Proton – IGCSE Chemistry Definition
IGCSE Chemistry definition of proton: a positively charged subatomic particle in the nucleus. Covers relative charge, mass, and how proton number defines elements.
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 proton is the positively charged subatomic particle found in the nucleus of every atom. Its number defines the element: every atom of carbon has exactly 6 protons; every atom of oxygen has exactly 8. This concept is tested throughout the 0620 syllabus, from atomic structure to isotopes to ion formation.
The 0620 definition
A proton is a subatomic particle with a relative charge of +1 and a relative mass of 1, located in the nucleus of an atom.
Key facts
| Property | Value |
|---|---|
| Relative charge | +1 |
| Relative mass | 1 |
| Location | Nucleus |
| Role | Defines the element; determines the atomic number |
The proton number (atomic number)
The number of protons in the nucleus is called the atomic number (Z). It is the smaller number shown next to each element on the periodic table.
- The atomic number defines the element. All atoms with 6 protons are carbon. All atoms with 26 protons are iron.
- In a neutral atom, the number of protons equals the number of electrons.
- Changing the number of protons changes the element entirely — this only happens in nuclear reactions, not in chemical reactions.
Protons and ions
When an atom forms an ion, the number of protons does not change. Only electrons are gained or lost.
- Na atom: 11 protons, 11 electrons (neutral)
- Na+ ion: 11 protons, 10 electrons (positive because protons outnumber electrons)
This is why an ion has a charge: the imbalance between the fixed number of protons and the changed number of electrons.
Protons and isotopes
Isotopes of an element always have the same number of protons. What differs is the number of neutrons. For example, both Cl-35 and Cl-37 have 17 protons.
Worked exam question
An atom is represented as the notation showing mass number 27 and atomic number 13 for aluminium, Al. (a) State the number of protons. (1) (b) State the number of neutrons. (1) (c) The aluminium ion has a charge of 3+. Explain why. (2)
Mark scheme
(a) 13 [1]
(b) 27 - 13 = 14 [1]
(c) The aluminium atom has lost 3 electrons [1]; it now has 13 protons but only 10 electrons, so there is an overall positive charge of 3+ / more protons than electrons [1]
Common exam mistakes
- Saying an atom “gains protons” to become a positive ion. The proton number never changes in chemical reactions. Positive ions form by losing electrons.
- Confusing proton number with mass number. The proton number (atomic number) is the smaller number; the mass number (protons + neutrons) is the larger number.
- Writing that isotopes have different numbers of protons. Isotopes have the same number of protons — that is what makes them the same element. They differ in neutrons.
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