AQA GCSE Physics 4.4 Atomic structure: a complete overview of the atom, isotopes, radiation, half-life, fission and fusion

What topic 4.4 actually demands

Atomic structure runs from the inside of the atom to the energy of the stars. AQA tests two linked skills: precise recall of the model, the particles and the radiations, and confident reasoning with nuclear notation, decay equations and half-life.

This guide walks through all five dot points of the topic, then sets out the exam patterns AQA repeats. Each dot point has a matching page with practice questions; this overview ties them together.

The atom and how the model developed

An atom has a tiny nucleus of protons ($+1$) and neutrons ($0$) surrounded by electrons ($-1$) in energy levels. The atom is about $1 \times 10^{-10}\,m$ across, with a nucleus around $10000$ times smaller holding almost all the mass. The alpha scattering experiment, in which most alpha particles passed through gold foil but a few bounced back, replaced the plum pudding model with the nuclear model.

Isotopes and ions

The atomic number is the number of protons (it defines the element); the mass number is protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons (same atomic number, different mass numbers). An ion is a charged atom: losing electrons gives a positive ion, gaining them gives a negative ion.

Radiation and decay equations

Radioactive decay is random. Alpha is a helium nucleus (strongly ionising, least penetrating), beta is a fast electron (moderate), and gamma is an electromagnetic wave (weakly ionising, most penetrating); some nuclei emit a neutron. Emitting alpha lowers the mass number by $4$ and the atomic number by $2$; emitting beta raises the atomic number by $1$; gamma changes neither.

Half-life, contamination and irradiation

The half-life is the time for the activity (or number of undecayed nuclei) to halve; after $n$ half-lives, $\left(\frac{1}{2}\right)^n$ remains. Contamination is radioactive atoms on or in an object; irradiation is exposure to radiation from outside without becoming radioactive.

Fission and fusion

Fission splits a large unstable nucleus (such as uranium-235) into two smaller nuclei after absorbing a neutron, releasing energy and more neutrons that drive a chain reaction, controlled in a reactor. Fusion joins light nuclei to form a heavier one, releasing energy; it powers the stars but needs very high temperatures and pressures.

How topic 4.4 is examined

A typical AQA profile for Atomic structure:

• Short answer. Relative charges and sizes, defining isotopes and ions, and the properties of each radiation.
• Reasoning. Explaining the alpha scattering evidence and the difference between contamination and irradiation.
• Calculations. Counting protons, neutrons and electrons, balancing decay equations, and half-life problems.
• Extended answers. Describing fission, the chain reaction and the conditions needed for fusion.

Check your knowledge

A mix of recall and calculation questions covering topic 4.4. Attempt them under timed conditions, then check against the solutions.

1. State the relative charges of a proton, a neutron and an electron. (2 marks)
2. Explain what the alpha scattering experiment showed about the atom. (3 marks)
3. Define the term isotope. (2 marks)
4. An atom has atomic number $19$ and mass number $39$. State its numbers of protons, neutrons and electrons. (3 marks)
5. State the nature and penetrating power of beta radiation. (2 marks)
6. A nucleus emits an alpha particle. State the change in its mass and atomic numbers. (2 marks)
7. A source of activity $480\,Bq$ has a half-life of $3$ days. Find its activity after $9$ days. (3 marks)
8. Explain why nuclear fusion needs very high temperatures. (2 marks)