Giant ionic structures
- Ionic compunds have giant structures which are arranged in layers froming a regular 3D structure
- There are strong electrostatic forces which act in all directions holding the ions firmly together and therefore it takes huge amounts of energy to break the ionic bonds down.
- So all ionic compunds have high melting and boiling points
- Once an ionic compund has been broken down (either molten or in solution) the ions are free to move and so the compound can carry electrical charge meaning it can conduct heat and electricity
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- The atoms within a molecule are held together by strong covalent bonds but the covalent bonds act only between the atoms it has bonded to and therefore simple molecules have little attraction for each other
- Simple molecules therefore have very low melting and boiling points
- The intermolecular forces (forces of attraction between the molecules) are very weak so they are easily broken down by heating hence the fact that they have low melting and boiling points.
- Some of the smallest molecules such as C2 and CH4 are gases at room temperature. Others have stronger covalent bonds and are liquds at room temperature such as Br2 and C6H14 and some are solids with low melting points such as I2.
- Molecules can not conduct electricity because each molecule has no overall charge so they cannot carry electrical charge
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Giant covalent structures
- Atoms of some elements can form large covalent structures sometimes refered to as macromolecules. This is because in groups 4,5 and 6 each atom can bond to several other atoms forming huge structures which take huge amounts of energy to break down and therefore they have very high melting and boiling points
- Diamond is a form of carbon in which each carbon atom covalently bonds to 4 other carbon atoms froming a huge regular 3D arrangment and making diamond hard and transparent. The properties of diamond are very similar to the properties of silica
- Graphite is a different form of carbon in which each carbon atom covalently bonds to 3 other carbon atoms in 2D hexagonal layers. The intermolecular forces between the different layers are very weak and therefore the layers easily slide over each other making graphite grey and slippery. When carbon atoms bond together one electron in the highest energy level from each atom delocalises and free to move between the atoms. This enables graphite to carry electrical charge therefore enabling it to conduct electricity.
- Fullerenes is also made up of carbon atoms which each bond to 3 other carbons froming hexagons which join together to form 3D cage like strucutres of varying numbers of carbon atoms some of which are nano-sized. There are many applications for fullerenes such as drug delivery into body, lubricants, catalysts and reinforcing metals.
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Giant metallic structures
- Metal atoms are arranged in regular layers
- Because of this, if a force is applied to the metal the layers can slide over each other bending the shape without breaking the structure making them malleable and ductile
- This makes metals useful as wires, rods and sheet materials
- Alloys are mixtures of metal or mixtures of a metal with other elements such as carbon.
- The different atoms in the alloy distort the regular arrangment of atoms making it more difficult for the layers to slide over each other
- This makes alloys harder than the standard metals
- Shape memory alloys are alloys which can be bent and deformed in to new shapes but return to the original shape upon heating.
- There are many applications of shape memory alloys such as some types of stiching, wound dressings and dental braces
- When metal atoms pack together the electrons in the highest energy level delocalise so they are free to move through the solid.
- This makes metals very good conductors of heat and electricity because the electrons can move through the solid transfering energy very quickly.
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Properties of polymers
- The properties of polymers depend on the monomers used to make them and the reaction conditions.
- Low density (LD) polyethene and high density (HD) polyethene are both made from ethene but are made using different reaction conditions and a different catalyst
- HD polyethene is stronger than LD polyethene and has a higher softening and melting and boiling point.
- There are thermosoftening polymers and thermosetting polymers
- Thermosoftening polymers soften when heated so can be remoulded, but they harden again upon cooling. This is because they are made up of polymer chains which are tangled together. They have weak intermolecular forces which easily break down when heated causing the polymer to soften. However when cooled the intermolecular forces bring the polymer molecules back together again causing the polymer to harden again.
- Thermosetting polymers set hard once they are first moulded and are not softened by heating. This is because the strong covalent bonds form cross links between their polymer chains holding the polymer chains in position.
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- Nano-science is a new and exciting area of chemistry and scientists are finding many uses for nano-particles
- Nano-particles are particles which are between 1 and 100 nanometers in size
- The nano-particles behave differently to the bulk substances they are made from
- A nano-metre is one billionth of a metre
- The small size gives them a huge surface area
- This makes them very useful as highly selective sensors, catalysts, cosmetics such as deodrants and sun creams, and to give construction materials useful properties
- As nano-particles are being used more and more frequently there is a greater risk that they will soon find their way into the air and our bodies which could have unpredictable consequences on our health and the environment
- Therefore more research needs to be done to find out there effects
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