What is What is the mass number??

The total number of protons + neutrons in an atom

Atomic Structure — GCSE Chemistry Revision

Free taster

5 of 6 sections open

The basics

The Incredible Shrinking Journey

📖 The Incredible Shrinking Journey

Imagine you could shrink yourself down — smaller than a grain of sand, smaller than a cell, smaller than a virus. Keep shrinking past molecules until you reach the atom. At this scale, something extraordinary happens: you discover that what seems like solid matter is actually mostly empty space! If an atom were the size of a football stadium, the nucleus would be a marble at the centre, and the electrons would be gnats buzzing around the stands. Yet this tiny marble contains almost ALL the atom's mass. Welcome to the quantum world, where the rules are beautifully strange.

What are nucleons?: Particles in the nucleus — protons and neutrons together

Hotspot diagram

Structure of the atom — click each part to zoom in and explore.

Bohr model of a sodium atom showing nucleus, three electron shells, and electrons as glowing blue dots. Click each part to explore.

1
Nucleus —
2
Shell 1 (2 electrons) —
3
Shell 2 (8 electrons) —
4
Outer electron (Shell 3) —

Figure 1: Structure of the atom — click each part to zoom in and explore.

Key terms

Chemistry glossary

What are nucleons?: Particles in the nucleus — protons and neutrons together
What is the mass number?: The total number of protons + neutrons in an atom

Spotlight

The History of Atomic Models

Think of an atom like a football stadium. The nucleus is a marble at the centre circle — tiny but incredibly dense, holding almost all the mass. The electrons are like flies buzzing around the stands, kilometres away from the centre. The rest? Empty space! This is why most alpha particles went straight through Rutherfo

Exam tip

Earn the mark scheme marks

🧠 Memory Aids

PEN = Protons, Electrons, Neutrons — the three subatomic particles.

For charges: "ProPlus" = Protons are Positive. Electrons are the opposite (negative). Neutrons are Neutral.

For isotopes: "Same Pro, Different Neu" — Same number of Protons, Different number of Neutrons.

Atomic model order: "Thomson Tasted Rutherford's Brilliant Cake" (Thomson → Thomson's plum pudding → Rutherford → Bohr → Chadwick).

Now try it yourself

Quiz · Question 1 of 28

What does the atomic number of an element tell you?

Tap an answer to check it

This topic in real past papers

Every real exam question we've found on atomic structure, with a full worked answer.

AQA Paper 1

Describe the plum pudding model, describe three differences between the plum pudding model and the nuclear model, write the subatomic particles in discovery order, or describe the change Bohr made to the nuclear model.

2 marks

AQA Paper 1

Calculate the relative atomic mass (Ar) of an element given the mass numbers and percentage abundances of its isotopes.

2 marks

Next topic →

Electronic Configuration