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Radioactivity, Fission and Fusion Revise the following: Sources of background radiation Basic atomic structure Isotopes History of the atom Modern atom Effect of alpha and beta radiation on nuclei • Nuclear Fission • Nuclear Fusion • • • • • Sources of background radiation A reminder about atoms - complete • Atoms may lose or gain ________to form charged particles called ions. • All atoms of a particular element have the _____ number of protons. • The total number of protons in an atom is called its _______number. • The total number of protons and neutrons in an atom is called its _____number. Early ideas about the atom: The plum pudding model • A scientist called JJ Thomson suggested that an atom is a positively-charged sphere with negative electrons distributed throughout it • Thomson’s model became known as the plum pudding model, because the electrons in the atom were thought to be like raisins in a plum pudding. Rutherford, Geiger and Marsden: The nuclear model of the atom - 1 Rutherford, Geiger and Marsden: The nuclear model of the atom - 1 Rutherford, Geiger and Marsden: The nuclear model of the atom - 1 Rutherford, Geiger and Marsden: The nuclear model of the atom - 1 The modern atom: The nuclear model The nucleus is where most of the mass of the atom is found. It contains protons and neutrons. The electrons orbit the nucleus in layers called shells. • In an atom the number of electrons is equal to the number of protons in the nucleus. • The atom has no net electrical charge Plum pudding Vs nuclear model Plum pudding Nuclear model Atom is solid sphere Atom is mostly empty space Negative electrons distributed throughout the atom Positive charge throughout the atom Electrons orbit the nucleus No nucleus Contains a nucleus Positive charge concentrated in the centre Isotopes Alpha radiation What happens during alpha decay? An alpha particle consists of two protons and two neutrons. It is the same as a helium nucleus. When an atom’s nucleus decays and releases an alpha particle, it loses two protons and two neutrons. mass number decreases by 4 238 U 92 234 Th 90 + 4 α 2 atomic number decreases by 2 The number of protons has changed, so the decayed atom has changed into a new element. Beta radiation What happens during beta decay? A beta particle consists of a high energy electron, which is emitted by the nucleus of the decaying atom. When an atom’s nucleus decays and releases a beta particle, a neutron turns into a proton, which stays in the nucleus, and a high energy electron, which is emitted. mass number remains the same 14 C 6 14 N 7 + β atomic number increases by 1 The decayed atom has gained a proton and so has changed into a new element. Nuclear fission fission + Uranium 235 + + neutron uranium 236 Strontium 90 + + Xenon 144 + neutrons • Nuclear fission is the splitting of an atomic nucleus • There are two fissionable substances in common use in nuclear reactors, uranium 235 and plutonium 239 • For fission to occur the uranium 235 or plutonium 239 nucleus must first absorb a neutron • The nucleus undergoing fission splits into two smaller nuclei and 2 or 3 neutrons and energy is released • The neutrons may go on to start a chain reaction What is a chain reaction? Nuclear fission results in a chain reaction because each time a nucleus splits it releases more neutrons, which can go on and cause more fission reactions to occur... and so on. + This is why a chain reaction releases a lot of energy so rapidly. If a chain reaction is uncontrolled, heat builds up very quickly. A chain reaction must be controlled to maintain a steady output of heat. Nuclear fusion + deuterium + + tritium fusion helium + neutron • Nuclear fusion is the joining of two atomic nuclei to form a larger one. • Nuclear fusion is the process by which energy is released in stars