No atoms are created or destroyed during a chemical reaction. This allows calculations to be made on the quantities of substances that react and of the products made.

It is often useful in chemical calculations to know the number of atoms or molecules present. The mole, mol, is the unit for the amount of substance.

One mole contains the same number of particles as there are atoms in 12 g of carbon-12. As atoms and molecules are so small, the number of particles in a mole is very large. This number is:

- called the Avogadro constant
- equal to 60,000,000,000,000,000,000,000 (6.0 × 10
^{23}) atoms, molecules or formula units

The Avogadro constant = 6.0 × 10^{23}

The number of particles of a substance can be calculated using:

- the Avogadro constant, 6.0 × 10
^{23} - the number of moles

number of particles = 6.0 × 10^{23} x number of moles

Calculate the number of water molecules in 0.5 mol of water.

Number of water molecules = 6.0 × 10^{23} x 0.5

= 3.0 × 10^{23}

It is important to be clear about the particles involved. For example, 3.0 × 10^{23} water molecules contain 9.0 × 10^{23} atoms. This is because a water molecule, H_{2}O, contains three atoms.

- Question
Calculate the number of oxygen atoms in 0.5 moles of oxygen molecules, O

_{2}.Number of atoms = 2 × the number of molecules

Number of molecules = 0.5 × 6.0 × 10

^{23}So number of atoms = 2 × 0.5 × 6.0 × 10

^{23}= 6.0 x 10^{23}

- Question
Give the answer in standard form.

3 x 6.0 x 10

^{23}= 18 × 10^{23}Standard form has a single digit multiplied by the power of ten, so in standard form the number of molecules is 1.8 × 10

^{24}.