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ChemistryGCSEAQA & EdexcelYears 9–11

GCSE Chemistry Atomic Structure Notes

Everything is made of atoms. Knowing the particles inside an atom and how they are arranged underpins the whole of chemistry.

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Key concepts

Proton
Positive particle in the nucleus. Relative mass 1, charge +1. The number of protons (atomic number) defines the element.
Neutron
Neutral particle in the nucleus. Relative mass 1, charge 0. Changing neutron number gives isotopes.
Electron
Negative particle in shells around the nucleus. Relative mass ~1/2000, charge −1. Arranged in energy levels.
Isotopes
Atoms of the same element with the same protons but different numbers of neutrons, e.g. carbon-12 and carbon-14.
  • Atomic number = number of protons. Mass number = protons + neutrons.
  • Atoms are neutral because the number of protons equals the number of electrons.
  • Electron shells fill 2, 8, 8 for the first 20 elements.
  • The radius of an atom is about 1 × 10⁻¹⁰ m; the nucleus is around 1/10 000 of that.

Worked example

An atom has atomic number 11 and mass number 23. State its numbers of protons, neutrons and electrons.

  1. 1Protons = atomic number = 11.
  2. 2Electrons = protons (neutral atom) = 11.
  3. 3Neutrons = mass number − atomic number = 23 − 11 = 12.

Answer: 11 protons, 11 electrons, 12 neutrons

Common mistakes

✗ Confusing mass number with the number of neutrons.

✓ Neutrons = mass number − atomic number, not the mass number itself.

✗ Thinking isotopes have different numbers of protons.

✓ Isotopes have the same protons (same element) but different neutrons.

✗ Writing electron configuration as 2,8,8,2 incorrectly for calcium.

✓ Fill 2, then 8, then 8, then the rest: calcium (20) is 2,8,8,2.

Quick check

Try these, then reveal the answers.

What does the atomic number tell you?Reveal answer

The number of protons in the atom.

How many neutrons are in an atom with mass number 19 and atomic number 9?Reveal answer

10 neutrons

What is the electron configuration of sodium (atomic number 11)?Reveal answer

2,8,1

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Exam tips

  • Command words: 'define' needs a precise definition, 'describe' needs what happens, 'explain' needs why.
  • Use the periodic table provided — the big number is the mass number, the small number is the atomic number.
  • When asked about isotopes, always mention 'same protons, different neutrons'.

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Key terms explained

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Frequently asked questions

What is an isotope?+

An atom of the same element with the same number of protons but a different number of neutrons, giving it a different mass number.

Why is an atom electrically neutral?+

Because it has equal numbers of positively charged protons and negatively charged electrons, so the charges cancel out.

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