Chemistry - C2.2 - Structure and Properties - Revision Cards in GCSE Chemistry

C2.2.1 - Giant Ionic Structures

Ionic compounds - giant structure in which many strong electrostatic forces hold ions together = solid at room temperature - lot of energy needed to overcome ionic bonds and cause melting = ionic compounds have high melting and boiling points
Melted ionic compound - ions are free to move = carry electrical charge - liquids conduct electricity
Some dissolve in water - water molecules split up lattice - ions free to move in these solutions = conduct electricity

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Atoms within molecules held together by strong covalent bonds - only act between atoms in the molecule = little attraction for other single molecules
Simple molecule substances - low melting and boiling point, don't conduct electricity - no overall charge, so can't carry electrical charges
Forces of attraction between molecules - intermolecular forces - are weak and are overcome when substance melts or boils = substances made of small molecules have low melting and boiling points
Smallest molecules = weakest intermolecular forces = gas
Larger molecules = stronger intermolecular forces = liquid

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Some form several covalent bonds = giant covalent structure - every atom joined to several others by covalent bonds - lots of energy needed to break down lattice = high melting and boiling points
Diamond - form of carbon, regular 3D structure, atoms covalently bond to 4 others = hard and transparent - SiO2 (silica) has simliar structure
Graphite - form of carbon, atoms covalently bonded to 4 other carbons - no covalent bonds between layers = layers slide = slippery and grey
Graphite - flat sheet of hexagons, one electron from each atom is delocalised = conducts - weak intermolecular forces = layers slide
Fullerenes - large molecules formed from hexagonal rings of carbon - rings join to form cage-like structure with different nos of carbon atoms - scientists finding applications for fullerenes eg. drug delivery into body, lubricants, catalysts and reinforcing materials

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Metal atoms - arranged in layers, force applied = layers slide - move into new position without breaking apart - stretches/bends into new shape - useful as wires, rods and sheet materials
Alloys - mixture of metals or metal + other elements - different sized atoms distort layers = more difficult to slide = harder than pure metal
Shape memory alloys = bent/deformed to different shape and when heated return to original shape - used in many ways eg. braces
Metal structures - delocalised electrons = good conductors - delocalised electrons move throughout lattice and can transfer energy quickly

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Polymer properties depend on monomers and conditions used to make it
Poly(propene) - made from propene, softens at higher temperature than poly(ethene) - made from ethene
Low density(LD) poly(ethene) and high density(HD) poly(ethene) made using different catalysts and reaction conditions - HD poly(ethene) has higher softening temperature and is stronger
Poly(ethene) - thermosoftening polymer - made of individual polymer chains tangled together - when heated: softens, when cool: hardens
Thermosetting polymers - don't change shape when heated, set hard when first moulded: covalent bonds form cross links between polymer chains
Weak forces in thermosoftening polymers - heated = intermolecular forces break down: softens, cooled = intermolecular forces bring molecules together: hardens

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When atoms are arranged into very small particles they behave differently than materials made of the same atoms
Nanometre = one billionth of metre - nanoparticles = few nanometres - contain few hundred atoms arranged in particular way - small size = large surface area and new properties that make them useful materials
Nanotechnology - uses nanoparticles as highly selective sensors, efficient catalysts, new coatings and cosmetics eg. sun screen and deodorants, and give construstive materials special properties
Nanoparticles used more = high risk of getting into air and bodies - unpredictable consequences for health and environment - more research needed to find out effects

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