Atomic structure and radiation

Revision notes

Atoms contain a small positively charged nucleus of protons and neutrons surrounded by electrons. Isotopes are atoms of the same element with the same proton number but different numbers of neutrons. Evidence from alpha-particle scattering led to the nuclear model, which replaced the earlier plum-pudding model. Unstable nuclei decay randomly and may emit alpha, beta or gamma radiation. Nuclear equations conserve mass number and atomic number. Half-life is the time for the number of undecayed nuclei, activity or count rate to fall to half its value. Radioactive contamination is the unwanted presence of radioactive material, whereas irradiation is exposure to nuclear radiation; an irradiated object does not automatically become radioactive. The penetration and ionising properties of alpha, beta and gamma radiation determine their hazards and suitable applications.

Atomic and nuclear structure

Atoms contain a small nucleus of protons and neutrons with electrons outside it. Proton number identifies the element. Isotopes are atoms of the same element with the same proton number but different neutron numbers.

Unstable nuclei decay randomly and can emit alpha, beta or gamma radiation. Nuclear equations must conserve nucleon number and charge.

Ionising radiation

Alpha, beta and gamma differ in penetration and ionising ability. These properties determine suitable shielding and help explain why different radiations present different hazards.

Contamination means radioactive material is present where it is not wanted. Irradiation means exposure to radiation from a source. An irradiated object does not automatically become radioactive.

Activity and half-life

Activity is the rate of nuclear decay. Half-life is the time for the number of undecayed nuclei, activity or a proportional count rate to fall to half its value. Individual decay events are random, but large samples show predictable statistical behaviour.

Exam technique

For half-life questions, count the number of complete halvings. When comparing radiation types, link the physical property to the consequence rather than giving an isolated list of facts.

Radiation risk and interpretation

Risk depends on more than the name of the radiation. Consider how strongly it ionises, how far it can penetrate, whether the source is outside or inside the body, how long exposure lasts and how far the source is from tissue. A good explanation links these factors rather than stating that one radiation is always the 'most dangerous'.

When using decay data, look for multiplicative change rather than a constant subtraction. Radioactive decay is exponential, so equal time intervals correspond to equal fractions remaining, not equal amounts lost.