Explain how electrons are arranged in an atom, and describe how this arrangement changes when an atom has a greater number of electrons.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Atoms contain sub-atomic particles arranged in a specific structure. The electrons in an atom occupy shells at different distances from the nucleus.
Model answer (5 marks)
Electrons are arranged in shells (energy levels) around the nucleus. The first shell, closest to the nucleus, can hold a maximum of 2 electrons. The second and third shells can each hold a maximum of 8 electrons. Electrons fill shells from the innermost outwards, occupying the lowest energy level first. When an atom has more electrons, the first shell becomes full, then the second shell starts to fill, and once the second shell is full a new outer shell begins to fill.
Examiner tips
- Use the exact terms ‘shell’ and ‘energy level’. Mention the 2‑electron rule for the first shell and the 8‑electron rule for the second and third. Explain the ‘fill from the innermost shell outwards’ rule. Show the change in arrangement as electrons increase – new outer shell starts to fill.
- common_mistakes
- :
- Saying electrons are in ‘orbitals’ instead of shells. Forgetting the 2‑electron limit for the first shell. Not stating that shells fill from the lowest energy level first.
Mark scheme (5 marks)
- Electrons occupy shells (energy levels) around the nucleus
- The first shell (closest to the nucleus) can hold a maximum of 2 electrons
- The second and third shells can each hold a maximum of 8 electrons
- Shells fill from the innermost shell outwards / electrons fill the lowest energy level first
- As the number of electrons increases, a new (outer) shell begins to fill once the previous shell is full
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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