Electron Configuration – IGCSE Chemistry Definition

Electron configuration describes how electrons are arranged in energy levels (shells) around the nucleus of an atom. At IGCSE level, the configuration is written as a sequence of numbers separated by commas, where each number gives the count of electrons in a shell, starting from the innermost. For example, aluminium (13 electrons) has the configuration 2, 8, 3.

Filling rules

The first shell holds a maximum of 2 electrons. The second shell holds a maximum of 8. The third shell also holds a maximum of 8 at IGCSE level (though it can hold more at higher levels of study). Electrons always fill the lowest available shell first before moving to the next one.

Examples: hydrogen 1, helium 2, lithium 2,1, carbon 2,4, neon 2,8, sodium 2,8,1, argon 2,8,8, calcium 2,8,8,2.

Link to the periodic table

The period number tells you how many shells are occupied. Sodium is in Period 3 and has three occupied shells (2, 8, 1). The group number tells you the number of outer-shell electrons. Sodium is in Group I and has 1 outer electron. This connection is a powerful exam shortcut: you can read off the electron configuration directly from the element’s position in the periodic table.

Elements in the same group have the same number of outer electrons, which is why they show similar chemical properties.

Exam context

Drawing or stating electron configurations is tested on both Paper 2 (multiple choice) and Paper 4 (structured). On Paper 4 you may also be asked to draw the electrons on a diagram of concentric circles (shells) around a nucleus. Mark schemes require the correct number of electrons in each shell. A common extension question asks you to predict the ion charge from the electron configuration.

Worked exam question

Phosphorus has atomic number 15. (a) State the electron configuration of a phosphorus atom. (1 mark) (b) Predict the charge of a phosphide ion. Explain your answer. (2 marks)

(a) 2, 8, 5 [1]

(b) 3- [1]. Phosphorus has 5 outer electrons and needs to gain 3 electrons to achieve a stable configuration of 2, 8, 8 (like argon) [1].

Common mistakes

Candidates sometimes put too many electrons in the first shell (e.g. writing 8,8,1 for sodium instead of 2,8,1). The first shell maximum is always 2. Another error is confusing electron configuration with the number of protons, neutrons, or the mass number. The configuration describes only the electrons.