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P2.5 What Happens When Radioactive Substances Decay, And The Uses And Dangers Of Their Emissions P2.5.1 Atomic Structure P2.5.2 Atoms And Radiation The basic structure of an atom is a small central nucleus composed of protons and neutrons surrounded by electrons. The number of electrons is equal to the number of protons in the nucleus. The atom has no overall electrical charge. Atoms may lose or gain electrons to form charged particles called ions. The atoms of an element always have the same number of protons, but have a different number of neutrons for each isotope. The total number of protons in an atom is called its atomic number. The total number of protons and neutrons in an atom is called its mass number. Some substances give out radiation from the nuclei of their atoms all the time, whatever is done to them; they are radioactive. Background radiation is all around us. Some of it comes from natural sources and some comes from artificial sources. Natural background radiation includes: cosmic rays - radiation that reaches the Earth from space; rocks and soil - some rocks are radioactive and give off radioactive radon gas; living things - plants absorb radioactive materials from the soil and these pass up the food chain. For most people, natural sources contribute the most to their background radiation dose. However, human activity has added to background radiation by creating and using artificial sources of radiation. These include radioactive waste from nuclear power stations, radioactive fallout from nuclear weapons testing and medical x-rays. Artificial sources account for about 15 per cent of the average background radiation dose. Photographic film goes darker when it absorbs radiation. The more radiation the film absorbs, the darker it is when it is developed. The Geiger-Muller tube detects radiation. Each time it absorbs radiation, it transmits an electrical pulse to a counting machine. This makes a clicking sound or displays the count rate. There are three main types of radiation emitted from radioactive atoms. These are alpha, beta and gamma radiation. An alpha particle is identical to the nucleus of a helium atom, which comprises two protons and two neutrons. Beta radiation consists of high energy electrons emitted from the nucleus. These electrons have not come from the electron shells or energy levels around the nucleus. Instead, they form when a neutron splits into a proton and an electron. The electron then shoots out of the nucleus at high speed. Gamma radiation is very short wavelength - high frequency - electromagnetic radiation. This is similar to other types of electromagnetic radiation such as visible light and x-rays, which can travel long distances. Alpha radiation is the least penetrating. It can be stopped - or absorbed - by just a sheet of paper. Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium. Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead or many metres of concrete. Alpha particles are positively charged, beta particles are negatively charged and gamma radiation is electrically neutral. This means that alpha radiation and beta radiation can be deflected by magnetic and electric fields, but gamma radiation is not deflected. Beta particles are negatively charged so they will be attracted towards a positively charged plate. Positive alpha particles will be attracted towards a negatively charged plate. When radiation collides with molecules in living cells it can damage them. If the DNA in the nucleus of a cell is damaged, the cell may become cancerous. The cell then divides rapidly and causes serious health problems. The greater the dose of radiation, the greater the chance the cell will become cancerous. However, very high doses of radiation can kill the cell completely. We use this property of radiation to kill cancer cells, and also harmful bacteria and other micro-organisms. If the radioactive source is inside the body, perhaps after being swallowed or breathed in alpha radiation is the most dangerous because it is easily absorbed by cells and beta and gamma radiation are not as dangerous because they are less likely to be absorbed by a cell and will usually just pass right through it. If the radioactive source is outside the body alpha radiation is not as dangerous because it is unlikely to reach living cells inside the body and beta and gamma radiation are the most dangerous sources because they can penetrate the skin and damage the cells inside. The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. This is called radioactive decay. A half-life is the time it takes for the number of nuclei of the isotope in a sample to halve. Different radioactive isotopes have different half-lives. The half-life of carbon-14 is 5,715 years, but the half-life of francium-223 is just 20 minutes. Two protons and two neutrons are lost from a nucleus when it emits an alpha particle. The atomic mass number decreases by 4. The atomic number decreases by 2. A new element is formed that is two places lower in the Periodic Table than the original element. In beta decay, a neutron changes into a proton plus an electron. The proton stays in the nucleus. The electron leaves the atom with high energy as a beta particle. The nucleus has one more proton and one less neutron when it emits a beta particle. The atomic mass number stays the same. The atomic number increases by 1. A new element is formed that is one place higher in the Periodic Table than the original table. 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