The structure of the atom

An atom has a tiny, dense, positive nucleus of protons and neutrons, with electrons orbiting in shells. Almost all the mass is in the nucleus, which is far smaller than the whole atom.

Ions: a neutral atom has equal protons and electrons. Gain an electron → negative ion; lose one → positive ion.

Atomic number, mass number, isotopes

We label a nucleus with the symbol ^A_ZX:

Isotopes are atoms of the same element (same proton number Z) with a different number of neutrons — so a different mass number A. Carbon-12 and carbon-14 are isotopes of carbon.

Watch out: isotopes differ in neutrons, never in protons. Change the proton number and you have a different element.

Counting the nucleus

How we know the nucleus is there

Geiger and Marsden fired positive alpha particles at very thin gold foil. Most passed straight through, but a few were deflected and a tiny number bounced almost straight back.

Conclusion: the atom is mostly empty space; its positive charge and mass are concentrated in a tiny central nucleus.

Alpha, beta and gamma

Unstable nuclei emit three kinds of ionising radiation. They differ in what they are, how strongly they ionise, and how far they penetrate:

• Alpha (α) — a helium nucleus (2 protons + 2 neutrons), charge +2. Most ionising, least penetrating; range a few cm in air.
• Beta (β⁻) — a fast electron from the nucleus, charge −1. Moderately ionising and penetrating.
• Gamma (γ) — a high-energy electromagnetic wave, no charge, no mass. Least ionising, most penetrating; huge range in air.

Common mistake: the most ionising radiation (alpha) is the least penetrating — it dumps all its energy quickly, so it can't travel far. Don't assume "most powerful = goes furthest".

Match the radiation

Tap the radiation that fits each clue.

Deflection in a field

Because α is positive and β is negative, a magnetic or electric field bends them in opposite directions. Gamma has no charge, so it is not deflected at all.

Alpha & beta decay equations

In every nuclear equation the total mass number and the total atomic number are conserved (they balance on both sides).

Why Z rises in beta decay: the mass number stays the same because a neutron (mass 1) becomes a proton (mass 1), but the number of protons goes up by one, so Z increases by 1.

Your turn — alpha decay

Your turn — beta decay

Detecting it & background radiation

Ionising radiation is detected by a photographic film badge (darkens with exposure) or a Geiger–Müller (GM) tube (clicks / counts each time radiation enters it).

Everywhere on Earth there is low-level background radiation. Its sources are:

Exam tip: always subtract the background count rate before doing a half-life calculation.

Activity and half-life

The activity of a source is the number of decays per second, measured in becquerels (Bq). It decreases over time as nuclei decay. The half-life is the time taken for the activity (or number of undecayed nuclei) to halve.

Half-life is probabilistic. We can't say which nucleus will decay or when — decay is random. Half-life is only the average time for half a large sample to decay. It is constant and never depends on how many nuclei have already gone.

Your turn — find the half-life

Your turn — count rate later

Putting radiation to work

The type and half-life are chosen to suit the job:

• Smoke detectors — a long half-life alpha source; smoke absorbs the alpha and drops the current, triggering the alarm.
• Thickness control — a beta source through metal/paper sheet; if the detected count changes, the thickness has changed.
• Sterilising equipment & food — gamma kills bacteria, even through sealed packaging.
• Medical tracers — short half-life gamma emitters (e.g. technetium-99m) are tracked through the body.
• Treating tumours & radioactive dating (e.g. carbon-14 dating of once-living material).

Pattern to learn: need it to get out of the body or packaging → gamma. Need it stopped easily (smoke alarm) → alpha. Need a medical tracer → short half-life so it doesn't irradiate the patient for long.

Match use to radiation

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

Dangers and safe handling

Ionising radiation can damage cells and tissue and cause mutations in living organisms (which may lead to cancer). Two key terms:

• Irradiation — being exposed to a source from outside. The object does not become radioactive, and it stops the moment the source is removed.
• Contamination — radioactive material gets onto or inside an object. It stays radioactive while the source is there — usually more dangerous because exposure continues.

Reduce risk by less time near the source, more distance, and shielding (lead, dense materials). Radioactive waste from reactors is sealed and buried deep underground after cooling.

Don't confuse them: an irradiated object is safe to handle once away from the source; a contaminated object carries the source with it and keeps emitting.

Contamination or irradiation?

Splitting the atom — fission

In fission, a slow (thermal) neutron strikes a uranium-235 nucleus and splits it into two radioactive daughter nuclei, a few neutrons, and a lot of energy (as kinetic energy of the products). Those neutrons can split more U-235 — a chain reaction.

In a reactor: control rods (e.g. boron) absorb neutrons to keep the rate steady, and a moderator (e.g. water) slows the neutrons so they are absorbed by U-235. Thick shielding (concrete/steel) absorbs escaping radiation.

Inside the reactor

Joining nuclei — fusion

Fusion is the opposite of fission: two small nuclei join to make a larger one. A small amount of mass is lost and converted to a release of energy. Fusion is how the Sun and other stars release energy.

Why fusion is so hard: both nuclei are positive, so they repel (electrostatic repulsion). Only at the very high temperatures and pressures inside stars do they move fast enough to fuse.

Fission vs fusion

The essentials to know

You've covered all of Edexcel International GCSE Section 7 — Radioactivity and particles. Press Finish to see your score.