Atomic Structure, Radioactivity and Half-Life
The Model of the Atom
Our understanding of atomic structure has developed over time. The first model was the solid sphere model proposed by Dalton in the early 1800s. He imagined atoms as tiny, indivisible balls.
In 1897, J.J. Thomson discovered the electron and proposed the plum pudding model — a sphere of positive charge with negative electrons scattered throughout, like plums in a pudding.
In 1909, Rutherford, Geiger and Marsden fired alpha particles at a thin gold foil. Most passed straight through, but some were deflected and a few bounced back. This proved the atom has a tiny, dense, positive nucleus with electrons orbiting at a distance — the nuclear model.
Bohr refined the nuclear model by proposing that electrons orbit the nucleus at specific distances in fixed energy levels (shells). Niels Bohr’s model explained why atoms emit and absorb specific frequencies of electromagnetic radiation.
Later, James Chadwick discovered the neutron in 1932, completing our picture of the atom: a nucleus of protons and neutrons, surrounded by electrons in energy levels.
Radioactive Decay
Some atomic nuclei are radioactive — they are unstable and emit radiation to become more stable. This process is called radioactive decay and is random. You cannot predict when a particular nucleus will decay.
There are three types of nuclear radiation: alpha (\(\alpha\)) particles, beta (\(\beta\)) particles, and gamma (\(\gamma\)) rays. Each has different properties, penetrating power, and ionising ability.
Alpha (\(\alpha\)) — two protons and two neutrons (a helium nucleus). Strongly ionising, stopped by paper or a few centimetres of air. The atomic number decreases by 2 and the mass number decreases by 4.
Beta (\(\beta\)) — a high-speed electron emitted when a neutron turns into a proton. Moderately ionising, stopped by a few millimetres of aluminium. The atomic number increases by 1, mass number stays the same.
Gamma (\(\gamma\)) — an electromagnetic wave with no mass or charge. Weakly ionising, only reduced significantly by several centimetres of lead or thick concrete. The atomic number and mass number are unchanged.
Half-Life
The half-life of a radioactive isotope is the time it takes for half the unstable nuclei to decay, or for the count rate (activity) to fall to half its original value.
After one half-life, half the original nuclei remain. After two half-lives, a quarter remain. After three half-lives, one-eighth remain, and so on.
A sample has an activity of 800 Bq. After 30 minutes, the activity is 100 Bq. Calculate the half-life.
800 → 400 → 200 → 100 = three half-lives
Three half-lives = 30 minutes, so one half-life = \(\frac{30}{3} = 10\) minutes.
Uses and Hazards of Radiation
Nuclear radiation has useful applications: gamma rays are used in medical tracers and to sterilise equipment; beta radiation is used in thickness gauges; alpha sources are used in smoke detectors.
Contamination occurs when radioactive material gets onto or inside the body. This is more dangerous because the source stays close to cells. Irradiation is exposure to radiation from an outside source — once the source is removed, the exposure stops.
Precautions include: keeping radioactive sources in lead-lined containers, using tongs to handle sources, wearing protective clothing, and minimising exposure time.