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neurologyYou need to know

• A proton has relative mass 1, charge +1, a radius of about `1 \times 10^{-15}` m, and is found in the nucleus of an atom.
• A neutron has relative mass 1, charge 0, a radius of about `1 \times 10^{-15}` m, and is found in the nucleus of an atom.
• An electron has a very small relative mass of about `1/1836`, charge -1, is much smaller than a proton or neutron, and is found in energy levels around the nucleus.
• The atomic number of an element is the number of protons in each atom of that element, and it also equals the number of electrons in a neutral atom.
• The mass number of an atom is the total number of protons and neutrons in its nucleus.
• An ion is an atom or group of atoms that has gained or lost electrons, so it has an overall electrical charge.
• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
• Some isotopes are unstable because their nuclei can decay and emit ionising radiation to become more stable.
• Thomson discovered the electron and proposed the plum pudding model, in which negative electrons were embedded in a ball of positive charge.
• Rutherford’s alpha scattering experiment showed that atoms are mostly empty space and have a tiny, dense, positively charged nucleus.
• Bohr proposed that electrons orbit the nucleus in fixed energy levels or shells.
• Chadwick discovered the neutron, which explained the extra mass in the nucleus that could not be due to protons alone.
• Alpha radiation is a helium nucleus with charge +2 and mass 4; it is strongly ionising, travels only a few centimetres in air, and is stopped by paper or skin.
• Beta radiation is a high-speed electron emitted from the nucleus with charge -1 and very small mass; it is moderately ionising, travels a few metres in air, and is stopped by thin aluminium.
• Gamma radiation is high-energy electromagnetic radiation with no mass and no charge; it is weakly ionising, travels far through air, and is reduced by thick lead or concrete.
• Radioactive decay is random because it is impossible to predict when a particular unstable nucleus will decay, although the activity of a large sample follows a pattern.
• Half-life is the time taken for the number of undecayed nuclei, or the activity of a radioactive source, to fall to half its original value.
• Irradiation happens when an object is exposed to radiation from a source, while contamination happens when radioactive material gets onto or inside an object or organism.
• Background radiation is always present and comes from natural sources such as cosmic rays, rocks, soil, radon gas and living things, and from artificial sources such as medical imaging and nuclear waste.
• Nuclear fission is the splitting of a large unstable nucleus into smaller nuclei, while nuclear fusion is the joining of two light nuclei to form a heavier nucleus.

• Use atomic number, mass number and ionic charge to calculate the numbers of protons, neutrons and electrons in atoms and ions.
• Explain Rutherford’s alpha scattering observations by linking most alpha particles passing straight through to empty space, some deflections to a positive nucleus, and rare rebounds to a tiny dense nucleus.
• Complete alpha decay equations by reducing the mass number by 4 and the atomic number by 2, while conserving total mass number and total charge.
• Complete beta minus decay equations by keeping the mass number the same and increasing the atomic number by 1, while showing an emitted electron.
• Complete neutron emission equations by reducing the mass number by 1 and keeping the atomic number the same.
• Determine half-life from a graph by reading the time taken for activity or count rate to halve between two values.
• Calculate remaining activity after a number of half-lives using repeated halving, and express the result as a fraction or ratio of the original activity.
• Compare irradiation and contamination hazards by considering radiation type, exposure time, whether the source enters the body, and how easily the source can be removed.
• Select suitable radiation protection measures, including reducing exposure time, increasing distance, using shielding, using tongs, wearing protective clothing, and storing sources in lead-lined containers.
• Evaluate medical uses of ionising radiation by balancing benefits such as imaging or destroying tumours against risks such as cell damage, mutations and cancer.