Atomic Structure — AQA Combined Science: Trilogy
This unit covers the structure of the atom, how the model developed, radioactivity, nuclear radiation and half-life.
The structure of the atom
Atoms have a radius of about 1 × 10⁻¹⁰ m. They consist of:
- a tiny central nucleus containing protons (positive) and neutrons (neutral), where almost all the mass is concentrated;
- electrons (negative) arranged in energy levels (shells) around the nucleus.
The nucleus is around 1/10 000 the size of the whole atom.
- Atomic number = number of protons (defines the element).
- Mass number = protons + neutrons.
- Isotopes have the same number of protons but different numbers of neutrons.
Electrons can move to a higher energy level if they absorb electromagnetic radiation, and fall back, emitting radiation. If an atom loses or gains electrons it becomes a charged ion.
Development of the atomic model
The model changed as new evidence appeared:
- Atoms were thought to be tiny indivisible spheres.
- The discovery of the electron led to the plum pudding model.
- The alpha particle scattering experiment (most alpha particles passed straight through, a few deflected strongly) showed the atom has a small, dense, positively charged nucleus surrounded by empty space — the nuclear model.
- Niels Bohr showed electrons orbit at specific distances (energy levels).
- Later work identified protons, and James Chadwick found evidence for the neutron.
Radioactive decay
Some atomic nuclei are unstable and decay randomly, giving out radiation. This is a random process — we cannot predict when a particular nucleus will decay.
Activity is the rate at which a source decays, measured in becquerels (Bq). The count-rate is the number of decays detected per second (e.g. by a Geiger–Müller tube).
Types of nuclear radiation
| Radiation | What it is | Penetration | Ionising power |
|---|---|---|---|
| Alpha (α) | helium nucleus (2 protons + 2 neutrons) | stopped by paper/skin | most ionising, short range |
| Beta (β) | a fast electron from the nucleus | stopped by a few mm of aluminium | moderate |
| Gamma (γ) | electromagnetic wave | stopped by thick lead/concrete | least ionising, very penetrating |
(Neutron emission can also occur.)
Nuclear equations
Decay changes the nucleus:
- Alpha decay — mass number decreases by 4, atomic number decreases by 2.
- Beta decay — a neutron turns into a proton and an electron; mass number stays the same, atomic number increases by 1.
- Gamma emission — energy is released; mass and atomic numbers are unchanged.
Nuclear equations must balance — the total mass numbers and the total atomic numbers must be equal on both sides.
Half-life
The half-life of a radioactive isotope is the time taken for:
- the number of unstable nuclei in a sample to halve, or equivalently
- the count-rate (activity) to fall to half its initial value.
After each half-life, the activity halves again (½, ¼, ⅛, …). You should be able to use a graph or work out how many half-lives have passed to find the remaining activity or fraction.
Worked example: if a source has a half-life of 2 hours and starts at 800 Bq, after 6 hours (3 half-lives) the activity is 800 → 400 → 200 → 100 Bq.
Contamination and irradiation
- Irradiation — exposing an object to radiation from a source. The object does not become radioactive.
- Contamination — getting radioactive atoms onto or into an object, which then continues to emit radiation.
Both can be hazardous. Precautions include shielding, keeping a distance, limiting exposure time, and wearing protective clothing. Scientific findings on radiation risk are published and peer reviewed so they can be checked.
Exam tips
- Learn the three radiations with their nature, penetration and ionising power.
- Be able to balance simple alpha and beta nuclear equations.
- Define half-life carefully (number of nuclei OR activity halving) and practise half-life calculations.
- Distinguish irradiation (exposure, not made radioactive) from contamination (radioactive atoms present).
- Remember radioactive decay is a random process.