Relative Atomic Mass and Isotopes — AQA GCSE Chemistry

Isotopes are atoms of the same element with different numbers of neutrons. Relative atomic mass takes their abundance into account.

Isotopes

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, and therefore different mass numbers.

Because they have the same number of electrons, isotopes have the same chemical properties, but slightly different physical properties (e.g. density).

Example: chlorine has two isotopes, chlorine-35 and chlorine-37.

Relative atomic mass (Ar)

The relative atomic mass (Ar) of an element is the average mass of its atoms, taking into account the abundance of each isotope.

$$A_r = \frac{\sum(\text{isotope mass} \times \text{abundance})}{100}$$

Worked example

Chlorine is 75% chlorine-35 and 25% chlorine-37. $$A_r = \frac{(35 \times 75) + (37 \times 25)}{100} = \frac{2625 + 925}{100} = 35.5$$

This is why chlorine's relative atomic mass is 35.5 — not a whole number.

Exam tips

• Define isotopes precisely: same protons, different neutrons.
• Isotopes have the same chemical properties (same electrons).
• Learn the relative atomic mass calculation and practise it.
• A non-whole-number Ar (like 35.5) reflects the mix of isotopes.