Inside the atom

Atoms are about 1 × 10⁻¹⁰ m across, but the nucleus at the centre is around 10 000 times smaller (~1 × 10⁻¹⁴ m). The nucleus holds the protons and neutrons; the electrons occupy energy levels around it.

Watch out: an atom is neutral because the number of protons equals the number of electrons. Lose or gain electrons and it becomes a charged ion.

Numbers, notation & isotopes

• Atomic number (Z) = number of protons (it defines the element).
• Mass number (A) = number of protons + neutrons.
• Number of neutrons = A − Z.
• Isotopes are atoms of the same element (same Z) with different numbers of neutrons (different A).

Common slip: isotopes do not differ in protons or electrons — only in neutrons. They behave the same chemically but can differ in nuclear stability.

Counting the nucleus

How the model developed

Our picture of the atom changed as new evidence arrived:

• Plum pudding model — a ball of positive charge with electrons dotted through it (no nucleus).
• Alpha-scattering experiment — alpha particles fired at thin gold foil. Most passed straight through, but a few were deflected and some bounced back.
• Nuclear model — this proved the atom is mostly empty space with a tiny, dense, positively charged nucleus.

Electron energy levels

Electrons occupy fixed energy levels around the nucleus. They can move between levels by gaining or losing energy as electromagnetic radiation:

• Absorption — an electron absorbs EM radiation and jumps up to a higher energy level.
• Emission — an electron emits EM radiation and drops down to a lower energy level.

HTThe refined Bohr model — electrons orbiting at set distances — is Higher-tier only.

Three types of nuclear radiation

An unstable nucleus becomes more stable by emitting ionising radiation. There are three kinds:

• Alpha (α) — a helium nucleus: 2 protons + 2 neutrons. Charge +2.
• Beta (β) — a fast electron emitted when a neutron turns into a proton. Charge −1.
• Gamma (γ) — high-energy electromagnetic radiation from the nucleus. No mass, no charge.

Key misconception: alpha is the most ionising but the least penetrating (it bumps into atoms easily). Gamma is the least ionising but the most penetrating. Ionising power and penetration run in opposite orders.

Which radiation?

Tap the type of radiation that matches each clue.

Nuclear decay equations

In any decay, the total mass number and the total charge (atomic number) are conserved — they must balance on both sides.

Remember: in beta decay the mass number stays the same (the electron has mass number 0), but the atomic number goes up by 1 because a neutron became a proton.

Your turn — alpha decay

Your turn — beta decay

Activity & half-life

Radioactive decay is a random process — you cannot say which nucleus decays next. But for a large sample, the rate is predictable. The activity (measured in becquerels, Bq) is the number of decays per second.

The half-life is the time taken for the number of unstable nuclei (or the count-rate) to halve.

HTMisconception: half-life is probabilistic, not a countdown for one atom. It does not mean the sample is gone after two half-lives — after 2 half-lives a quarter remains; after 3, an eighth.

Your turn — half-life

Half-life from a fraction

Background radiation

We are exposed to low-level background radiation all the time. It comes from natural and man-made sources:

When measuring a source's activity, you must subtract the background count first to get the true reading.

Hazards: irradiation vs contamination

Ionising radiation is dangerous because it can damage cells and DNA. There are two distinct hazards:

• Irradiation — being exposed to radiation from an outside source. The object/person does not become radioactive, and it stops when the source is removed.
• Contamination — radioactive atoms get onto or into an object. They keep decaying there, so the hazard continues.

Safety: keep your distance, limit time of exposure, and use shielding (lead, concrete) and protective clothing. Store sources in lead-lined boxes.

HTDon't mix them up: irradiation ≠ contamination. Inside the body, an alpha source is the most dangerous (highly ionising, all energy dumped locally). Outside the body, gamma/beta are more hazardous because alpha can't penetrate skin.

Uses of radioactivity

The properties of each radiation make it useful in different ways:

• Medical tracers — a gamma source with a short half-life is swallowed/injected and tracked outside the body (e.g. PET scans).
• Sterilisation — gamma kills microbes on medical equipment and food without heating them.
• Treating cancer — targeted gamma "radiotherapy" destroys tumour cells.
• Smoke detectors — an alpha source; smoke absorbs the alpha and trips the alarm.
• Radioactive dating — carbon-14 dating of once-living material; uranium dating of rocks.
• Thickness control — a beta source monitors the thickness of paper/foil in a factory.

Why those choices? The penetration of each type decides its job — gamma to pass out of the body, alpha (short range) for a smoke alarm, beta for thickness gauges (partly absorbed by the sheet).

Match the use to the radiation

Tap a use on the left, then its best-matched radiation on the right.

Nuclear fission

Nuclear fission is the splitting of a large, unstable nucleus (e.g. uranium-235) into two smaller nuclei, releasing 2 or 3 neutrons, gamma rays and a lot of energy.

HTIn a reactor a chain reaction must be controlled: control rods (e.g. boron) absorb spare neutrons to set the rate, and a moderator slows neutrons down so they are absorbed.

Nuclear fusion

Nuclear fusion is the joining of two small, light nuclei (e.g. hydrogen) to make one larger nucleus, releasing energy. This is the process that powers stars, including the Sun.

HTFusion needs extremely high temperature and pressure so nuclei move fast enough to overcome the electrostatic repulsion between their positive charges — which is why it is so hard to do on Earth.

Fission or fusion?

Irradiation or contamination?

The key facts to know

You've covered all four parts of Eduqas Topic 9. Press Finish to see your score.