MYP Chemistry: Isotopes (Intro) — FAQs

Isotopes trip students up because the concept fuses two ideas at once: atoms of the same element can have different masses yet behave identically in reactions. In years of marking this topic, the confusion is almost always about muddling mass number with atomic number — not the idea of isotopes itself.

An isotope is an atom of the same element with the same number of protons but a different number of neutrons, which changes its mass number. Carbon-12 and carbon-14 are both carbon — six protons each — but carbon-14 carries two extra neutrons in its nucleus.

Isotopes differ in neutron number and therefore mass; ions differ in electron number and therefore charge. An atom becomes an isotope by gaining or losing neutrons in the nucleus, while it becomes an ion by gaining or losing electrons in the outer shells — two completely separate changes.

Atomic number is fixed because it counts protons, and protons define which element you're looking at — change the proton count and you've changed the element completely. Mass number counts protons plus neutrons, and neutrons can vary without touching the element's identity, which is exactly what allows isotopes to exist.

Subtract the atomic number (proton number) from the mass number — that's the entire calculation. For chlorine-37, the atomic number is 17, so neutrons = 37 − 17 = 20. Get that order right and every neutron-counting question becomes routine arithmetic rather than a mystery.

Multiply each isotope's mass number by its percentage abundance, add the results, then divide by 100. For chlorine — 75% chlorine-35 and 25% chlorine-37 — the sum is (35 × 75 + 37 × 25) ÷ 100 = 35.5, which matches the relative atomic mass shown on the periodic table.

Show your working every single time — Criterion A (Knowing and Understanding) rewards clear method as much as the final number. State the formula, substitute the values, then give a labelled answer. Examiners can't award full credit for a correct number that appears from nowhere.

The error I mark most often is swapping mass number and atomic number in the standard notation, or forgetting the smaller number always sits at bottom left. Students also confuse "isotope of element X" with "compound containing element X" — isotopes are still the pure element, just with a different neutron count.

Yes — isotopes sit within the "Knowing and Understanding" strand described in the MYP Sciences guide and appear regularly in the on-screen MYP eAssessment taken by students completing the MYP certificate. Expect short calculation items — neutron number, relative atomic mass — rather than long extended-response questions on this specific topic.

Isotope questions mostly sit under Criterion A: Knowing and Understanding, where you state, calculate or explain using correct scientific vocabulary. They can also feed into Criterion C: Processing and Evaluating if a task asks you to interpret isotope abundance data presented in a table or graph.

Yes, and getting the basics solid now genuinely saves time later. According to the IB, the current DP Chemistry guide (first exams 2025) reintroduces isotopes for mass spectrometry calculations at both SL and HL, where you interpret spectra to find relative atomic mass rather than just apply a simple formula.

Not really — the isotope content is nearly identical between the two. What differs is assessment style: IGCSE tends to test isotopes with recall-based short questions, while MYP Chemistry's criteria (A-D) expect your child to also explain reasoning and evaluate data, which can feel harder even when the underlying facts are the same.

Ask your child to explain, out loud and in their own words, the difference between mass number and atomic number. If they can't, that's the actual gap — not the maths. Isotope calculations are genuinely simple once the vocabulary is secure, so vocabulary is where home revision time pays off most.

Short, topic-specific practice beats generic revision for a narrow topic like this one. On RevisionPrep, the MYP Chemistry Topical Worksheets isolate isotope calculations with full mark schemes, and the Revision Notes give the exact vocabulary — mass number, atomic number, relative atomic mass — examiners expect in Criterion A responses.