important ideas in GCSE Chemistry and is required by the Edexcel specification (1CH0).

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Why Do We Need the Mole?

Atoms and molecules are incredibly small. Even a tiny grain of salt contains billions upon billions of particles. It would be impractical to count individual atoms, so chemists use a special counting unit called the mole (symbol: mol).

The mole is to chemistry what the "dozen" is to eggs — a convenient way to count a fixed number of items.

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What Is a Mole?

One mole of any substance contains exactly 6.02 × 10²³ particles.

This number is called Avogadro's constant (sometimes written as Avogadro's number).

$N_A = 6.02 \times 10^{23} \text{ mol}^{-1}$NA​=6.02×1023 mol−1

So:

• 1 mol of carbon atoms contains 6.02 × 10²³ carbon atoms
• 1 mol of water molecules contains 6.02 × 10²³ water molecules
• 1 mol of sodium chloride formula units contains 6.02 × 10²³ NaCl units