THE REVISION 
GCSE Chemistry
GCSE Chemistry atomic structure, sorted for the exam
Last updated August 2026
Atomic structure is protons, neutrons and electrons, and how the numbers on the periodic table tell you how many of each an atom has. Get that straight and most of the exam questions on this topic answer themselves.
What's actually inside an atom
An atom has a tiny, dense nucleus at its centre, made of protons and neutrons, with electrons moving around it in shells. Protons carry a positive charge, electrons a negative one, and neutrons no charge at all — which is why a normal atom, with equal protons and electrons, has no overall charge.
- Atomic number — the number of protons. It's also the number of electrons in a neutral atom, and it's what makes an element that element.
- Mass number — protons plus neutrons added together. Subtract the atomic number from it and you've got the neutrons.
- Isotopes — atoms of the same element with the same number of protons but a different number of neutrons, so they have different mass numbers.
Electron shells and the periodic table
Electrons fill shells in a set order — 2, then 8, then 8 again — always filling the shell closest to the nucleus first. That pattern is the whole reason the periodic table is shaped the way it is: the group number tells you how many electrons are in the outer shell, and the period tells you how many shells are occupied. It's also worth knowing the model didn't arrive fully formed — from Dalton's solid spheres, through J.J. Thomson's "plum pudding" model, to Rutherford's gold foil experiment revealing a small charged nucleus, and finally Bohr's fixed electron shells. Exam papers like asking you to explain what each experiment showed.
Questions students actually ask
What's the difference between atomic number and mass number? Atomic number counts protons only. Mass number counts protons and neutrons together, so it's always the bigger of the two (unless there are no neutrons at all, as in ordinary hydrogen).
How do isotopes affect relative atomic mass? Most elements exist as a mix of isotopes in nature, so the relative atomic mass on the periodic table is a weighted average, not a whole number — that's why chlorine's is 35.5, not 35 or 37.
Why don't electrons just fall into the nucleus? They occupy fixed energy levels rather than orbiting freely, and moving between shells means gaining or losing a specific amount of energy — which is also what happens during ionisation.