The parts of an atom

An atom has a tiny central nucleus of protons and neutrons, surrounded by electrons in energy levels (shells):

Analogy — a marble in a stadium: if the whole atom were a football stadium, the nucleus would be a tiny marble at the centre and the electrons a faint blur out by the stands. The diagram above is squashed for clarity; in reality the atom is mostly empty space.

Misconception: the nucleus is incredibly tiny (~1/10000 of the atom) yet it holds almost all of the mass. Electrons take up nearly all the volume but contribute hardly any mass.

Which particle?

Counting the particles

Every nucleus is described by two numbers:

• Atomic (proton) number — the number of protons. It's what makes an element that element.
• Mass number — the total number of protons + neutrons.

Isotopes are atoms of the same element (same number of protons) with a different number of neutrons — so different mass numbers.

Ions form when an atom gains or loses electrons, giving it an overall negative or positive charge.

Electrons sit in energy levels. An electron can move to a higher level by absorbing EM radiation, and falls back by emitting EM radiation.

Your turn — count the neutrons

The atomic model over time

Our picture of the atom changed as new evidence appeared — a great example of how scientific models are revised:

• Dalton — atoms are tiny solid spheres that can't be divided.
• Thomson — discovered the electron; proposed the "plum pudding" model (a ball of positive charge with electrons dotted in it).
• Rutherford — the alpha-scattering experiment → the nuclear model (a tiny, positive, dense nucleus).
• Bohr — electrons orbit in fixed shells / energy levels at set distances.
• Chadwick — discovered the neutron.

Reading the evidence

Unstable nuclei decay

Some isotopes have unstable nuclei that give out radiation to become more stable. This radioactive decay is completely random — you can't predict which nucleus will decay or when.

• Activity — the rate at which a source decays, measured in becquerel (Bq). 1 Bq = 1 decay per second.
• Count-rate — the number of decays detected per second by a Geiger–Müller tube.

Because decay is random, we work with large numbers of nuclei and talk about probability and averages rather than individual nuclei.

Analogy — popcorn: radioactive decay is like popcorn kernels popping in a pan. You cannot predict which kernel pops next (or exactly when), but you can say roughly how many pop each second on average. Rolling a big bucket of dice and removing every six is the same idea: each die is unpredictable, but the rate is reliable.

Misconception: you cannot predict which individual nucleus will decay, or when a given nucleus will go. Half-life describes the average behaviour of huge numbers of nuclei, not a countdown timer on any single one.

Alpha, beta, gamma (and neutron)

An unstable nucleus can emit four kinds of radiation. They differ in what they are, how ionising they are, and how far they penetrate:

• Alpha (α) — a helium nucleus (2 protons + 2 neutrons). Very ionising, short range, stopped by paper / skin.
• Beta (β) — a high-speed electron from the nucleus. Moderately ionising, stopped by a few mm of aluminium.
• Gamma (γ) — a high-energy EM wave. Weakly ionising, long range, only reduced by thick lead or concrete.
• Neutron (n) — a neutron emitted from the nucleus.

Analogy — bowling ball vs dart: an alpha particle is like a heavy, slow bowling ball — it smashes into everything it meets, so it ionises a lot, but it loses its energy almost at once and is stopped very quickly. Gamma is like a tiny dart thrown through a crowd — it barely touches anything (weakly ionising) so it travels a long way and is hard to stop. Beta sits in between.

Misconception: alpha is the MOST ionising but the LEAST penetrating. "Most dangerous" depends on where the source is: outside the body, alpha is harmless (skin stops it); inside the body it is the most harmful because all that ionising happens in your cells.

Name the radiation

Tap the radiation (α, β or γ) that best matches each described property.

What decay does to the nucleus

When a nucleus decays, the mass and atomic numbers must balance on both sides:

• Alpha decay — loses an α particle (2 protons, 2 neutrons): mass number −4, atomic number −2.
• Beta decay — a neutron turns into a proton + the emitted electron: mass number unchanged, atomic number +1.
• Gamma — only energy leaves; mass and atomic numbers are unchanged.

Beta example: carbon-14 (6 protons) → nitrogen-14 (7 protons). The mass number stays 14; the proton number goes up by 1.

Balancing a decay

How fast a source decays

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

Your turn — activity after n half-lives

Your turn — fraction remaining

Your turn — finding the half-life

Two ways radiation reaches you

This distinction is a common exam trap:

• Irradiation — an object is exposed to radiation from an outside source. The object does NOT become radioactive itself, and irradiation stops the moment the source is removed.
• Contamination — radioactive atoms get onto or into an object. Those atoms keep decaying, so the object stays a radiation hazard until they are removed.

Wearing a lead apron protects against irradiation; washing your hands removes contamination. Contamination is especially dangerous if the source emits alpha — very ionising, and now inside the body.

Misconception: being irradiated does NOT make an object radioactive — it is just exposed to rays, like being lit by a torch; switch the source off and it is safe to handle. Contamination is different: the radioactive atoms themselves are now on or in the object, and they keep decaying until removed. (One handy check: contamination is the only one where you carry the source away with you.)

Irradiation or contamination?

Tap an example, then tap the box it belongs in.

Background radiation & uses

Background radiation is the low-level radiation around us all the time. Its main sources are radon gas, rocks & soil, cosmic rays, food and drink, and medical procedures.

Radioactivity is also genuinely useful:

• Medical tracers — a gamma source is swallowed/injected and tracked through the body.
• Sterilising equipment and food using gamma rays.
• Treating cancer with targeted radiation (radiotherapy).
• Smoke alarms use a small alpha source.

Peer review: when scientists publish findings about radiation (or anything else), other scientists check the work before it is accepted. This helps catch mistakes and keeps conclusions trustworthy. (Physics only.)

Choosing the radiation

Splitting a big nucleus

Nuclear fission is the splitting of a large, unstable nucleus (such as uranium-235). It usually happens when the nucleus absorbs a neutron:

In a reactor, control rods absorb spare neutrons to slow the reaction, and a moderator slows the neutrons so they cause more fissions. This keeps the chain reaction steady and safe.

Controlling a reactor

Joining light nuclei

Nuclear fusion is the opposite of fission: two light nuclei join to form a heavier nucleus, releasing energy. This is what powers stars like our Sun.

Fusion needs extremely high temperature and pressure to force the positively charged nuclei close enough to join — which is why it is so hard to use on Earth.

Fission or fusion?

The key facts to know

You've covered both parts of AQA 4.4 — atoms & isotopes, and atoms & nuclear radiation. Press Finish to see your score.