WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.776 Unit S C 3: Atomic structure. 00:00:09.433 --> 00:00:15.469 Dalton modelled atoms as tiny spheres and proposed that elements contain characteristic atoms. 00:00:15.619 --> 00:00:21.751 Later evidence showed atoms have smaller particles and isotopes of an element can have different masses. 00:00:24.433 --> 00:00:25.700 J. 00:00:25.850 --> 00:00:27.117 J. 00:00:27.267 --> 00:00:30.359 Thomson discovered the negatively charged electron. 00:00:30.509 --> 00:00:35.417 His plum-pudding model placed electrons within a spread-out region of positive charge. 00:00:38.100 --> 00:00:43.433 In the alpha-scattering experiment, most alpha particles passed straight through thin gold foil. 00:00:43.583 --> 00:00:47.797 Some changed direction (were deflected), and very few bounced back. 00:00:47.947 --> 00:00:51.836 This did not fit a model with positive charge spread throughout the atom. 00:00:54.500 --> 00:00:59.141 Most particles passing through suggested the atom is mostly empty space. 00:00:59.291 --> 00:01:05.645 Strong deflections suggested a small, dense, positively charged nucleus containing most of the mass. 00:01:05.795 --> 00:01:10.213 Evidence supported a small positive nucleus, replacing plum pudding. 00:01:14.867 --> 00:01:21.575 Later developments established protons, neutrons and electrons occupying energy levels around the nucleus. 00:01:21.725 --> 00:01:26.801 New evidence can change a scientific model; a model is not a literal photograph. 00:01:29.467 --> 00:01:38.948 Atoms are roughly ten to the power minus 10 metres across; nuclei are much smaller, roughly ten to the power minus 15 to ten to the power minus 14 metres. 00:01:39.098 --> 00:01:46.072 Simple diagrams hugely exaggerate nuclear size and do not represent electrons as real orbiting planets. 00:01:48.733 --> 00:01:52.884 Protons have relative charge plus 1 and relative mass 1. 00:01:53.034 --> 00:01:56.304 Neutrons have charge 0 and relative mass 1. 00:01:56.454 --> 00:01:58.267 Both are in the nucleus. 00:02:00.933 --> 00:02:12.205 Electrons have relative charge minus 1 and relative mass about 1 divided by 1836, usually treated as negligible in mass-number calculations. 00:02:12.355 --> 00:02:15.702 They occupy shells or energy levels around the nucleus. 00:02:15.852 --> 00:02:20.960 Protons and neutrons are nuclear; electrons occupy surrounding energy levels. 00:02:25.633 --> 00:02:29.384 A neutral atom has equal numbers of protons and electrons. 00:02:29.534 --> 00:02:33.513 Charges cancel; neutrons do not supply negative charge. 00:02:36.167 --> 00:02:40.595 Atomic number, Z, is the number of protons and identifies the element. 00:02:40.745 --> 00:02:44.505 Every carbon atom has six protons, whatever its isotope. 00:02:47.167 --> 00:02:51.989 Mass number, A, is the total number of protons and neutrons in one nucleus. 00:02:52.139 --> 00:02:53.973 It is always a whole number. 00:02:54.123 --> 00:02:59.935 Relative atomic mass is different: it is an average that takes account of the element’s isotopes. 00:03:02.600 --> 00:03:14.641 In sodium twenty-three notation, the upper number is mass number 23 and the lower number is atomic number 11: 11 protons, 12 neutrons, and 11 electrons for the neutral atom. 00:03:14.791 --> 00:03:19.205 Mass number belongs to one isotope, not a weighted isotope average. 00:03:23.867 --> 00:03:27.700 Number of neutrons equals mass number minus atomic number. 00:03:27.850 --> 00:03:31.958 For carbon-14: 14 minus 6 equals 8 neutrons. 00:03:34.633 --> 00:03:38.521 For a neutral atom, number of electrons equals atomic number. 00:03:38.671 --> 00:03:42.039 Always check whether the question instead describes an ion. 00:03:44.700 --> 00:03:49.980 An ion forms when electrons are lost or gained; its proton count remains the same. 00:03:50.130 --> 00:03:52.754 Changing protons would change the element. 00:03:55.433 --> 00:04:00.483 A sodium ion, N A, charge plus, has 11 protons but 10 electrons. 00:04:00.633 --> 00:04:06.245 A chloride ion, C L, charge minus, has 17 protons but 18 electrons. 00:04:08.900 --> 00:04:16.471 For a positive ion, subtract its charge number from the neutral atom’s electron count: a 2 plus ion has lost two electrons. 00:04:16.621 --> 00:04:21.468 For a negative ion, add the charge number: a 2 minus ion has gained two. 00:04:21.618 --> 00:04:24.815 Forming these ions does not change the neutron count. 00:04:27.467 --> 00:04:34.591 If an atom has 13 protons and 14 neutrons, its mass number is 27 and the element is aluminium. 00:04:34.741 --> 00:04:37.374 Its neutral electron count is 13. 00:04:40.033 --> 00:04:47.381 Isotopes are atoms of the same element with the same proton number but different neutron numbers, so their mass numbers differ. 00:04:50.033 --> 00:04:57.552 Carbon-12, carbon-13 and carbon-14 each have six protons but six, seven and eight neutrons respectively. 00:04:57.702 --> 00:04:59.259 They are all carbon. 00:04:59.409 --> 00:05:01.840 Same element, different neutron numbers. 00:05:06.500 --> 00:05:14.381 Neutral isotopes of an element have the same number of electrons and electronic arrangement, giving very similar chemical behaviour. 00:05:17.033 --> 00:05:24.275 Isotopes can differ in physical properties because their masses differ; some isotopes are radioactive, but not all are. 00:05:26.933 --> 00:05:34.217 Isotopes are not ions: isotope differences concern neutrons, while ordinary ion differences concern electrons. 00:05:36.900 --> 00:05:40.757 An element can occur as a naturally occurring mixture of isotopes. 00:05:40.907 --> 00:05:45.117 Their proportions affect the relative atomic mass in the periodic table. 00:05:47.800 --> 00:05:56.141 Relative atomic mass, A R, is the weighted mean mass of atoms of an element compared with one twelfth of the mass of a carbon-12 atom. 00:05:56.291 --> 00:05:57.784 It has no unit. 00:06:00.467 --> 00:06:07.634 For isotope percentages, A R equals sum of (isotope mass times percentage abundance) divided by 100. 00:06:07.784 --> 00:06:12.839 At GCSE, use mass numbers as isotope masses when supplied that way. 00:06:15.500 --> 00:06:29.985 For chlorine with 75 percent chlorine-35 and 25 percent chlorine-37: A R equals (35 times 75 plus 37 times 25) divided by 100 equals 35.5. 00:06:30.135 --> 00:06:34.807 The result is nearer 35 because chlorine-35 is more abundant. 00:06:39.467 --> 00:06:45.395 For relative abundances rather than percentages, divide the weighted total by the total abundance. 00:06:45.545 --> 00:06:53.417 A 3 to 1 ratio gives (35 times 3 plus 37 times 1) divided by 4 equals 35.5. 00:06:56.100 --> 00:07:00.635 An A R that is not a whole number does not mean an atom has part of a neutron. 00:07:00.785 --> 00:07:06.639 It is an average of the masses of different isotopes, taking account of how common each isotope is. 00:07:09.300 --> 00:07:14.761 Check that your answer lies between the isotope masses and nearer the more abundant isotope. 00:07:14.911 --> 00:07:19.077 An ordinary unweighted average works only when abundances are equal. 00:07:21.733 --> 00:07:23.908 That completes Atomic structure. 00:07:24.058 --> 00:07:27.403 Revisit the notes and test yourself on the revision website.