Simple Molecules and Giant Covalent Structures

C2aL5 Simple Molecules and Giant Covalent Structures
Key Words Attraction - pulling together. Compound - at least two elements chemically combined. Covalent Bond - bond in which electrons are shared. Diamond - carbon atoms arranged in a structure with strong bonds between layers, making a very hard substance. Electrons - tiny little particles with negative charge. Graphite - carbon arranged in layers with weak bonds between them. Giant covalent structure - lots of atoms bonding to each other with covalent bonds. Minerals - naturally occurring chemical compounds. Molecule - two or more atoms covalently bonded. Silicon dioxide - compound of silicon and oxygen.



Grade E Molecules are combinations of two or more atoms that are covalently bonded. Gases like oxygen are molecules, consisting of two atoms of the same element. Carbon dioxide is a molecule consisting of three atoms, two of which are oxygen, and one is carbon. Simple molecules have the following properties: They don't conduct electricity; They have low melting points or boiling points; They are mostly liquids or gases at room temperature; Solid molecules are often soft, waxy, and brittle; They are usually insoluble in water, but do dissolve in other liquids. Some non-metals form solids, because the atoms make covalent bonds with other atoms. These are giant covalent structures. For example, carbon can form giant covalent structures in the form of graphite, or diamond. Silicon dioxide is a compound that forms giant covalent structures. Properties of giant covalent structure are: They don't conduct electricity (except graphite); They have high melting points; They are very hard; They are insoluble in water.
Grade C Some simple molecules like sulphur are solid at room temperature. It has a regular crystal structure. This is because sulphur atoms form ring structures consisting of 8 atoms with strong covalent bonds. Between the groups of molecules there are weak forces of attraction. Only a little energy is needed to break such bonds, so sulphur melts at 119 ^oC. When simple molecules are solid, liquid, or gas, they cannot conduct electricity, as there are no charged particles. This is true when they are dissolved as well. Carbon has two giant structure forms. The first is graphite. The carbon forms three single bonds within the layers. This makes a tessellation of regular hexagons. In the picture above, they appear squashed to give the 3-D effect. These bonds are strong. However the fourth electron is free and available, so graphite can conduct electricity. The forces of attraction between layers are quite weak, so the layers can slide across each other easily. Unlike sulphur, where the atoms go around in groups of 8, carbon atoms go around in groups of many millions. Therefore graphite does not melt so easily. The other form of carbon is diamond. In this material, carbon makes bonds with four other carbon atoms in an arrangement like this: The basic subunit is a tetrahedron. In a diamond, each carbon bonds to four other carbons like this, so a large, long range structure is formed. The covalent bonds in three dimensions make the material very hard, as a lot of energy is needed to break them. Also the melting point is very high. However there are no spare electrons to allow electricity to be conducted. It is possible to convert graphite to industrial diamonds by squashing it very hard indeed with pressures of several million atmospheres. Silicon dioxide has a similar tetrahedral structure to diamond. Although not as hard as diamond, silicon dioxide is, nevertheless, very hard because of the strong covalent bonds. It is a mineral found widely in rocks, sometimes forming attractive clear crystals of quartz. It is very resistant to weathering, and often ends up transported as sand.
Grade A Large simple molecules like plant and animal oils are used to make perfume. They have a lovely smell. They are mixed with ethanol, which evaporates readily, and carries the smell away. to others Diamonds can be used to cut metals, stone, and concrete. They use industrial diamonds, not valuable gemstones. Both discs for the angle grinder started at the same size. The one on the left wore down to that size after cutting about 10 cm of concrete. The one on the right is diamond studded and has cut several metres of concrete without wear. The one on the left cost £4 (5 Euros), while the one on the right cost about £20 (25 Euros). No brainer, isn't it? Diamond gems can be tested to see if they cut glass. A real diamond will; a fake will not.