Atom – IGCSE Chemistry Definition
IGCSE Chemistry definition of atom: the smallest particle of an element that can take part in a chemical reaction. Covers structure, subatomic particles, and notation.
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 atom is the fundamental building block of all matter. Every element, compound, and mixture is made of atoms, and understanding atomic structure is essential for almost every topic on the 0620 syllabus — from bonding and the periodic table to stoichiometry and electrochemistry.
The 0620 definition
An atom is the smallest particle of an element that can take part in a chemical reaction.
Structure of the atom
An atom consists of:
- A central nucleus containing protons (positive charge) and neutrons (no charge)
- Electrons (negative charge) orbiting the nucleus in electron shells
| Subatomic particle | Relative charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | 1/1836 (negligible) | Electron shells |
In a neutral atom, the number of protons equals the number of electrons, so the overall charge is zero.
Key numbers
- Atomic number (Z): the number of protons in the nucleus. This defines the element — all atoms of the same element have the same atomic number.
- Mass number (A): the total number of protons + neutrons in the nucleus.
- Number of neutrons = mass number - atomic number
For example, an atom of sodium (Na) with atomic number 11 and mass number 23 has 11 protons, 11 electrons, and 23 - 11 = 12 neutrons.
Electron arrangement
Electrons fill shells starting from the innermost:
- 1st shell: holds up to 2 electrons
- 2nd shell: holds up to 8 electrons
- 3rd shell: holds up to 8 electrons (at IGCSE level)
The electron arrangement of sodium (11 electrons) is 2, 8, 1.
Worked exam question
An atom has 17 protons and a mass number of 35. (a) State the atomic number. (1) (b) Calculate the number of neutrons. (1) (c) State the number of electrons. (1) (d) Write the electron arrangement. (1)
Mark scheme
(a) 17 [1]
(b) 35 - 17 = 18 neutrons [1]
(c) 17 electrons (same as protons in a neutral atom) [1]
(d) 2, 8, 7 [1]
Common exam mistakes
- Confusing atomic number with mass number. Atomic number = protons only. Mass number = protons + neutrons.
- Forgetting that a neutral atom has equal numbers of protons and electrons. If the question says “atom” (not “ion”), the charges balance.
- Writing electron arrangements incorrectly — the first shell holds a maximum of 2, not 8. An arrangement starting “8, …” is always wrong.
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