Atomic Structure
Principles of Chemistry · 1 question type
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 9% of your exam marks.
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 electronic configuration 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
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.