WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.969 Unit C C 4: The periodic table. 00:00:09.633 --> 00:00:18.293 Mendeleev arranged elements mainly by increasing relative atomic mass, placing elements with similar chemical properties in the same columns. 00:00:20.967 --> 00:00:24.720 He sometimes changed the mass order to preserve chemical groupings. 00:00:24.870 --> 00:00:31.844 Tellurium was placed before iodine so iodine joined the other halogens despite its lower relative atomic mass. 00:00:34.500 --> 00:00:40.258 Mendeleev left gaps for undiscovered elements and predicted their properties from nearby elements. 00:00:40.408 --> 00:00:43.968 Later discoveries matching predictions supported his table. 00:00:46.633 --> 00:00:50.905 Mendeleev worked before scientists knew about protons and atomic number. 00:00:51.055 --> 00:00:54.763 Ordering elements by mass could not explain every exception. 00:00:57.433 --> 00:01:02.382 Moseley’s X-ray evidence helped show that elements should be ordered by atomic number. 00:01:02.532 --> 00:01:08.835 The modern table follows increasing proton number, solving the problems caused by ordering only by mass. 00:01:08.985 --> 00:01:12.320 New evidence explained why proton-number order works. 00:01:17.000 --> 00:01:26.631 Tellurium has atomic number 52 and iodine 53, so their modern order agrees with Mendeleev's chemical placement without reversing atomic-number order. 00:01:29.300 --> 00:01:33.487 Rows in the periodic table are periods and vertical columns are groups. 00:01:33.637 --> 00:01:36.314 Atomic number increases across a period. 00:01:38.967 --> 00:01:46.356 Elements in the same main group have similar chemical properties because their atoms have the same number of outer-shell electrons. 00:01:49.033 --> 00:01:53.429 For main groups 1, 7, group number gives the number of outer electrons. 00:01:53.579 --> 00:02:01.875 GCSE group 0 contains noble gases with complete outer shells; it is also numbered group 18 in another convention. 00:02:02.025 --> 00:02:08.446 A simplified table: group 0 is at the right; omitted elements are not empty gaps in the full table. 00:02:13.100 --> 00:02:19.519 Period number tells you the number of occupied electron shells in the simple model for these main-group elements. 00:02:22.200 --> 00:02:26.722 Metals occupy the left and centre; non-metals are mainly on the right. 00:02:26.872 --> 00:02:31.226 Hydrogen is a non-metal despite being on the left above group 1. 00:02:33.900 --> 00:02:39.189 Metals usually conduct electricity and form positive ions by losing electrons. 00:02:39.339 --> 00:02:45.566 Many non-metals form negative ions or share electrons; not every non-metal conducts like graphite. 00:02:48.233 --> 00:02:52.767 The group predicts related chemical behaviour, not identical properties. 00:02:52.917 --> 00:02:57.427 Reactivity trends differ between groups and need the appropriate explanation. 00:03:00.100 --> 00:03:03.937 Electrons occupy energy levels or shells around the nucleus. 00:03:04.087 --> 00:03:08.259 For a neutral atom, the electron total equals its atomic number. 00:03:10.933 --> 00:03:20.062 For the first 20 elements, use a simple filling pattern of up to 2 in the first shell, then 8 in the second, then 8 in the third before starting the fourth. 00:03:20.212 --> 00:03:22.943 This is not a universal shell-capacity rule. 00:03:25.600 --> 00:03:29.881 Electronic configurations list electrons from inner to outer shell. 00:03:30.031 --> 00:03:37.309 Sodium, Z equals 11, is two, eight, one; the numbers add to 11. 11 electrons: period 3, group 1. 00:03:41.967 --> 00:03:44.373 Helium is 2 and neon two, eight. 00:03:44.523 --> 00:03:50.762 Their full outer shells explain membership of group 0; helium's first shell is full with just two electrons. 00:03:53.433 --> 00:03:58.303 Magnesium, Z equals 12, is two, eight, two: period 3 and group 2. 00:03:58.453 --> 00:04:06.397 Chlorine, Z equals 17, is two, eight, seven: period 3 and group 7. 17 electrons: period 3, group 7. 00:04:11.067 --> 00:04:18.171 Potassium, Z equals 19, is two, eight, eight, one and calcium, Z equals 20, is two, eight, eight, two. 00:04:18.321 --> 00:04:24.171 Both have four occupied shells and are in period 4. 20 electrons: period 4, group 2. 00:04:28.833 --> 00:04:33.985 Dot-and-shell diagrams show electron counts, not actual electron paths or scale. 00:04:34.135 --> 00:04:38.333 Include all electrons when asked for an atom's complete configuration. 00:04:41.000 --> 00:04:48.909 An atom with configuration two, eight, three has three occupied shells and three outer electrons: period 3, group 3, aluminium. 00:04:51.567 --> 00:04:56.373 An atom with configuration two, seven is fluorine: period 2, group 7. 00:04:56.523 --> 00:05:01.664 Chlorine below it has an extra occupied shell but the same number of outer electrons. 00:05:04.333 --> 00:05:08.719 Elements below one another in a group have more occupied shells down the group. 00:05:08.869 --> 00:05:12.164 Their shared outer-electron count connects their chemistry. 00:05:14.833 --> 00:05:21.861 To identify an element from a configuration, add all electrons for a neutral atom and find that atomic number. 00:05:22.011 --> 00:05:24.827 Do not use mass number as the electron count. 00:05:27.500 --> 00:05:34.303 Use the table and configuration together: check electron total, occupied-shell count and outer-electron count. 00:05:34.453 --> 00:05:38.967 Group 0 is the exception to a literal outer-electron group-number rule. 00:05:41.633 --> 00:05:51.679 Electronic arrangement relates to bonding: a group 1 metal commonly loses one outer electron, while a group 7 non-metal commonly gains one or shares one electron. 00:05:54.333 --> 00:05:56.573 That completes The periodic table. 00:05:56.723 --> 00:06:00.069 Revisit the notes and test yourself on the revision website.