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neurologyYou need to know

• A proton has relative mass 1, relative charge +1, is found in the nucleus, and has a radius of about `1 \times 10^{-15}` m.
• A neutron has relative mass 1, relative charge 0, is found in the nucleus, and has a radius of about `1 \times 10^{-15}` m.
• An electron has relative mass about `1/1836`, relative charge -1, is found in electron shells around the nucleus, and is much smaller than a proton or neutron.
• An atom has a radius of about `1 \times 10^{-10}` m, while the nucleus has a radius of about `1 \times 10^{-14}` m and contains almost all the mass of the atom.
• The atomic number of an element is the number of protons in each atom, and the mass number is the total number of protons and neutrons.
• A neutral atom has the same number of electrons as protons, while an ion is formed when an atom loses or gains electrons.
• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, and some isotopes are unstable and radioactive.
• 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 most alpha particles passed straight through gold foil, but a few were deflected or bounced back, proving that atoms are mostly empty space with a small, dense, positively charged nucleus.
• Bohr proposed that electrons occupy fixed energy levels or shells around the nucleus, and Chadwick discovered the neutron in the nucleus.
• Alpha, beta and gamma radiation are emitted by unstable nuclei as they change to become more stable, and unstable nuclei can also emit high-energy neutrons.
• Alpha radiation is a helium nucleus with charge +2, is strongly ionising, travels only a few centimetres in air, and is stopped by paper or skin.
• Beta radiation is a fast-moving electron with charge -1, is moderately ionising, travels a few metres in air, and is stopped by a thin sheet of aluminium.
• Gamma radiation is electromagnetic radiation with no mass and no charge, is weakly ionising, travels a long distance in air, and is reduced by thick lead or concrete.
• In alpha decay, the mass number decreases by 4 and the atomic number decreases by 2; in beta-minus decay, the mass number stays the same and the atomic number increases by 1; in neutron emission, the mass number decreases by 1 and the atomic number stays the same.
• Half-life is the time taken for the number of unstable nuclei in a sample, or the activity of a sample, to fall to half its original value.
• Radioactive decay is random, so it is impossible to predict when a particular unstable nucleus will decay, but the half-life of a large sample can be measured.
• Background radiation is always present and comes from sources including radon gas, cosmic rays, rocks, building materials, food and medical procedures; radiation dose is measured in sieverts.
• Irradiation means exposure to radiation from a source outside the body, while contamination means radioactive material gets onto or inside an object or living tissue.
• Nuclear fission is the splitting of a large unstable nucleus into smaller nuclei with energy released, while nuclear fusion is the joining of small nuclei to form a larger nucleus with energy released.

• Determine the numbers of protons, neutrons and electrons in atoms and ions using atomic number, mass number and ionic charge from the periodic table.
• Explain Rutherford’s alpha scattering experiment by linking each observation to a conclusion about atomic structure, including empty space and a small positive nucleus.
• Complete balanced nuclear decay equations for alpha decay, beta-minus decay and neutron emission by conserving total mass number and total atomic number.
• Identify the half-life of a radioactive substance from a decay graph by reading the time taken for the activity or count rate to halve.
• Calculate the remaining activity or number of nuclei after whole numbers of half-lives using fractions, ratios or percentages of the original amount.
• Compare the hazards of alpha, beta, gamma and neutron radiation by considering ionising power, penetration, range in air, and whether the source is inside or outside the body.
• Describe protection methods against ionising radiation, including reducing exposure time, increasing distance, using suitable shielding, wearing protective clothing, and preventing contamination.
• Evaluate medical uses of ionising radiation by weighing benefits such as imaging and destroying tumours against risks such as cell damage, mutations and increased cancer risk.
• Describe induced nuclear fission as a neutron being absorbed by a large nucleus, causing it to split, release energy, produce smaller nuclei and emit more neutrons.
• Represent a fission chain reaction using a labelled diagram showing incoming neutrons, splitting nuclei, released energy, daughter nuclei and further emitted neutrons.