WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.063 Unit C C 7: Types of substance. 00:00:09.733 --> 00:00:18.051 Classify substances by structure and bonding: ionic lattices, simple molecules, giant covalent networks and metallic lattices. 00:00:18.201 --> 00:00:22.453 The pattern of bonds and mobile charge carriers explains their properties. 00:00:25.133 --> 00:00:34.956 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. 00:00:37.633 --> 00:00:44.512 Simple molecular substances generally have low melting and boiling points because intermolecular attractions are weak. 00:00:44.662 --> 00:00:48.085 They usually lack mobile charge carriers and conduct poorly. 00:00:50.767 --> 00:00:55.333 Giant covalent structures have a large network of strong covalent bonds. 00:00:55.483 --> 00:01:00.676 Breaking these bonds needs a lot of energy, so these substances have high melting points. 00:01:00.826 --> 00:01:03.450 They are not made of separate small molecules. 00:01:06.100 --> 00:01:12.036 Most metals are shiny, solid, fairly dense and good electrical conductors, with high melting points. 00:01:12.186 --> 00:01:19.721 These are trends, not absolutes: mercury is liquid at room temperature and alkali metals have relatively low melting points. 00:01:22.400 --> 00:01:25.395 Use several measured properties to infer structure. 00:01:25.545 --> 00:01:30.479 Appearance or solubility alone rarely identifies a bonding type reliably. 00:01:33.133 --> 00:01:36.460 Allotropes are different structural forms of the same element. 00:01:36.610 --> 00:01:40.890 Diamond and graphite are both pure carbon but have very different properties. 00:01:43.567 --> 00:01:50.946 In diamond each carbon atom forms four strong covalent bonds to other carbons in a three-dimensional giant network. 00:01:51.096 --> 00:01:55.322 Local unit only, not an isolated molecule or a flat structure. 00:02:00.000 --> 00:02:07.709 The strong rigid network makes diamond very hard and gives it a high melting or sublimation temperature under appropriate conditions. 00:02:07.859 --> 00:02:10.675 It is useful in cutting tools and drill tips. 00:02:13.333 --> 00:02:22.772 Diamond does not normally conduct electricity because its outer electrons are involved in bonds and there are no mobile delocalised electrons or ions. 00:02:25.433 --> 00:02:33.740 Diamond is insoluble in water: dissolving it would require disrupting a giant bonded network, not just separating small molecules. 00:02:36.400 --> 00:02:40.680 A diagram with a central carbon and four neighbours shows a local unit. 00:02:40.830 --> 00:02:45.587 The bonds continue through the material; diamond is not a five-atom molecule. 00:02:48.267 --> 00:02:53.642 In graphite each carbon atom forms three covalent bonds in flat hexagonal layers. 00:02:53.792 --> 00:02:58.230 Strong bonds within the layers give a high melting or sublimation temperature. 00:03:00.900 --> 00:03:05.009 Attractions between layers are relatively weak, so layers can slide. 00:03:05.159 --> 00:03:09.650 This makes graphite soft and useful as a lubricant and in pencil cores. 00:03:09.800 --> 00:03:13.740 Layer sections are schematic; bonds extend beyond the drawing. 00:03:18.400 --> 00:03:26.463 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. 00:03:26.613 --> 00:03:32.287 These electrons carry charge, so graphite conducts electricity and can be used for electrodes. 00:03:34.967 --> 00:03:39.376 Do not explain graphite conduction by mobile ions or sliding layers. 00:03:39.526 --> 00:03:42.363 The charge carriers are delocalised electrons. 00:03:45.033 --> 00:03:50.130 Graphene is a single layer of carbon atoms in a hexagonal network, one atom thick. 00:03:50.280 --> 00:03:55.293 It is strong, light and conducts electricity because of delocalised electrons. 00:03:55.443 --> 00:04:00.171 A small section of a continuous layer; edges are not the bulk bonding pattern. 00:04:04.833 --> 00:04:08.950 Graphene differs from graphite in the number of layers, not in the element. 00:04:09.100 --> 00:04:13.232 A rolled graphene-like sheet forms the structure of a carbon nanotube. 00:04:15.900 --> 00:04:18.938 Fullerenes have hollow carbon cages or tubes. 00:04:19.088 --> 00:04:26.797 Buckminsterfullerene, C 60, is a molecule of sixty carbon atoms in a cage containing pentagonal and hexagonal rings. 00:04:26.947 --> 00:04:30.812 Different structures of the same element explain different properties. 00:04:35.467 --> 00:04:41.674 The covalent bonds within a C 60 molecule are strong, but attractions between separate molecules are weaker. 00:04:41.824 --> 00:04:46.437 Molecular fullerene solids differ from diamond's continuous covalent network. 00:04:49.100 --> 00:04:56.095 Hollow cages can enclose other substances; their structures are investigated for uses such as delivering substances. 00:04:56.245 --> 00:05:01.503 Use potential applications carefully rather than assuming every application is routine. 00:05:04.167 --> 00:05:10.714 Carbon nanotubes are long, hollow carbon structures that are very strong and have useful electrical properties. 00:05:10.864 --> 00:05:17.199 They can strengthen other materials and, because of their large surface area, provide a support for catalysts. 00:05:19.867 --> 00:05:24.026 A ball-and-stick cage model exaggerates gaps and bond thickness. 00:05:24.176 --> 00:05:28.751 Its purpose is to show connectivity and shape, not literal atom sizes. 00:05:31.433 --> 00:05:39.037 A metal can be modelled as a regular lattice of positive metal ions surrounded by delocalised electrons from outer shells. 00:05:41.700 --> 00:05:48.920 Metallic bonding is the strong electrostatic attraction between positive metal ions and delocalised electrons. 00:05:49.070 --> 00:05:53.561 These electrons can move throughout the metal rather than belonging to one atom. 00:05:56.233 --> 00:06:02.909 Delocalised electrons move through the metal and carry electric charge, so metals conduct as solids and liquids. 00:06:03.059 --> 00:06:07.349 Electrons and lattice vibrations also transfer thermal energy. 00:06:07.499 --> 00:06:11.989 Electrostatic attraction to mobile electrons holds the metal together. 00:06:16.667 --> 00:06:21.967 Layers of metal ions can slide while attraction to the delocalised electrons remains. 00:06:22.117 --> 00:06:27.502 Metals are malleable, meaning they can be hammered into shape, and ductile, meaning drawn into wires. 00:06:27.652 --> 00:06:32.266 Layer sliding can occur without the brittle charge alignment of an ionic crystal. 00:06:36.933 --> 00:06:42.937 Many metals have high melting points because strong metallic attractions need much energy to overcome. 00:06:43.087 --> 00:06:49.689 Melting-point and conductivity comparisons depend on the particular metal, not a universal one-factor rule. 00:06:52.367 --> 00:06:54.777 Alloys mix a metal with other elements. 00:06:54.927 --> 00:07:00.697 Different-sized atoms can disrupt sliding of layers, often making an alloy harder than a pure metal. 00:07:00.847 --> 00:07:03.577 Alloy behaviour depends on its composition. 00:07:06.233 --> 00:07:12.803 Dot-and-cross diagrams show the electrons involved in bonding, but do not show the real three-dimensional shape. 00:07:12.953 --> 00:07:18.562 Ball-and-stick models show how atoms join and are arranged, using sticks to represent bonds. 00:07:21.233 --> 00:07:26.044 A flat lattice or layer diagram omits part of a three-dimensional structure. 00:07:26.194 --> 00:07:31.302 Space-filling models show relative occupied space but can hide bonding details. 00:07:33.967 --> 00:07:39.470 The metal model explains mobile electrons but static pictures omit particle motion. 00:07:39.620 --> 00:07:43.829 Real solid ions vibrate rather than remaining completely motionless. 00:07:46.500 --> 00:07:51.512 Explain a use by connecting structure to property: graphite has mobile electrons, 00:07:51.662 --> 00:07:53.475 so it can be an electrode; 00:07:53.625 --> 00:07:56.868 diamond has four bonds per carbon in a rigid network, 00:07:57.018 --> 00:07:59.109 so it can cut hard materials. 00:08:01.767 --> 00:08:07.974 When comparing unknown materials, consider melting point, conduction in different states and solubility. 00:08:08.124 --> 00:08:12.605 Check exceptions such as conducting graphite before assigning a bonding type. 00:08:15.267 --> 00:08:17.421 That completes Types of substance. 00:08:17.571 --> 00:08:20.917 Revisit the notes and test yourself on the revision website.