Avogadro Constant – IGCSE Chemistry Definition

The Avogadro constant connects the microscopic world of atoms and molecules to the macroscopic quantities you measure in the lab. On the 0620 syllabus (Supplement), you need to state its value and use it to convert between moles and numbers of particles. It is commonly tested alongside mole calculations on Paper 4.

The 0620 definition

The Avogadro constant is the number of particles in one mole of a substance. Its value is 6.02 x 10²³ per mole (mol⁻¹).

“Particles” can mean atoms, molecules, ions, or any specified entity. One mole of carbon atoms contains 6.02 x 10²³ atoms. One mole of water molecules contains 6.02 x 10²³ molecules.

Key relationships

Number of particles = number of moles x 6.02 x 10²³

Number of moles = number of particles / 6.02 x 10²³

Example calculations

How many molecules are in 2 moles of CO₂?

Number of molecules = 2 x 6.02 x 10²³ = 1.204 x 10²⁴

How many moles is 3.01 x 10²³ atoms of iron?

Moles = 3.01 x 10²³ / 6.02 x 10²³ = 0.5 mol

Be careful with atoms vs molecules

One mole of water (H₂O) contains:

• 6.02 x 10²³ molecules of H₂O
• 2 x 6.02 x 10²³ = 1.204 x 10²⁴ atoms of hydrogen (because each molecule has 2 H atoms)
• 6.02 x 10²³ atoms of oxygen

The question will specify whether it asks for atoms or molecules — read carefully.

Worked exam question

0.25 mol of magnesium contains how many atoms? (The Avogadro constant is 6.02 x 10²³ mol⁻¹) (2)

Mark scheme

Number of atoms = 0.25 x 6.02 x 10²³ [1]; = 1.505 x 10²³ atoms [1]

Common exam mistakes

• Confusing atoms with molecules. If asked “how many atoms in 1 mol of H₂?” the answer is 2 x 6.02 x 10²³ = 1.204 x 10²⁴ (two atoms per molecule), not 6.02 x 10²³.
• Forgetting the correct value. It is 6.02 x 10²³ — the exponent 23 is important. The value will normally be given on the exam paper, but know what it means.
• Multiplying instead of dividing (or vice versa). Moles to particles: multiply by 6.02 x 10²³. Particles to moles: divide by 6.02 x 10²³.