Bonding and Structure of Matter

In Chemistry, there are several different types of bonding that you need to know about.

Ionic bonds as drawn at GCSE

Firstly, ionic bonds. These are made up from ions (charged atoms so a metal loses the electrons in its outer shell to make a positive ion and a non metal fills its outer shell to make a negative ion). These oppositely charged ions are electrostatically attracted to each other and pull them close. This force is very strong and it explains why ionic solids have such a high melting temperature. Ionic solids cannot conduct electricity as they do not have free moving ions, they are fixed and will only conduct electricity if melted or dissolved in water.

Covalent bonds drawn at GCSE

Secondly, we have covalent bonding. This occurs with non metals and as they have no metals to give up their electrons and fill their outer shells, they are forced to share them with a like minded non metal. This sharing allows each atom to have a full outer shell. As they do not form ions, there is very little attraction between the discreet molecules so covalent substances have very low melting temperatures.

The third type is still covalent, however, it does not form discreet molecules, instead a much larger structure is formed called a giant covalent structure. These include diamond, graphite and sand (silicon dioxide). They have high melting points and still do not conduct electricity except graphite which has delocalized electrons between the layers in its structure.

Metals are the final example to look at. Metals, in their elemental state, have a specific and very different form of bonding unique to metals. Each atom in the metal loses its outer valency electrons and all of the positive metal ions remain fixed in a regular pattern and are held by their attraction to the delocalized electrons freely moving around. These free moving electrons explain why metals are such good conductors of electricity and heat.

Table salt is the best example of a giant ionic structure and it has the perfect stereotypical properties. It dissolves in water and has a very high melting point. The reason that it has such a high melting point is because the ions that make it are held tightly together in what is known as a lattice (alternating positive and negative ions in a regular arrangement strongly attracted to each other). The strength of this gives the compound the very high melting temperature.

Because the ions are fixed and cannot move, the solids are poor conductors of heat and do not conduct electricity. When the compound is dissolved in water, the ions are free to move around to areas that electrically attract them, meaning that this solution will conduct electricity. Likewise, when the solid is heated enough and it melts (breaking the lattice) the ions are free to move and will conduct electricity. When bauxite (aluminium ore: aluminium oxide) is electrolyzed, it must first be melted to allow it to conduct electricity. To reduce the energy required to melt it, it is mixed with cryolite which allows it to melt at a much lower temperature.

Simple molecules are molecules that can be isolated and are not a part of a giant structure. Examples are methane, ammonia and oxygen. Their bonds are between the atoms of the molecule and there are only weak attractions between each molecule (inter-molecular forces). They do not form ions in any state and this is why pure water does not conduct electricity. These weak forces cause the compounds to have low melting temperatures and they do not conduct as each molecule has no overall charge.

Giant covalent structures have very different properties from simple covalent ones, if you picture the examples of silicon dioxide (sand) and two allotropes of carbon: diamond and graphite, you can instantly picture the differences. They all have very high boiling and melting points. Diamond is strongly bonded in a fixed pattern making it very hard whereas graphite is covalently bonded in layers that are not fixed together meaning that one layer can slide over another allowing to to be used as a lubricant. Graphite also has delocalized electrons between they layers allowing it to conduct electricity. The last carbon allotrope is called a Fullerene (C60), they are cage like structures which look very similar to footballs (soccer balls).

The final giant structure is the giant metallic structure. The atoms in a metal are equally sized and this regular pattern makes it easy for the atoms to be stretched over one another which is why metals can easily be bent and shaped. If you add another sized atom into the metal, by making it an alloy, different sized and shaped atoms/ions are introduced which makes it harder for the atoms to move over each other. This makes the alloy harder than the original element. These are not strictly atoms at all, they lose their outer electrons making them ions. The positive metal ions are attracted to the delocalised electrons moving around the ions which gives them their relatively high melting temperature, more importantly, the delocalised electrons allow the metal to conduct electricity and heat.

These properties are put to use with memory alloys. When heated, they remember their original shapes and surgeons and dentists use this to straighten bones or teeth.

Thermosetting and thermosoftening plastics

It may be worth rereading the section on polymers first then here we go with more. The original monomer determines the properties of the final polymer. The atoms that are attached to the carbons in the monomer will attract or repel nearby atoms which will determine how closely the chains will sit. They may also form bonds across them called bridges. Sometimes, identical monomers can form very different polymers because of the conditions in which they were formed. LDPE and HDPE are low density poly ethene and high density poly ethene. The difference is achieved with the presence of a catalyst and extra pressure (for HDPE) during manufacture.
Thermosoftening polymers soften easily if heated and are made up from a web of chains like a plate of spaghetti. Thermosetting polymers do not soften when heated right up until they begin to burn, This is because of the strong cross links between the polymer chains.

If you are studying seperate science, you will also need to know about nanoscience! This is the study of small particles between 1 and 100 nanometers across. Despite being made largely of carbon, these tiny structures behave nothing like their larger counterparts. They are very much a cutting edge in chemistry and material science development. Their uses range from a coating that makes windows self clean, better anti-aging creams and delivery of pharmaceutical chemicals in the body. The ball and tube shapes of some of the particles makes them ideal to trap unwanted biological or chemical particles or to encapsulate a chemical while it is transported to the area that needs it.

Key words and terms for this topic: ion, ionic, covalent, giant structure, lattice, delocalised, discreet molecules, intermolecular forces, giant structures, macromolecule, fullerene, nanotubes, graphene, alloy, shape memory alloy, thermosetting, thermosoftening, nanoscience
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