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neurologyYou need to know

• A proton has relative mass 1 and charge +1, a neutron has relative mass 1 and charge 0, and an electron has very small relative mass about `\frac{1}{1836}` and charge −1.
• Protons and neutrons are found in the nucleus, while electrons occupy energy levels or shells around the nucleus.
• The radius of an atom is about `1 \times 10^{-10}` metres, and the radius of a nucleus is about `1 \times 10^{-14}` metres, so most of an atom is empty space.
• The atomic number is the number of protons in an atom, and the mass number is the total number of protons and neutrons in the nucleus.
• In a neutral atom, the number of electrons is equal to the number of protons.
• An ion is an atom or group of atoms that has gained or lost electrons, giving it an overall electric charge.
• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
• Some isotopes are unstable; unstable isotopes have nuclei that decay and emit radiation to become more stable.
• Thomson discovered the electron and proposed the plum pudding model, in which electrons were embedded in a sphere of positive charge.
• Rutherford’s alpha scattering experiment showed that most alpha particles passed straight through gold foil, some were deflected, and a very small number bounced back.
• Rutherford concluded from alpha scattering that atoms are mostly empty space, positive charge is concentrated in a tiny nucleus, and the nucleus contains most of the atom’s mass.
• Bohr proposed that electrons orbit the nucleus in fixed energy levels, and Chadwick discovered the neutron.
• Alpha, beta and gamma radiation can be emitted by unstable nuclei as they decay to become more stable, and high-energy neutrons can also be emitted from some unstable nuclei.
• Alpha radiation is a helium nucleus with charge +2, has high ionising power, travels only a few centimetres in air, and is stopped by paper or skin.
• Beta radiation is a fast-moving electron emitted from the nucleus with charge −1, has moderate ionising power, travels about a metre in air, and is stopped by thin aluminium.
• Gamma radiation is electromagnetic radiation with no mass and no charge, has low ionising power, travels a long distance in air, and is only 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.
• 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.
• Radioactive decay is random because it is impossible to predict exactly when a particular unstable nucleus will decay.
• Background radiation is always present and comes from sources such as cosmic rays, rocks, soil, buildings, food, medical procedures and nuclear accidents; radiation dose is measured in sieverts.

• Determine the numbers of protons, neutrons and electrons in atoms and ions using the atomic number, mass number and charge shown in the periodic table or nuclear notation.
• Explain Rutherford’s alpha scattering experiment by linking each observation to a conclusion about the atom’s empty space, tiny nucleus, positive charge and concentrated mass.
• Complete nuclear decay equations by conserving total mass number and total atomic number for alpha decay, beta-minus decay and neutron emission.
• Identify the half-life of a radioactive isotope from a decay graph by finding the time taken for activity or count rate to halve.
• Calculate the remaining activity or fraction after a number of half-lives using repeated halving, for example `\frac{1}{2}`, `\frac{1}{4}`, `\frac{1}{8}` of the original amount.
• Distinguish irradiation from contamination by stating whether the radioactive material is outside the body causing exposure or has been transferred onto or into an object or person.
• Compare the hazards of alpha, beta and gamma radiation by considering ionising power, penetration, range in air and whether the source is inside or outside the body.
• Select suitable safety measures for ionising radiation, including reducing exposure time, increasing distance, using shielding, wearing protective clothing and using tongs or remote handling.