GCSE Combined Science Atomic structure and radioactivity revision

Every atom is a nucleus of protons (charge +1) and neutrons (charge 0), with electrons (charge −1) arranged at set distances called energy levels. The numbers are tiny: a typical atom has a radius of about 1 × 10⁻¹⁰ m, and the nucleus is about 10 000 times smaller — yet the nucleus carries almost the whole mass. Atomic number Z counts protons and fixes which element an atom is; mass number A counts protons plus neutrons, so the neutron count is always A − Z: for ²³⁵₉₂U, 235 − 92 = 143 neutrons.

Isotopes are atoms of the same element with different numbers of neutrons — ³⁵₁₇Cl and ³⁷₁₇Cl both behave as chlorine because chemical behaviour depends on the 17 electrons, not the neutron count. Ions are the charged leftovers of electron transfer: losing an electron gives a positive ion; gaining one gives a negative ion. The nucleus never changes in ordinary chemistry.

The model has a history. Thomson's plum pudding pictured electrons dotted through a positive ball — until Geiger and Marsden fired alpha particles at gold foil. Most sailed straight through: the atom is mostly empty space. A few bounced straight back: they had met something tiny, dense and positively charged — the nucleus. Bohr then fixed electrons into shells, and Chadwick (1932) completed the picture by discovering the neutron.

Isotopes whose nuclei are unstable shed the excess energy by radioactive decay, emitting radiation until they reach a stable arrangement. Decay is random — no measurement can say which nucleus will decay next — and spontaneous: nothing you do (heating, squeezing, dissolving) changes it. The activity of a source is the number of decays per second, measured in becquerels (Bq): a 640 Bq source decays 640 times each second.

The pattern to remember: the more strongly a radiation ionises, the more quickly it is stopped — an alpha particle tears electrons off atoms so enthusiastically that the first sheet of paper finishes it. A source pushed through paper, then aluminium, then lead tells you its identity: only lead reducing the count means gamma; paper stopping it means alpha. These comparisons power half the radiation questions you will meet.

Nuclear equations bookkeep the decay: the totals of A and of Z must match on both sides. In alpha emission the nucleus loses a ⁴₂He particle, so the daughter has A − 4 and Z − 2: ²²²₈₆Rn → ²¹⁸₈₄Po + ⁴₂He. In beta-minus emission a neutron turns into a proton plus an electron — the electron bolts out as the beta particle — so A stays put and Z rises by 1: ⁹⁰₃₈Sr → ⁹⁰₃₉Y + ⁰₋₁e. The emitted electron never orbited the atom; it is created in the decay itself.

The half-life is the time for half the unstable nuclei in a sample to decay — equivalently, for the activity (or corrected count rate) to halve. Halving compounds: a source at 960 Bq with a 15-hour half-life falls to 480 Bq after 15 h, 240 Bq after 30 h and 120 Bq after 45 h (three half-lives). On a decay curve the half-life is the constant horizontal spacing between halvings; the curve falls towards zero but — because decay is random — never quite arrives.

Every measurement carries a background count from radiation that is always with us: radon gas seeping from rocks and soil (the largest natural contributor in many UK homes), cosmic rays, radioactive isotopes in food and building materials, plus smaller man-made contributions from medical procedures and the nuclear industry. Because the detector counts background even with no source present, measured readings must be corrected by subtraction: a detector reading 68 s⁻¹ with a source and 12 s⁻¹ without has a corrected rate of 68 − 12 = 56 s⁻¹.

Contamination means radioactive material itself gets where it should not be — dust on a bench, a tracer in the blood — and it keeps irradiating from inside until removed or decayed. Irradiation means being exposed to radiation from a source elsewhere; step away or shield it and the exposure stops instantly. Hence the hazard twist: alpha is the most dangerous radiation inside the body (strongly ionising, nowhere to escape) while gamma is the most dangerous outside (it reaches you through air and clothing). Safe handling means shielding, distance and short exposure times.

The uses exploit the same properties. Medical tracers are gamma-emitting isotopes with short half-lives: gamma escapes the body for the detector, and a short half-life clears the dose quickly. Radiotherapy focuses radiation to kill cancer cells; gamma sterilises sealed single-use instruments because it penetrates the packaging and kills bacteria without heating. Radiocarbon dating works because living things absorb carbon-14 while alive; after death the fraction remaining (halving every ~5 700 years) dates the remains.