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| Question | Answer |
| Simple Molecules | Covalent bonds between non-metals. These bonds are very strong |
| Covalently bonded substances fall into two main types: 1.Simple molecules 2.Giant covalent structures | 1. Simple Molecules contain only a few atoms held together by strong covalent bonds. An example is carbon dioxide (CO2), the molecules of which contain one atom of carbon bonded with two atoms of oxygen. |
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Properties of simple molecular substances |
| Low melting and boiling points - This is because the weak intermolecular forces break down easily. | Non-conductive - Substances with a simple molecular structure do not conduct electricity. This is because they do not have any free electrons or an overall electric charge. |
| Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds. All have very strong bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. Simple molecular substances are gases, liquids or solids with low melting and boiling points. | Macromolecules |
| Macromolecules have giant covalent structures. They contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds. | Their atoms are arranged into giant lattices, which are strong structures because of the many bonds involved. |
| Substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken e.g. Graphite. | Diamond |
| Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure. | As a result, diamond is very hard and has a high melting point. It does not conduct electricity. |
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Graphite |
| Graphite is a form of carbon in which the carbon atoms form layers. | Each carbon atom in a layer is joined to only three other carbon atoms.The layers can slide over each other because there are no covalent bonds between them. This makes graphite much softer than diamond. |
| It is used in pencils and as a lubricant. Graphite conducts electricity. | |
| Silica | Silica, which is found in sand, has a similar structure to diamond. It is also hard and has a high melting point. However, it contains silicon and oxygen atoms instead of carbon atoms. |
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Polymers |
| A polymer is a large molecule formed by many identical smaller molecules (monomers). | Polymers have properties which depend on the chemicals they are made from, and the conditions in which they are made. For example, poly(ethene) can be low-density or high-density depending upon the catalyst and reaction condition used to make it. |
| Thermosoftening polymers | Thermosoftening polymers soften when heated and can be shaped when hot. The shape will harden when it is cooled, but can be reshaped when heated up again. |
| Poly(ethene) is a thermosoftening polymer. Its tangled polymer chains can uncoil and slide past each other, making it a flexible material. |
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| Thermosetting polymers | Once moulded, they do not soften when heated and they cannot be reshaped. |
| Vulcanised rubber is a thermoset used to make tyres. Its polymer chains are joined together by cross-links, so they cannot slide past each other easily. |
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| Ionic compounds | Ionic bonds form when a metal reacts with a non-metal. Metals form positive ions, while non-metals form negative ions. |
| Ionic bonds are the electrostatic forces of attraction between oppositely charged ions. |
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| Their melting and boiling points are very high because ionic bonds are very strong so a lot of energy is needed to break them. | Ionic compounds conduct electricity when they are dissolved in water or when they are melted. This is because their ions are free to move and carry the current. |
| However, ionic compounds do not conduct electricity when they are solid. This is because their ions cannot move around in their lattice structure. | Metals |
| Metals are malleable (they can be bent and shaped). This is because they consist of layers of atoms. These layers can slide over one another when the metal is bent, hammered or pressed. |
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| Metals form giant structures in which electrons in the outer shells of the metal atoms are free to move. These are called 'delocalised electrons' | The metallic bond is the force of attraction between these free electrons and metal ions. Metallic bonds are strong, so metals can maintain a regular structure and usually have high melting and boiling points. |
| Metals are good conductors of electricity and heat. This is because the free electrons can move throughout the metal. | |
| Alloys | An alloy is a mixture of two or more elements, where at least one element is a metal. Many alloys are mixtures of two or more metals. |
| Alloys contain atoms of different sizes. These different sizes distort the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal. |
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| Fullerenes | Carbon exists as graphite and diamond, but it can also form fullerenes. These are cages and tubes with different number of carbon atoms. |
| Fullerenes may be used for drug delivery systems in the body, in lubricants and as catalysts. |
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