WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.895 Unit S C 4: The periodic table. 00:00:09.567 --> 00:00:18.226 Mendeleev arranged elements mainly by increasing relative atomic mass, placing elements with similar chemical properties in the same columns. 00:00:20.900 --> 00:00:24.653 He sometimes changed the mass order to preserve chemical groupings. 00:00:24.803 --> 00:00:31.777 Tellurium was placed before iodine so iodine joined the other halogens despite its lower relative atomic mass. 00:00:34.433 --> 00:00:40.191 Mendeleev left gaps for undiscovered elements and predicted their properties from nearby elements. 00:00:40.341 --> 00:00:43.901 Later discoveries matching predictions supported his table. 00:00:46.567 --> 00:00:50.838 Mendeleev worked before scientists knew about protons and atomic number. 00:00:50.988 --> 00:00:54.696 Ordering elements by mass could not explain every exception. 00:00:57.367 --> 00:01:02.315 Moseley’s X-ray evidence helped show that elements should be ordered by atomic number. 00:01:02.465 --> 00:01:08.768 The modern table follows increasing proton number, solving the problems caused by ordering only by mass. 00:01:08.918 --> 00:01:12.253 New evidence explained why proton-number order works. 00:01:16.933 --> 00:01:26.564 Tellurium has atomic number 52 and iodine 53, so their modern order agrees with Mendeleev's chemical placement without reversing atomic-number order. 00:01:29.233 --> 00:01:33.421 Rows in the periodic table are periods and vertical columns are groups. 00:01:33.571 --> 00:01:36.247 Atomic number increases across a period. 00:01:38.900 --> 00:01:46.289 Elements in the same main group have similar chemical properties because their atoms have the same number of outer-shell electrons. 00:01:48.967 --> 00:01:53.362 For main groups 1, 7, group number gives the number of outer electrons. 00:01:53.512 --> 00:02:01.809 GCSE group 0 contains noble gases with complete outer shells; it is also numbered group 18 in another convention. 00:02:01.959 --> 00:02:08.379 A simplified table: group 0 is at the right; omitted elements are not empty gaps in the full table. 00:02:13.033 --> 00:02:19.453 Period number tells you the number of occupied electron shells in the simple model for these main-group elements. 00:02:22.133 --> 00:02:26.655 Metals occupy the left and centre; non-metals are mainly on the right. 00:02:26.805 --> 00:02:31.159 Hydrogen is a non-metal despite being on the left above group 1. 00:02:33.833 --> 00:02:39.122 Metals usually conduct electricity and form positive ions by losing electrons. 00:02:39.272 --> 00:02:45.499 Many non-metals form negative ions or share electrons; not every non-metal conducts like graphite. 00:02:48.167 --> 00:02:52.700 The group predicts related chemical behaviour, not identical properties. 00:02:52.850 --> 00:02:57.361 Reactivity trends differ between groups and need the appropriate explanation. 00:03:00.033 --> 00:03:03.871 Electrons occupy energy levels or shells around the nucleus. 00:03:04.021 --> 00:03:08.193 For a neutral atom, the electron total equals its atomic number. 00:03:10.867 --> 00:03:19.995 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. 00:03:20.145 --> 00:03:22.876 This is not a universal shell-capacity rule. 00:03:25.533 --> 00:03:29.815 Electronic configurations list electrons from inner to outer shell. 00:03:29.965 --> 00:03:37.243 Sodium, Z equals 11, is two, eight, one; the numbers add to 11. 11 electrons: period 3, group 1. 00:03:41.900 --> 00:03:44.307 Helium is 2 and neon two, eight. 00:03:44.457 --> 00:03:50.695 Their full outer shells explain membership of group 0; helium's first shell is full with just two electrons. 00:03:53.367 --> 00:03:58.237 Magnesium, Z equals 12, is two, eight, two: period 3 and group 2. 00:03:58.387 --> 00:04:06.330 Chlorine, Z equals 17, is two, eight, seven: period 3 and group 7. 17 electrons: period 3, group 7. 00:04:11.000 --> 00:04:18.105 Potassium, Z equals 19, is two, eight, eight, one and calcium, Z equals 20, is two, eight, eight, two. 00:04:18.255 --> 00:04:24.104 Both have four occupied shells and are in period 4. 20 electrons: period 4, group 2. 00:04:28.767 --> 00:04:33.919 Dot-and-shell diagrams show electron counts, not actual electron paths or scale. 00:04:34.069 --> 00:04:38.266 Include all electrons when asked for an atom's complete configuration. 00:04:40.933 --> 00:04:48.842 An atom with configuration two, eight, three has three occupied shells and three outer electrons: period 3, group 3, aluminium. 00:04:51.500 --> 00:04:56.307 An atom with configuration two, seven is fluorine: period 2, group 7. 00:04:56.457 --> 00:05:01.597 Chlorine below it has an extra occupied shell but the same number of outer electrons. 00:05:04.267 --> 00:05:08.652 Elements below one another in a group have more occupied shells down the group. 00:05:08.802 --> 00:05:12.097 Their shared outer-electron count connects their chemistry. 00:05:14.767 --> 00:05:21.795 To identify an element from a configuration, add all electrons for a neutral atom and find that atomic number. 00:05:21.945 --> 00:05:24.761 Do not use mass number as the electron count. 00:05:27.433 --> 00:05:34.237 Use the table and configuration together: check electron total, occupied-shell count and outer-electron count. 00:05:34.387 --> 00:05:38.900 Group 0 is the exception to a literal outer-electron group-number rule. 00:05:41.567 --> 00:05:51.613 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. 00:05:54.267 --> 00:05:56.507 That completes The periodic table. 00:05:56.657 --> 00:06:00.002 Revisit the notes and test yourself on the revision website.