Atomic Structure
What's covered
Key facts
Background Radiation
Background radiation is the low-level ionising radiation that is always present around us, from natural and artificial sources.
Half-Life
Carbon-14 has a half-life of about 5,730 years; it is taken up by living organisms and decays predictably after death, allowing radiocarbon dating of once-living material.
Nuclear Fission and Fusion
A chain reaction in a fission reactor is when released neutrons trigger further fissions, sustaining the reaction.
Radioactive Decay
An alpha particle (helium nucleus) carries a charge of +2.
Uses and Dangers of Radiation
Alpha radiation is dangerous if inhaled or ingested because the alpha source sits inside the body where the strong ionisation directly damages cells.
Background Radiation
When measuring a radioactive source, the background count rate must be subtracted from the total measured count to find the source's true activity.
Half-Life
After n half-lives, the activity remaining is the original divided by 2ⁿ; after 3 half-lives it is 1/8 of the original.
Nuclear Fission and Fusion
Control rods absorb neutrons to control the rate of fission.
Radioactive Decay
Alpha decay reduces the mass number by 4 and the atomic number by 2.
Uses and Dangers of Radiation
Alpha particles are large (2 protons + 2 neutrons), carry a 2+ charge, and are strongly ionising — they lose energy quickly to surrounding atoms, so they only travel a few cm in air and are stopped by paper or skin.
Sample questions
A taste of the 54 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
List three sources of background radiation.
- •Burning fossil fuels (CO₂), car exhaust fumes (NOx), and mobile phone signals (microwaves)
- ✓Radon gas from rocks, cosmic rays from space, and medical X-ray imaging.
- •Sunlight (visible spectrum), wind energy (kinetic), and electrical appliances (low-voltage AC)
- •UV light (from the Sun), infrared heat (from radiators), and chemical reactions in the body (digestion)
A radioactive sample has an initial activity of 800 Bq and a half-life of 2 hours. What is the activity after 6 hours?
- ✓100 Bq
- •200 Bq
- •400 Bq
- •50 Bq
What is a chain reaction in a nuclear fission reactor?
- •Electrons released during fission flow through a circuit to generate power
- •Fusion of hydrogen nuclei triggered by the heat from fission
- •Neutrons released by fission decay into protons and electrons
- ✓Released neutrons trigger further fissions, sustaining the reaction
Which radiation is stopped by paper?
- ✓Alpha
- •Beta
- •Gamma
- •X-ray
Why is alpha radiation more ionising but less penetrating than gamma?
- •Alpha has a higher frequency than gamma but is absorbed by the atmosphere
- ✓Alpha is large, 2+ charged, and ionises rapidly, losing energy quickly
- •Alpha is more ionising because it travels faster than gamma rays do
- •Alpha is positively charged so it is attracted to negatively charged materials
What is the largest source of UK background radiation?
- •Cosmic rays from outer space that penetrate the atmosphere
- •Medical X-rays and radioactive tracers used in hospitals
- •Nuclear power stations releasing radiation during normal operation
- ✓Radon gas, which seeps from rocks and soil
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