WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.021 Unit S C 7: Types of substance. 00:00:09.700 --> 00:00:18.018 Classify substances by structure and bonding: ionic lattices, simple molecules, giant covalent networks and metallic lattices. 00:00:18.168 --> 00:00:22.419 The pattern of bonds and mobile charge carriers explains their properties. 00:00:25.100 --> 00:00:34.923 Ionic substances have strong attractions between ions and usually high melting points; they conduct when molten or dissolved because ions move, but not as solids. 00:00:37.600 --> 00:00:44.479 Simple molecular substances generally have low melting and boiling points because intermolecular attractions are weak. 00:00:44.629 --> 00:00:48.052 They usually lack mobile charge carriers and conduct poorly. 00:00:50.733 --> 00:00:55.299 Giant covalent structures have a large network of strong covalent bonds. 00:00:55.449 --> 00:01:00.643 Breaking these bonds needs a lot of energy, so these substances have high melting points. 00:01:00.793 --> 00:01:03.417 They are not made of separate small molecules. 00:01:06.067 --> 00:01:12.003 Most metals are shiny, solid, fairly dense and good electrical conductors, with high melting points. 00:01:12.153 --> 00:01:19.687 These are trends, not absolutes: mercury is liquid at room temperature and alkali metals have relatively low melting points. 00:01:22.367 --> 00:01:25.362 Use several measured properties to infer structure. 00:01:25.512 --> 00:01:30.446 Appearance or solubility alone rarely identifies a bonding type reliably. 00:01:33.100 --> 00:01:36.427 Allotropes are different structural forms of the same element. 00:01:36.577 --> 00:01:40.857 Diamond and graphite are both pure carbon but have very different properties. 00:01:43.533 --> 00:01:50.913 In diamond each carbon atom forms four strong covalent bonds to other carbons in a three-dimensional giant network. 00:01:51.063 --> 00:01:55.289 Local unit only, not an isolated molecule or a flat structure. 00:01:59.967 --> 00:02:07.676 The strong rigid network makes diamond very hard and gives it a high melting or sublimation temperature under appropriate conditions. 00:02:07.826 --> 00:02:10.642 It is useful in cutting tools and drill tips. 00:02:13.300 --> 00:02:22.739 Diamond does not normally conduct electricity because its outer electrons are involved in bonds and there are no mobile delocalised electrons or ions. 00:02:25.400 --> 00:02:33.707 Diamond is insoluble in water: dissolving it would require disrupting a giant bonded network, not just separating small molecules. 00:02:36.367 --> 00:02:40.647 A diagram with a central carbon and four neighbours shows a local unit. 00:02:40.797 --> 00:02:45.553 The bonds continue through the material; diamond is not a five-atom molecule. 00:02:48.233 --> 00:02:53.609 In graphite each carbon atom forms three covalent bonds in flat hexagonal layers. 00:02:53.759 --> 00:02:58.197 Strong bonds within the layers give a high melting or sublimation temperature. 00:03:00.867 --> 00:03:04.976 Attractions between layers are relatively weak, so layers can slide. 00:03:05.126 --> 00:03:09.617 This makes graphite soft and useful as a lubricant and in pencil cores. 00:03:09.767 --> 00:03:13.707 Layer sections are schematic; bonds extend beyond the drawing. 00:03:18.367 --> 00:03:26.430 In graphite, one outer electron from each carbon atom is delocalised: it is free to move through the layers rather than belonging to one bond. 00:03:26.580 --> 00:03:32.254 These electrons carry charge, so graphite conducts electricity and can be used for electrodes. 00:03:34.933 --> 00:03:39.343 Do not explain graphite conduction by mobile ions or sliding layers. 00:03:39.493 --> 00:03:42.330 The charge carriers are delocalised electrons. 00:03:45.000 --> 00:03:50.097 Graphene is a single layer of carbon atoms in a hexagonal network, one atom thick. 00:03:50.247 --> 00:03:55.259 It is strong, light and conducts electricity because of delocalised electrons. 00:03:55.409 --> 00:04:00.137 A small section of a continuous layer; edges are not the bulk bonding pattern. 00:04:04.800 --> 00:04:08.917 Graphene differs from graphite in the number of layers, not in the element. 00:04:09.067 --> 00:04:13.199 A rolled graphene-like sheet forms the structure of a carbon nanotube. 00:04:15.867 --> 00:04:18.905 Fullerenes have hollow carbon cages or tubes. 00:04:19.055 --> 00:04:26.764 Buckminsterfullerene, C 60, is a molecule of sixty carbon atoms in a cage containing pentagonal and hexagonal rings. 00:04:26.914 --> 00:04:30.779 Different structures of the same element explain different properties. 00:04:35.433 --> 00:04:41.641 The covalent bonds within a C 60 molecule are strong, but attractions between separate molecules are weaker. 00:04:41.791 --> 00:04:46.403 Molecular fullerene solids differ from diamond's continuous covalent network. 00:04:49.067 --> 00:04:56.062 Hollow cages can enclose other substances; their structures are investigated for uses such as delivering substances. 00:04:56.212 --> 00:05:01.469 Use potential applications carefully rather than assuming every application is routine. 00:05:04.133 --> 00:05:10.681 Carbon nanotubes are long, hollow carbon structures that are very strong and have useful electrical properties. 00:05:10.831 --> 00:05:17.166 They can strengthen other materials and, because of their large surface area, provide a support for catalysts. 00:05:19.833 --> 00:05:23.993 A ball-and-stick cage model exaggerates gaps and bond thickness. 00:05:24.143 --> 00:05:28.717 Its purpose is to show connectivity and shape, not literal atom sizes. 00:05:31.400 --> 00:05:39.004 A metal can be modelled as a regular lattice of positive metal ions surrounded by delocalised electrons from outer shells. 00:05:41.667 --> 00:05:48.887 Metallic bonding is the strong electrostatic attraction between positive metal ions and delocalised electrons. 00:05:49.037 --> 00:05:53.527 These electrons can move throughout the metal rather than belonging to one atom. 00:05:56.200 --> 00:06:02.875 Delocalised electrons move through the metal and carry electric charge, so metals conduct as solids and liquids. 00:06:03.025 --> 00:06:07.315 Electrons and lattice vibrations also transfer thermal energy. 00:06:07.465 --> 00:06:11.956 Electrostatic attraction to mobile electrons holds the metal together. 00:06:16.633 --> 00:06:21.933 Layers of metal ions can slide while attraction to the delocalised electrons remains. 00:06:22.083 --> 00:06:27.469 Metals are malleable, meaning they can be hammered into shape, and ductile, meaning drawn into wires. 00:06:27.619 --> 00:06:32.233 Layer sliding can occur without the brittle charge alignment of an ionic crystal. 00:06:36.900 --> 00:06:42.904 Many metals have high melting points because strong metallic attractions need much energy to overcome. 00:06:43.054 --> 00:06:49.655 Melting-point and conductivity comparisons depend on the particular metal, not a universal one-factor rule. 00:06:52.333 --> 00:06:54.744 Alloys mix a metal with other elements. 00:06:54.894 --> 00:07:00.663 Different-sized atoms can disrupt sliding of layers, often making an alloy harder than a pure metal. 00:07:00.813 --> 00:07:03.544 Alloy behaviour depends on its composition. 00:07:06.200 --> 00:07:12.770 Dot-and-cross diagrams show the electrons involved in bonding, but do not show the real three-dimensional shape. 00:07:12.920 --> 00:07:18.529 Ball-and-stick models show how atoms join and are arranged, using sticks to represent bonds. 00:07:21.200 --> 00:07:26.011 A flat lattice or layer diagram omits part of a three-dimensional structure. 00:07:26.161 --> 00:07:31.269 Space-filling models show relative occupied space but can hide bonding details. 00:07:33.933 --> 00:07:39.437 The metal model explains mobile electrons but static pictures omit particle motion. 00:07:39.587 --> 00:07:43.795 Real solid ions vibrate rather than remaining completely motionless. 00:07:46.467 --> 00:07:51.479 Explain a use by connecting structure to property: graphite has mobile electrons, 00:07:51.629 --> 00:07:53.442 so it can be an electrode; 00:07:53.592 --> 00:07:56.835 diamond has four bonds per carbon in a rigid network, 00:07:56.985 --> 00:07:59.075 so it can cut hard materials. 00:08:01.733 --> 00:08:07.941 When comparing unknown materials, consider melting point, conduction in different states and solubility. 00:08:08.091 --> 00:08:12.571 Check exceptions such as conducting graphite before assigning a bonding type. 00:08:15.233 --> 00:08:17.388 That completes Types of substance. 00:08:17.538 --> 00:08:20.883 Revisit the notes and test yourself on the revision website.