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Shell - GCSE Chemistry Definition

Reviewed by: Richard Boole

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In GCSE Chemistry, a shell (or electron shell) is a layer around the nucleus of an atom where electrons are found. Electrons are negatively charged particles that move in energy levels or shells around the nucleus, which contains protons and neutrons.

Each shell can hold a fixed maximum number of electrons, and electrons fill the inner shells first.

The typical order is:

  • 1st shell: up to 2 electrons

  • 2nd shell: up to 8 electrons

  • 3rd shell: up to 8 electrons (in GCSE Chemistry)

The number of electrons in the outer shell (also called the valence shell) determines how reactive an element is and how it forms chemical bonds.

Understanding shells is essential for explaining atomic structure, chemical bonding, and the Periodic Table.

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Richard Boole

Reviewer: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

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