Isotopes
Atoms of the same element with different numbers of neutrons, and relative atomic mass.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 2.3.1Define isotopes and explain why they have identical chemical properties.
- 2.3.2Calculate relative atomic mass from isotopic abundances.Supplement
5 minute read
Isotopes
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Because they have the same proton number, they are the same element and sit in the same place on the Periodic Table — but because their neutron numbers differ, they have different nucleon numbers and different masses.
Why isotopes react identically
Chemical reactions are governed by an atom's electrons, particularly the outer-shell electrons. Since isotopes of the same element have identical numbers of protons and therefore identical numbers of electrons, they have exactly the same electronic configuration — so they behave identically in chemical reactions, even though their masses differ.
Relative atomic mass
Because natural samples of an element are a mixture of its isotopes in fixed proportions, the relative atomic mass you look up on the Periodic Table is a weighted average of the isotope masses, weighted by how abundant each isotope is. This is why relative atomic masses are rarely whole numbers — chlorine's is 35.5, reflecting a natural mixture of about 75% ³⁵Cl and 25% ³⁷Cl.
Think of it like this
Isotopes are like identical twins who behave exactly alike in every social situation (chemistry, governed by electrons) but happen to have slightly different body weights (mass, governed by neutrons) — the difference in weight has no effect on how they interact with everyone else.
Worked examples
Method, step by step
Chlorine exists as 75% ³⁵Cl and 25% ³⁷Cl. Calculate its relative atomic mass.
- 1Ar = (75 × 35 + 25 × 37) / 100
- 2Ar = (2625 + 925) / 100
Ar = 35.5
Common misconceptions
- Thinking isotopes are different elements. They are the same element, since element identity is fixed entirely by proton number.
- Assuming a simple average of isotope masses gives the relative atomic mass. It must be weighted by the percentage abundance of each isotope, not just averaged equally.
- Believing isotopes have different chemical reactivity because they have different masses. Reactivity depends on electron arrangement, which is identical between isotopes.
In the exam
- When defining isotopes, both parts of the definition are needed for full marks: same proton number AND different neutron number.
- For weighted-average calculations, write out the full sum — (% × mass) + (% × mass), all divided by 100 — rather than trying to do it by inspection, since the method is worth marks independently of the final answer.