Radioactivity

• A radioactive substance contains unstable nuclei that become stable by emitting radiation.
• There are three main types of radiation from radioactive substances - α, β and γ
• Radioactive decay is a random event- you can't predict or influence when it will happen.
• Radioactive sources emit α, β, and γ radiation.
• Rutherford used α particles to probe inside atoms. He found that some of the α particles were scattered through large angles.
• The plum pudding model could not explain why some α particles were scattered through large angles.
• An atom has a small positively charged central nucleus where most of the atom's mass is located.
• The nuclear model of the atom correctly explained why some α particles scattered through large angles.
• Isotopes of an element are atoms with the same number of protons but different numbers of neutrons. So they have the same atomic nummber but different mass numbers.
• α radiation is stopped by paper and has a range of a few centimetres in air. It consists of particles, each composed of two protons and two neutrons. It has the greatest ionising power.
• β radiation is stopped by a thin sheet of metal and has a range of about one metre in air. It consists of fast-moving electrons emitted from the nucleus. It is less ionising than alpha radiation and more ionising than gamma radiation.
• γ radiation is stopped by thick lead and has an unlimited range in air. It consists of electromagnetic radiation.
• Alpha, beta and gamma radiation ionise substances they pass though. Ionisation in a living cell can damage or kill the cell.
• The half-life of a radioactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to halve.
• The count rate of a Geiger counter caused by a radioactive source decreases as the activity of the source decreases.
• The number of atoms of a radioactive isotope and the counr rate both decrease by half every half-life.