§ 5.1 The nuclear model of the atom
Key ideas
- An atom is a tiny, dense, positively charged nucleus of protons and neutrons, holding nearly all the mass, orbited by negative electrons; most of the atom is empty space.
- Charges: proton +1, electron −1, neutron 0. A neutral atom has equal numbers of protons and electrons.
- Extended: alpha-scattering evidence: most alpha particles passed straight through the gold foil (empty space), a few deflected (the nucleus is positive), and a very few bounced back (the nucleus is tiny, dense and massive).
- Nuclide notation: the proton number Z (bottom) counts protons and fixes the element; the nucleon number A (top) counts protons plus neutrons.
- Isotopes are atoms of the same element (same Z) with different numbers of neutrons (different A); they are chemically identical.
- Extended: fission splits a heavy nucleus into two lighter ones plus neutrons (nuclear reactors); fusion joins light nuclei into a heavier one (the Sun). Both release energy.
Equations
A = Z + Nnucleon number = protons + neutrons, so neutrons N = A − Zno unit
Fig. 1 · Lithium-7: Z = 3 protons (blue), N = 7 − 3 = 4 neutrons, and 3 orbiting electrons. The bottom number names the element; the top number counts every nucleon.
Fig. 2 · Alpha scattering (Extended): the three outcomes map directly onto the model: empty space, a positive nucleus, and a nucleus that is tiny, dense and massive.
Watch out: the top number A counts protons and neutrons. To find neutrons, always subtract: carbon-14 has 14 − 6 = 8 neutrons, not 14.
§ 5.2 Radioactivity
Key ideas
- Background radiation is always present: radon gas, rocks and soil, cosmic rays, food and drink, plus medical sources. Detect radiation with a Geiger-Müller tube, and subtract the background before analysing a source.
- Decay is random and spontaneous: no one can predict which nucleus will decay, or when.
- Alpha: a helium nucleus (2p + 2n), charge +2, the most ionising, stopped by paper or a few cm of air. Beta: a fast electron emitted when a neutron becomes a proton, stopped by a few mm of aluminium. Gamma: electromagnetic radiation, uncharged, the least ionising, only reduced by thick lead.
- Extended: in electric and magnetic fields, alpha and beta deflect opposite ways (opposite charges), beta far more (much lighter), and gamma not at all.
- Half-life: the time for half the radioactive nuclei in a sample to decay, or for the activity to fall by half.
- Match the use to the radiation: alpha in smoke alarms, beta for thickness control of foils and paper, gamma for sterilising and cancer treatment; medical tracers need short half-lives.
- Safety: ionising radiation damages or kills cells. Keep exposure time short, distance large (use tongs), and shield with lead; store sources in lead-lined boxes.
Equations
α decay: Z−2, A−4the nucleus loses two protons and two neutronsno unit
β decay: Z+1, A samea neutron becomes a proton plus the emitted electronno unit
N → N/2 → N/4 → N/8what remains after one, two, three half-livesno unit
Fig. 3 · Penetration increases as ionising power decreases: alpha is stopped by paper, beta by a few mm of aluminium, and gamma is only weakened by thick lead.
Fig. 4 · Deflection in an electric field (Extended): positive alpha drifts towards the negative plate, light negative beta swings sharply towards the positive plate, uncharged gamma is untouched.
Fig. 5 · The decay curve: every half-life the activity halves, so it falls to a half, then a quarter, then an eighth, but never quite reaches zero.
Watch out: after two half-lives a quarter of the sample remains, not none. Each half-life halves what is left, so the sample never simply "runs out" after two of them.