WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.042 Unit C P 6: Radioactivity. 00:00:09.700 --> 00:00:15.832 Thomson’s plum-pudding model described negative electrons embedded in spread-out positive charge. 00:00:15.982 --> 00:00:18.307 Later evidence changed this model. 00:00:20.967 --> 00:00:25.357 Rutherford’s team directed positive alpha particles at thin gold foil. 00:00:25.507 --> 00:00:31.938 Most passed through; some were deflected; very few were deflected through large angles or back towards the source. 00:00:34.600 --> 00:00:38.127 Most passing through showed the atom is mostly empty space. 00:00:38.277 --> 00:00:46.692 Large deflections showed positive charge and much of the mass concentrated in a tiny nucleus; the plum-pudding model could not explain them. 00:00:46.842 --> 00:00:50.839 A small concentrated nucleus explains rare large deflections. 00:00:55.500 --> 00:01:02.036 The nuclear model has positive protons and neutral neutrons in the nucleus, with negative electrons outside it. 00:01:02.186 --> 00:01:07.071 Rutherford’s original model did not yet include the later discovery of neutrons. 00:01:09.733 --> 00:01:23.301 Protons have relative charge plus 1 and mass about 1; neutrons charge 0 and mass about 1; electrons charge minus 1 and much smaller mass, about 1 divided by 1840 of a proton. 00:01:25.967 --> 00:01:31.821 Atomic number Z counts protons; mass or nucleon number A counts protons plus neutrons. 00:01:31.971 --> 00:01:34.233 Neutrons equals A minus Z. 00:01:34.383 --> 00:01:39.341 In a neutral atom, electrons equals protons; ions have unequal numbers. 00:01:39.491 --> 00:01:43.777 A neutral sodium-23 atom also has 11 electrons. 00:01:48.433 --> 00:01:52.595 Isotopes have the same proton number but different neutron numbers. 00:01:52.745 --> 00:01:57.251 Carbon-12, carbon-13 and carbon-14 are isotopes of carbon. 00:01:59.933 --> 00:02:03.536 Bohr’s model uses fixed electron energy levels or shells. 00:02:03.686 --> 00:02:11.151 Electrons absorb energy to move to higher levels and emit electromagnetic radiation when returning to lower levels. 00:02:13.833 --> 00:02:20.243 When electrons move between energy levels, different energy changes produce radiation of different frequencies. 00:02:20.393 --> 00:02:26.802 The separate lines in emission and absorption spectra show that the energy levels have fixed, distinct values. 00:02:26.952 --> 00:02:29.640 Different elements produce different line patterns. 00:02:32.300 --> 00:02:36.658 Ionisation is removal or addition of electrons to form ions. 00:02:36.808 --> 00:02:42.855 Radiation that removes an electron leaves a positive ion; it does not change the number of protons. 00:02:45.533 --> 00:02:51.782 Background radiation is radiation present around us even without a nearby experimental source. 00:02:51.932 --> 00:02:58.957 Natural sources include rocks, radon gas, cosmic radiation and naturally radioactive materials in food. 00:03:01.633 --> 00:03:08.737 Artificial sources include medical uses and smaller contributions from nuclear industry and past activities. 00:03:08.887 --> 00:03:12.115 Contributions vary with location and occupation. 00:03:14.767 --> 00:03:19.670 A Geiger, Müller tube and counter detect ionising radiation as pulses. 00:03:19.820 --> 00:03:24.907 Count rate is counts per second or minute; not every decay in the source is detected. 00:03:27.567 --> 00:03:33.016 Source activity is the number of nuclear decays per second, measured in becquerels (becquerels). 00:03:33.166 --> 00:03:39.009 One becquerels means one decay per second; activity and detector count rate are different quantities. 00:03:41.667 --> 00:03:44.811 Measure background without the source for a suitable time. 00:03:44.961 --> 00:03:51.841 Divide counts by time, then subtract this background count rate from the source-plus-background rate measured consistently. 00:03:54.500 --> 00:03:57.292 Radioactive counts fluctuate randomly. 00:03:57.442 --> 00:04:05.141 Count for longer and repeat to reduce relative random uncertainty; keep distance, detector position and counting time controlled. 00:04:07.800 --> 00:04:10.498 Photographic film darkens on exposure. 00:04:10.648 --> 00:04:17.528 Film badges and other dosimeters monitor exposure; they do not shield the wearer or replace limiting exposure time. 00:04:20.200 --> 00:04:24.669 An unstable nucleus can decay spontaneously, emitting radiation. 00:04:24.819 --> 00:04:30.175 Decay is random: it is impossible to predict when one particular nucleus will decay. 00:04:32.833 --> 00:04:38.526 An alpha particle is a helium nucleus: two protons and two neutrons, charge plus 2. 00:04:38.676 --> 00:04:44.340 Alpha is strongly ionising, short-range in air and stopped by paper or the outer skin layer. 00:04:47.000 --> 00:04:52.951 Beta minus is a fast electron emitted from the nucleus when a neutron changes into a proton. 00:04:53.101 --> 00:04:56.149 It is not an electron ejected from an outer shell. 00:04:58.800 --> 00:05:05.913 Beta plus is a positron, the positive counterpart of an electron, emitted when a proton changes into a neutron. 00:05:06.063 --> 00:05:11.365 Both beta types are moderately penetrating and ionising compared with alpha and gamma. 00:05:14.033 --> 00:05:20.869 A thin aluminium sheet can substantially stop beta radiation; required thickness depends on beta energy. 00:05:21.019 --> 00:05:23.579 Beta travels further in air than alpha. 00:05:26.233 --> 00:05:32.536 Gamma is high-frequency electromagnetic radiation emitted when a nucleus loses excess energy. 00:05:32.686 --> 00:05:36.505 It has no charge or rest mass and does not alter A or Z. 00:05:39.167 --> 00:05:43.491 Gamma is highly penetrating and weakly ionising compared with alpha. 00:05:43.641 --> 00:05:50.316 Thick lead or concrete reduces intensity substantially, rather than guaranteeing every gamma photon is stopped. 00:05:50.466 --> 00:05:55.779 These are qualitative comparisons; shielding reduces the radiation reaching a detector. 00:06:00.433 --> 00:06:06.928 A neutron can also be emitted from a nucleus: mass number falls by 1 and proton number stays unchanged. 00:06:07.078 --> 00:06:10.065 Different radiation types need suitable shielding. 00:06:12.733 --> 00:06:16.789 In alpha decay, A decreases by 4 and Z decreases by 2. 00:06:16.939 --> 00:06:27.636 For example, radium-226 (Z equals 88) becomes radon-222 (Z equals 86) plus helium-4 (Z equals 2). 00:06:30.300 --> 00:06:34.983 In beta-minus decay, A stays unchanged and Z increases by 1. 00:06:35.133 --> 00:06:41.531 The emitted electron has nucleon number 0 and charge number minus 1, keeping equation totals balanced. 00:06:44.200 --> 00:06:48.615 In beta-plus decay, A stays unchanged and Z decreases by 1. 00:06:48.765 --> 00:06:53.373 The emitted positron has nucleon number 0 and charge number plus 1. 00:06:56.033 --> 00:06:58.697 In gamma emission, A and Z stay the same. 00:06:58.847 --> 00:07:05.577 The nucleus has less energy, but it still has the same numbers of protons and neutrons: it is the same nuclide. 00:07:08.233 --> 00:07:14.759 Check nuclear equations by balancing total nucleon numbers and total charge numbers on both sides. 00:07:14.909 --> 00:07:18.730 The identity of the element is set by Z, not by A alone. 00:07:18.880 --> 00:07:22.504 Balance nucleon numbers and charge numbers in each equation. 00:07:27.167 --> 00:07:33.586 Half-life is the time for half the undecayed nuclei in a large sample to decay, or for the activity to halve. 00:07:33.736 --> 00:07:36.785 It does not mean half the total material vanishes. 00:07:39.467 --> 00:07:43.015 The activity decreases as fewer undecayed nuclei remain. 00:07:43.165 --> 00:07:49.121 After 1, 2 and 3 half-lives, the fraction remaining is one half, one quarter and one eighth. 00:07:51.800 --> 00:07:58.316 After n half-lives, remaining activity equals initial activity divided by two to the power n. 00:07:58.466 --> 00:08:02.978 Time elapsed equals number of half-lives times half-life duration. 00:08:05.633 --> 00:08:14.409 If activity starts at 80 becquerels and half-life is 5 years, activities after 5, 10 and 15 years are 40, 20 and 10 becquerels. 00:08:14.559 --> 00:08:19.514 Equal time intervals halve the activity; the curve is exponential, not linear. 00:08:24.167 --> 00:08:30.289 To read half-life from a graph, choose an activity, find when it falls to half, and subtract the times. 00:08:30.439 --> 00:08:35.215 Check several pairs and subtract background from measured count-rate data first. 00:08:37.867 --> 00:08:45.482 Half-life lets us predict how a large radioactive sample will behave overall, even though individual decays are random. 00:08:45.632 --> 00:08:48.095 Smaller samples show more variation. 00:08:48.245 --> 00:08:54.036 As fewer undecayed nuclei remain, fewer decay each second: the decrease is not a straight line. 00:08:56.700 --> 00:09:03.024 Ionising radiation can damage cells and D N A, causing mutations and increasing cancer risk. 00:09:03.174 --> 00:09:08.060 Effects depend on dose and exposure; not every exposure causes a tumour. 00:09:10.733 --> 00:09:17.581 Keep sources at a distance using tongs, minimise exposure time, use appropriate shielding and store them securely. 00:09:17.731 --> 00:09:22.287 Follow supervised handling procedures and never point a source towards someone. 00:09:24.967 --> 00:09:27.835 Irradiation means exposure to radiation. 00:09:27.985 --> 00:09:36.272 Contamination means radioactive material gets onto or into an object or person, so exposure can continue while it remains there. 00:09:36.422 --> 00:09:42.351 Contamination can continue irradiating the person until the material is removed or decays. 00:09:47.033 --> 00:09:52.205 Ordinary irradiation does not mean radioactive material has been deposited. 00:09:52.355 --> 00:09:58.817 Contamination may need removal and containment; merely moving away from the original source is insufficient. 00:10:01.500 --> 00:10:10.095 Alpha is particularly dangerous if an alpha-emitting material enters the body: strong ionisation affects nearby tissue without the skin barrier. 00:10:10.245 --> 00:10:13.785 Penetrating gamma is also an external-exposure hazard. 00:10:16.467 --> 00:10:25.863 Medical staff monitor dose and limit time near sources, use shielding and distance, and balance treatment or diagnostic benefit against radiation risk. 00:10:28.533 --> 00:10:30.773 That completes Radioactivity. 00:10:30.923 --> 00:10:34.269 Revisit the notes and test yourself on the revision website.