Electron – IGCSE Chemistry Definition
IGCSE Chemistry definition of electron: a negatively charged subatomic particle found in shells around the nucleus. Covers charge, mass, and electron arrangement.
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 electron is the subatomic particle responsible for all chemical bonding and reactivity. While protons and neutrons define what element an atom is, it is the electrons — specifically the outer shell electrons — that determine how it reacts. Understanding electron arrangement is critical for bonding, the periodic table, and ion formation.
The 0620 definition
An electron is a subatomic particle with a relative charge of -1 and a negligible relative mass (approximately 1/1836). Electrons are found in shells (energy levels) surrounding the nucleus.
Key facts
| Property | Value |
|---|---|
| Relative charge | -1 |
| Relative mass | 1/1836 (negligible — treated as 0 for mass calculations) |
| Location | Electron shells around the nucleus |
| Number in a neutral atom | Equal to the number of protons (= atomic number) |
Electron shells
Electrons occupy shells at increasing distances from the nucleus:
- 1st shell (closest to nucleus): maximum 2 electrons
- 2nd shell: maximum 8 electrons
- 3rd shell: maximum 8 electrons (at IGCSE level)
Shells fill from the innermost outward. The electron arrangement of an atom is written as the number of electrons in each shell, separated by commas.
| Element | Atomic number | Electron arrangement |
|---|---|---|
| Hydrogen | 1 | 1 |
| Carbon | 6 | 2, 4 |
| Oxygen | 8 | 2, 6 |
| Sodium | 11 | 2, 8, 1 |
| Chlorine | 17 | 2, 8, 7 |
| Calcium | 20 | 2, 8, 8, 2 |
Electrons and the periodic table
- The number of outer shell electrons = the group number (for main groups)
- The number of occupied shells = the period number
- Elements in the same group have the same number of outer electrons, so they have similar chemical properties
Worked exam question
An atom of phosphorus has 15 electrons. (a) Write its electron arrangement. (1) (b) In which group and period of the periodic table is phosphorus? (2) (c) How many electrons would a phosphorus atom need to gain to achieve a full outer shell? (1)
Mark scheme
(a) 2, 8, 5 [1]
(b) Group V / Group 15 [1]; Period 3 [1]
(c) 3 electrons [1]
Common exam mistakes
- Putting more than 2 electrons in the first shell. The first shell always holds a maximum of 2.
- Confusing the number of electron shells with the group number. The number of shells gives the period; the number of outer electrons gives the group.
- Treating electron mass as significant in calculations. Electron mass is negligible — the mass of an atom is essentially the mass of its protons and neutrons.
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