Neutron – IGCSE Chemistry Definition
IGCSE Chemistry definition of neutron: a neutral subatomic particle in the nucleus with relative mass 1. Covers role in isotopes and mass number calculations.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
The neutron is the uncharged subatomic particle in the nucleus. While it does not affect the chemical properties of an atom, it contributes to the atom’s mass and is the key to understanding isotopes. Neutron calculations are straightforward marks on every 0620 paper.
The 0620 definition
A neutron is a subatomic particle with no charge (relative charge = 0) and a relative mass of 1, found in the nucleus of an atom.
Key facts
| Property | Value |
|---|---|
| Relative charge | 0 (neutral) |
| Relative mass | 1 |
| Location | Nucleus |
| Role | Contributes to mass; different numbers of neutrons create isotopes |
Calculating the number of neutrons
Number of neutrons = mass number - atomic number
| Element | Mass number (A) | Atomic number (Z) | Neutrons (A - Z) |
|---|---|---|---|
| Carbon-12 | 12 | 6 | 6 |
| Carbon-14 | 14 | 6 | 8 |
| Sodium-23 | 23 | 11 | 12 |
| Iron-56 | 56 | 26 | 30 |
Neutrons and isotopes
Isotopes are atoms of the same element with different numbers of neutrons. Because neutrons have no charge, changing the number of neutrons does not affect:
- The number of electrons
- The electron arrangement
- The chemical properties
It does change the mass number and therefore affects physical properties such as density and rate of diffusion.
Example: hydrogen’s three isotopes
- Hydrogen (H-1): 0 neutrons
- Deuterium (H-2): 1 neutron
- Tritium (H-3): 2 neutrons
All three have 1 proton and 1 electron, so they react identically in chemical reactions.
Neutrons and nuclear stability
Neutrons help hold the nucleus together. Without neutrons, the positively charged protons would repel each other. In heavier elements, there are typically more neutrons than protons to maintain nuclear stability.
Worked exam question
Two atoms are represented as: Atom X has mass number 40 and atomic number 18. Atom Y has mass number 40 and atomic number 20. (a) Calculate the number of neutrons in each atom. (2) (b) Are these atoms isotopes of the same element? Explain. (2)
Mark scheme
(a) Atom X: 40 - 18 = 22 neutrons [1]; Atom Y: 40 - 20 = 20 neutrons [1]
(b) No, they are not isotopes [1]; they have different atomic numbers / different numbers of protons, so they are different elements (X is argon, Y is calcium) [1]
Common exam mistakes
- Confusing neutron number with mass number. The mass number includes both protons and neutrons. You must subtract to find the neutron number.
- Saying neutrons affect chemical properties. They do not — chemical properties depend on electrons, which are determined by the number of protons.
- Forgetting that hydrogen-1 has no neutrons. It is the only atom with zero neutrons — its nucleus is a single proton.
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