Electronic Configuration — Atomic Structure | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 9% of your exam marks.

stable

Medium

Stable9%

Subatomic particles and electronic configuration appear in every exam series.

Electrons occupy around the nucleus, filling from the innermost shell outwards
For the first 20 elements, the shell capacities are:
- 1st shell: up to 2 electrons
- 2nd shell: up to 8 electrons
- 3rd shell: up to 8 electrons
- 4th shell starts to fill once the 3rd has reached 8
An is written as the number of electrons in each shell, separated by commas, starting from the innermost shell:
- sodium (Z = 11): 2,8,1
- chlorine (Z = 17): 2,8,7
- calcium (Z = 20): 2,8,8,2

Reading an electronic configuration

The total of all the numbers equals the (= number of electrons)
The number of electrons in the outermost shell equals the element's group number for groups 1–7 (this is why same-group elements behave alike chemically)
The number of shells used equals the element's period number on the periodic table

Exam tip

Identifying the group of an element from its electronic configuration

What comes up: a 2-mark question gives an electronic configuration and asks which group of the periodic table the element belongs to, with an explanation.

Write (two marks): (1) state the group number; (2) because there are that many electrons in the outer shell — or state the full configuration. For example, an element with configuration 2,8,4 is in group 4 because it has 4 electrons in its outer shell (quoting the configuration itself is accepted as the explanation).

Watch out: you must give both the group number AND the reason to score 2 marks. Stating only the group number scores M1 but drops M2.