GCSE Chemistry Electron Configuration Notes
Electrons fill shells from the innermost outwards using 2, 8, 8. The outer-shell electrons determine an element's group and its chemistry.
Key concepts
- Shell
- An energy level where electrons are found around the nucleus.
- Filling rule
- Fill the inner shells first: 2 in the first, then up to 8 in the second, then up to 8 in the third for the first 20 elements.
- Outer electrons
- The number of outer electrons equals the group number (1–7) and determines reactivity.
- Group 0 (noble gases) have full outer shells and are unreactive.
- Group 1 metals have 1 outer electron and get more reactive down the group.
- Group 7 non-metals have 7 outer electrons and get less reactive down the group.
Worked example
Write the electron configuration of potassium (atomic number 19) and predict its group.
- 1Fill shell 1: 2 electrons.
- 2Fill shell 2: 8 electrons (10 used).
- 3Fill shell 3: 8 electrons (18 used).
- 4Place the remaining 1 in shell 4.
Answer: 2,8,8,1 — Group 1 (alkali metal).
Common mistakes
✗ Overfilling shell 3 for the first 20 elements.
✓ Stop at 8 in shell 3 and move to shell 4 for K and Ca.
✗ Confusing period with group.
✓ Group = outer electrons; period = number of shells.
✗ Writing the electrons as one big number, e.g. 288.
✓ Separate shells with commas: 2,8,8.
Quick check
Try these, then reveal the answers.
Electron configuration of oxygen (8)?Reveal answer
2,6
Which group has 7 outer electrons?Reveal answer
Group 7 (halogens).
Why is neon unreactive?Reveal answer
It has a full outer shell.
Exam tips
- Draw shells as concentric circles with electrons in pairs where possible.
- State the group and period from the configuration.
- Link full outer shells to lack of reactivity.
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Frequently asked questions
How do I work out an element's group?+
Count the outer-shell electrons — this equals the group number for groups 1–7.
Why is the 2,8,8 rule used at GCSE?+
It is a simplified model that works for the first 20 elements and lets you predict reactivity and bonding.
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