E.3.1—Isotopes
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- SL
Identify Isotopes
Define an isotope
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They share chemical identity but can have different physical properties.
Read the numbers
The proton number Z stays fixed within an element. Different isotopes have different nucleon numbers A, so their neutron numbers N=A−Z differ.
Common trap
Do not define isotopes only as atoms with different A and Z. The same proton number is essential; otherwise the atoms are different elements.
E.3.1 Exam Analysis
Assessment in practice
1 marksHow it is assessed
Questions define an isotope or use particle charge-to-mass comparisons in nuclear contexts.
Command terms
Outline / Identify
What earns marks
State same protons and different neutrons; do not replace the definition with only different mass numbers.
Watch for
Giving only “different mass numbers” without stating the same proton number.
Retrieve the SL Nuclear Model
Retrieve the nuclear structure
Isotopes differ in neutrons; mass defect becomes binding energy; the binding-energy curve explains why fusion and fission can release energy; and the strong force competes with electromagnetic repulsion.
Retrieve the decay model
Alpha, beta and gamma decays change A and Z differently. Radioactive decay is random but statistically predictable; use half-life, count-rate scaling and background correction carefully.