What an atom is made of

Every atom has a tiny, dense, positively charged nucleus at its centre, with negatively charged electrons orbiting around it. Almost all the mass sits in the nucleus, yet the nucleus is over 10 000 times smaller than the whole atom — so the atom is mostly empty space.

Relative properties: proton — mass 1, charge +1; neutron — mass 1, charge 0; electron — mass ≈ 1/2000 (0.0005), charge −1. A neutral atom has equal numbers of protons and electrons.

Inside the atom

Counting protons and nucleons

Two numbers describe any nucleus:

• Proton number Z (atomic number) — the number of protons. It fixes which element it is.
• Nucleon number A (mass number) — the total number of protons + neutrons.

So number of neutrons = A − Z. A nucleus written with these numbers is a nuclide, shown as:

Isotopes are atoms of the same element (same Z, same number of protons) but with a different number of neutrons, so a different nucleon number A. Carbon-12 and carbon-14 are isotopes of carbon.

Watch out: isotopes differ only in neutron number — never in proton number. Change Z and you change the element entirely.

Your turn — count the neutrons

The alpha-scattering experiment

How do we know the atom has a tiny central nucleus? Geiger and Marsden (directed by Rutherford) fired a beam of alpha particles at very thin gold foil and watched where they went.

What it tells us:

• Most pass straight through → the atom is mostly empty space; the nucleus is very small.
• A few are deflected back → the nucleus is very dense and holds nearly all the mass.
• The repulsion that turns them back → the nucleus is positively charged (it repels the positive α-particles).

Reading the scattering

Fission and fusion

Cambridge 0625 also asks you to state what is meant by these two nuclear processes:

• Nuclear fission — a large, unstable nucleus (e.g. uranium-235) is split into two smaller nuclei, releasing energy and 2 or 3 neutrons. This is the energy source in nuclear reactors.
• Nuclear fusion — two light nuclei join to form a larger nucleus, releasing energy. This is how the Sun and stars release energy.

Don't mix them up: fission = split a big nucleus; fusion = fuse small nuclei together. Both release energy.

Background radiation

Some ionising radiation is around us all the time — this is background radiation. It comes from both natural and man-made sources:

• Radon gas in the air (from rocks)
• Rocks and buildings (uranium and thorium in the ground)
• Cosmic rays from space
• Food and drink (e.g. potassium-40)
• Small man-made amounts from medical X-rays and nuclear industry

Radiation is detected with a Geiger–Müller (GM) tube connected to a counter. The count rate is the number of counts per second (or per minute). Because the count spreads out with distance, it falls as you move the source away.

Always subtract the background: the corrected count rate = measured count rate − background count rate. Forgetting this is a classic mistake in half-life questions.

Decay is random and spontaneous

An unstable nucleus throws out radiation to become more stable. This is radioactive decay, and it is:

• Spontaneous — it isn't triggered by anything outside; temperature, pressure and chemical state make no difference.
• Random — you cannot predict which nucleus will decay next, or when a given nucleus will decay. You can only give a probability.

Half-life isn't a countdown: a nucleus doesn't "know" how old it is. Decay is purely probabilistic — half-life is a statistical average over very many nuclei.

Alpha, beta and gamma

Unstable nuclei emit three kinds of ionising radiation. You must know their nature, how strongly they ionise, and how far they penetrate:

• Alpha (α) — a helium nucleus (2 protons + 2 neutrons), charge +2. Most ionising, but least penetrating — stopped by a sheet of paper or a few cm of air.
• Beta (β) — a fast-moving electron, charge −1. Moderately ionising and moderately penetrating — stopped by a few mm of aluminium.
• Gamma (γ) — a high-energy electromagnetic wave, no charge, no mass. Least ionising, but most penetrating — needs thick lead (or several cm) to reduce it.

Key misconception: alpha is the most ionising but the least penetrating. Being highly ionising means it gives up its energy quickly, so it can't travel far.

Deflection in a field

Because α and β are charged, they are deflected by electric and magnetic fields; γ has no charge, so it goes straight on. Their charges are opposite, so they bend in opposite directions. Alpha is heavy (+2) so it deflects only a little; beta is light (−1) so it deflects a lot.

Match each property to the right emission

Tap a property on the left, then tap the radiation it describes.

Which radiation gets stopped?

Tap a barrier statement, then tap the radiation it stops.

Nuclear decay equations

When a nucleus decays it changes into a different element. In every equation, the totals of A and of Z on each side must be equal (A and Z are conserved).

Alpha decay — the nucleus loses an alpha particle (a helium nucleus ⁴₂He), so A drops by 4 and Z drops by 2:

Beta decay — a neutron turns into a proton and emits an electron (⁰₋₁e). A is unchanged; Z goes up by 1:

Watch out: the β-particle counts as Z = −1 in the equation — that's how the proton number stays balanced when Z rises by 1. Gamma emission alone changes neither A nor Z.

Your turn — balance an alpha decay

Your turn — balance a beta decay

Half-life

The half-life of an isotope is the time taken for half the nuclei in a sample to decay — equivalently, the time for the count rate to fall to half its value.

Remember: subtract the background count rate first, then look for the time for the corrected count to halve. The shape is always the same regardless of how much you start with.

Your turn — find the half-life

Your turn — count after several half-lives

Your turn — subtract the background

Using radioactivity safely

The type of emission and the half-life decide what an isotope is good for:

• Smoke alarms — a long half-life alpha emitter (α is absorbed by smoke).
• Thickness control of sheets — a beta emitter (changes in thickness change how much β gets through).
• Sterilising equipment and irradiating food, and medical tracers / treating tumours — gamma emitters (γ is penetrating).

Radiation damages living cells (it can kill them or cause mutations leading to cancer), so sources must be handled with care:

• Handle with tongs, never bare hands — keep your distance.
• Keep exposure time short.
• Store sources in a lead-lined box; shield with lead or thick concrete.

Why distance and time? Dose falls as you move away and as exposure time drops — both keep the absorbed dose to a minimum.

Choosing the right source

What half-life really means

The key facts to know

You've covered all of Cambridge IGCSE Physics (0625) Topic 5 — the nuclear model and radioactivity. Press Finish to see your score.