Atomic Number – IGCSE Chemistry Definition
IGCSE Chemistry definition of atomic number: the number of protons in the nucleus. Covers how it defines elements, determines electron arrangement, and orders the periodic table.
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 atomic number is the single most important number for identifying an element. It tells you how many protons are in the nucleus, which in turn determines the number of electrons, the electron arrangement, and therefore all the chemical properties of the element. The periodic table is ordered by increasing atomic number.
The 0620 definition
The atomic number (Z), also called the proton number, is the number of protons in the nucleus of an atom.
Why the atomic number matters
| What it tells you | How |
|---|---|
| Which element the atom is | Each element has a unique atomic number |
| Number of electrons (neutral atom) | Electrons = protons in a neutral atom |
| Position on the periodic table | Elements are arranged by increasing atomic number |
| Electron arrangement | Determines how electrons fill the shells |
Atomic number on the periodic table
The atomic number is the smaller number displayed for each element. Elements are arranged in order of increasing atomic number, left to right and top to bottom.
Examples:
- Hydrogen: Z = 1
- Carbon: Z = 6
- Oxygen: Z = 8
- Sodium: Z = 11
- Chlorine: Z = 17
- Iron: Z = 26
Atomic number stays constant
The atomic number of an element never changes in a chemical reaction. This means:
- Isotopes of the same element always have the same atomic number (same number of protons, different number of neutrons)
- When ions form, the atomic number stays the same — only electrons are gained or lost
- The identity of the element is fixed by its proton count
Worked exam question
Element X has atomic number 20. (a) Identify element X. (1) (b) State the number of electrons in a neutral atom of X. (1) (c) Write the electron arrangement. (1) (d) In which group and period is element X? (2)
Mark scheme
(a) Calcium (Ca) [1]
(b) 20 [1]
(c) 2, 8, 8, 2 [1]
(d) Group II / Group 2 [1]; Period 4 [1]
Common exam mistakes
- Confusing atomic number with mass number. Atomic number = protons only. Mass number = protons + neutrons. The atomic number is always the smaller of the two.
- Thinking the atomic number changes when an ion forms. It does not. A Na+ ion still has atomic number 11 (11 protons) — it has just lost an electron.
- Writing that isotopes have different atomic numbers. They do not. Isotopes have the same atomic number but different mass numbers.
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