Curriculum fit: GCSE Chemistry and Combined Science (atomic structure and the periodic table). US: high school physical science (patterns in the periodic table).
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What Is It?
Electrons are arranged in shells around the nucleus. The first shell holds up to 2 electrons, and the second and third shells hold up to 8 each (for the first 20 elements). Electrons fill the innermost shells first.
The number of electrons in the outer shell decides how an element behaves chemically. Atoms gain, lose or share electrons to get a full outer shell, like the noble gases.
Key Formulas and Ideas
- Shell capacity (first 20 elements): 2, 8, 8, then 2 in the fourth shell for potassium and calcium.
- The group number equals the number of outer-shell electrons (for groups 1 to 7); noble gases have a full outer shell.
- Metals lose electrons to form positive ions. Non-metals gain electrons to form negative ions.
Worked Examples
Write a Configuration
Write the electron configuration of chlorine (atomic number 17).
- Chlorine has 17 electrons.
- First shell: 2. Second shell: 8. Remaining: 17 − 10 = 7 in the third.
✓ Answer 2, 8, 7
Metal Ion
Magnesium is 2, 8, 2. What ion does it form?
- It loses its 2 outer electrons to have a full shell.
- Losing two negative charges leaves a charge of 2+.
✓ Answer Mg²⁺ with configuration 2, 8
Non-Metal Ion
Oxygen is 2, 6. What ion does it form?
- It needs 2 more electrons for a full outer shell.
- Gaining two electrons gives a charge of 2−.
✓ Answer O²⁻ with configuration 2, 8
Common Mistakes to Avoid
- Putting more than 2 electrons in the first shell.
- Forgetting to use the atomic number as the total electron count.
- Writing the wrong charge: metals form positive ions and non-metals negative.
- Writing the ion charge with the number and sign the wrong way round, such as 2+ versus +2.
Practice Questions
Frequently Asked Questions
Why do noble gases not react?
They already have a full outer shell, so they have no tendency to gain, lose or share electrons.
How does electron arrangement relate to the periodic table?
The period number gives the number of shells and the group number gives the outer electrons.
Keep Going With These Guides
Atoms and the Periodic Table: Protons, Neutrons, Electrons
Learn atomic structure, atomic and mass number, isotopes and how the periodic table is organised, with worked examples.
KS3GCSEMiddle SchoolHigh SchoolIonic and Covalent Bonding: How Atoms Join, With Examples
Compare ionic and covalent bonding, write ionic formulae and explain properties such as melting point and electrical conductivity.
GCSEA-LevelHigh SchoolAPThe Reactivity Series: Displacement Reactions and Extraction
Order metals by reactivity, predict displacement reactions and link the reactivity series to how metals are extracted, with worked examples.
GCSEHigh SchoolSources and Review
Reviewed by StudyMorrow editorial review on 5 Nov 2026.