C2.2.1 Giant Ionic Structures
- An ionic compound consists of a giant structure of ions arranged in a lattice-the attractive electrostatic forces between oppositely charged ions act in all directions and are very strong, holding all the ions in the lattice together very tightly.
- Their strong electrostatic forces mean that they are solid at room temperature as it takes more energy to break the lattice up-they have high boiling and melting points.
- Ionic solid: Ions cannot move around and therefore cannot conduct electricity.
- Ionic liquid: When enough energy has been supplied to overcome the electrostatic forces between ions, ions can freely move around the liquid with their electrical charge, and can therefore conduct electricity.
- Ions in a solution: water molecules can help seperate ions and make them free to move around, and it can therefore conduct electricity.
1 of 7
C2.2.2 Simple Molecules
- Atoms in a molecule are held together by strong covalent bonds when they share electrons.
- A lot of substances made out of covalent bonds have low melting and boiling points-many can be liquids or gases at room temperature, or solid with low melting points. This is because the intermolecular forces are weak.
- Intermolecular forces-although covalent bonds are very strong, each molecule tends to be quite seperate from others, as th attraction between them is very small, so overcoming these forces doesn't take too much energy.
- A substance made of simple molecules doesn't conduct energy because molecules have no overall charge so cannot carry electrical charge, even if it is liquid at room temperature.
2 of 7
C2.2.3 Giant Covalent Structures Part 1
- Atoms of some elements can have several covalent bonds, then can join together in giant structures to make a macromolecule. Every atom in the structure is covalently bonded to other atoms.
- Atoms like these in giant lattices gives the lattice special properties, like for diamond and silica. These two giant lattices are very hard, have very hard melting points and are insoluble in water. Diamond is exceptionally hard-each carbon atom is joined to four others by covalent bonds.
- Carbon isn't always found as diamond-it can be found as graphite, where carbon atoms are covalently bonded to three other carbon atoms, forming hexagons in giant layers. There are no covalent bonds between layers, so the layers slide over each other easily, makeing graphite a soft and slippery-feeling material.
3 of 7
C2.2.3 Giant Covalent Structures Part 2
- Graphite-As each carbon atom forms only 3 covalent bonds, despite being able to have a maximum of 4 bonds, as they have four electrons on the outer shell, there is one spare electron on the outer shell of each carbon atom. These electrons delocalise and move freely, allowing graphite to conduct electricity, something most other covalent substances can't do.
- Carbon atoms can form a lot of different structures, structures of which the carbon atoms join together to make huge cages which can have lots of shapes.
- Shapes looking like balls, onions, tubes, doughnuts, corkscrews and cones can be made, from hexagonal rings of carbon atoms. These are called fullerenes.
- These were first discovered in 1985. The first one discovered was the buckyball, which had 60 carbon atoms, but now giant fullerenes which contain many thousands of carbon atoms can be made.
- Scientists can now put these inside carbon cages, which means it could be possible that they could be used to deliver dryugs to parts of the body. They will become important in nanoscience, for things like catalysts and lubricants.
4 of 7
C2.2.4 Giant Metallic Structures
- Metal atoms are arranged in layers, which means that the layers can slide over each other easily-metals can be bent and hammered into other shapes easily without breaking it.
- Alloys are mixtures of metals, apart from steels, which contain iron with particular amounts of carbon (non-metal). Different sized atoms in an alloy makes it more difficult for layers to slide over each other, making the substance harder.
- Properties-the delocalised electrons can move around the giant lattice, holding the ions together but also enabling the lattice to distort, which means that when the metal collides with something or is struck, the atoms just slip past each other without breaking the structure.
- Shape memory alloys are alloys with special properties. They can be shaped like all other metals, but when you heat them they can return to their original shape, so they seem to be able to 'remember' the orginal shape.
- Shape memory alloys can be used in health care. For example, in dental braces, or to hold broken bones in place as they heal.
5 of 7
C2.2.5 Properties of Polymers
- Polymers are made essentially from crude oil, made from monomers, such as poly(ethene).
- The properties of a polymer depend on which monomers were used to make it, and the conditions that were used.
- Polymer chains in nylon are made from two different monomers-one monomer has acidic groups at each end, and the other had basic group at each end. The polymer they make is very very different from poltymers made from hydrocarbons.
- There are 2 types of poly(ethene)-high density (HD) and low density (LD), both made from ethene monomers but formed under different conditions.
- LDPE-made from very high pressures and traces of oxygen. The chains are branched and can't pack closely together very well.
- HDPE-slightly raised pressure and a catalyst at 50ºC. The molecules are straighter and can pack closer together, giving it a higher softening temperature and making it stronger.
- Polymers are classified according to what happens to then when heated.
- Thermosoftening polymers-soften easily and reset when cooled, made of individual chains tangled together. Weak intermolecular forces, which means that when heated, they break and the polymer becomes soft. When cooled, intermolecular forces bring molecules back together so it hardens again.
- Thermosetting polymers-other polymers don't melt when heated. They have strong covalent bonds forming 'cross links' between their polymer chains. Their monomers make covalent bonds between polymer chains when first heated in order to shape them, which stops them softening. It takes really high temperatures before they soften.
6 of 7
- It is a new and exciting area of science. 1 nanometre (nm)=1 x 10 metres (one billionth of a metre).
- When atoms are arranged into very small particles they behave differently to ordinary materials made of the same atoms.
- It contains a few hundred atoms arranged in a particular way-the small sizes give them a very large surface area and new properties making them very useful materials.
- Glass can be coated with titanium oxide particles. Sunshine triggers a chemical reactions breaking down dirt on a window. When it rains, broken-down dirt is washed off.
- Titanium oxide & zinc oxide are used in sun screens-they block sun rays better than conventional UV absorbers.
- Nanoparticles in face creams are absorbed deeper into the skin.
- Latest techniques-nanocages of gold can be used to deliver drugs. It's been found that gold particles can be injected and absorbed by tumours (have thin leaky blood vessels and holes large enough for gold particles) but can't get into healthy cells. When a laser is directed at the tumour the gold nanoparticles absorb energy, warm up. The temperature of the tumour increases enough to change the properties of its proteins but barely warms surrounding tissue, destroying tumour cells without hurting healthy cells. There is also potential to use the cages to carry anti-cancer drugs.
- Developments: nanotubes are now being developed that can be used as nanowires, making it possible to make tiny electronic circuits. Nanotubes can be used to make highly sensitive selective sensors, such as detecting tiny traces of gas present in the breath of asthmatics before an attach.
- Risks: they can easily find their way into the air and in our bodies, resulting in unpredictable consequences for our health and the environment-more research needed.
7 of 7