IGCSE 0625 / Section 5.1.2 / Core

Same element, different weight.

Carbon is carbon whether its nucleus carries six neutrons or eight. Change the neutron count and you have a heavier version of the same element, chemically identical but a different isotope.

The key idea

Isotopes are atoms of the same element, so they have the same proton number, but they have different numbers of neutrons and so different nucleon numbers. Isotopes of an element are chemically identical.

Definition · learn the exact words

Isotopes are atoms of the same element with the same proton number but different numbers of neutrons, and so different nucleon numbers.

same Z, different number of neutrons

Isotopes of an element behave the same chemically, for example carbon-12 and carbon-14.

Section 01

Add a neutron.

Keep the protons fixed and change the neutrons to move between isotopes of the same element.

Stage 1 · Learn

Check isotopes

Four quick checks. Each correct answer earns XP and lights this skill on your star map.

Quick check+10 XP

Isotopes of an element have the same number of...

Quick check+10 XP

Isotopes of an element differ in their number of...

Quick check+10 XP

Isotopes of the same element have the same...

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Chemically, isotopes of an element are...

Section 02

What stays, what changes.

The proton number is fixed, so the element is the same; only the neutrons differ.

Examiner trap

Isotopes have the same number of protons, which is why they are the same element. It is the number of neutrons, not the protons, that differs between isotopes.

Stage 2 · Exam

Exam-style questions

Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 2.

Finish the four checks above to unlock the exam questions
Exam style+20 XP

Two atoms have the same proton number but different nucleon numbers. They are...

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Carbon-12 and carbon-14 differ in their number of...

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Which property is the same for all isotopes of an element?

Skill unlocked

Isotopes, mastered.

Isotopes are mapped. Keep the chain going.

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Go deeper · practice
Six original Cambridge-style questions
Isotopes as same-element atoms with equal proton numbers but different neutron numbers, and their identical chemistry. Attempt each, then reveal the worked solution.
Stage 3 · master the unit
Nuclear physics challenge
Mixed questions across the whole unit, each one worth XP. Start this only when you feel confident across every topic in the unit, not just isotopes.
Start the unit challenge →