This essay investigate's different chemical structures and how they are formed.

Graphite has a layer structure which is quite difficult to draw convincingly in three dimensions. The diagram below shows the arrangement of the atoms in each layer, and the way the layers are spaced:

The bonding in graphite

Each carbon atom uses three of its electrons to form simple bonds to its three close neighbors. That leaves a fourth electron in the bonding level. These "spare" electrons in each carbon atom become delocalized over the whole of the sheet of atoms in one layer. They are no longer associated directly with any particular atom or pair of atoms, but are free to wander throughout the whole sheet.

The physical properties of graphite

* has a high melting point, similar to that of diamond. In order to melt graphite, it isn't enough to loosen one sheet from another. You have to break the covalent bonding throughout the whole structure.

* has a soft, slippery feel, and is used in pencils and as a dry lubricant for things like locks.

* has a lower density than diamond. This is because of the relatively large amount of space that is "wasted" between the sheets.

* is insoluble in water and organic solvents - for the same reason that diamond is insoluble. Attractions between solvent molecules and carbon atoms will never be strong enough to overcome the strong covalent bonds in graphite.

Ionic Structures

In this section I wish into the relationships between the arrangement of the ions in a typical ionic solid like sodium chloride and its physical properties - melting point, boiling point, brittleness, solubility and electrical behavior.

The structure of a typical ionic solid - sodium chloride

Sodium chloride is taken as a typical ionic compound. Compounds like this consist of a giant lattice of ions. So sodium chloride is described as having a giant ionic structure.Giant means that you can't state exactly how many ions there are. There could be billions of sodium ions and chloride ions packed together, or trillions, or whatever - it simply depends how big the crystal is. That is different from, say, a water molecule which always contains exactly 2 hydrogen atoms and one oxygen atom - never more and never less.

A small representative bit of a sodium chloride lattice looks like this:

I can see that the sodium ions and chloride ions alternate with each other in each of the three dimensions. This diagram is easy enough to draw with a computer, but extremely difficult to draw convincingly by hand.

Only those ions joined by lines are actually touching each other. The sodium ion in the centre is being touched by 6 chloride ions.

Why is sodium chloride 6:6-co-ordinated?

The more attraction there is between the positive and negative ions, the more energy is released. The more energy that is released, the more energetically stable the structure becomes. That means that to gain maximum stability, you need the maximum number of attractions. So why does each ion surround itself with 6 ions of the opposite charge? That represents the maximum number of chloride ions that you can fit around a central sodium ion before the chloride ions start touching each other.

Sodium chloride crystals are brittle

If a stress is applied it shifts the ion layers slightly.

Ions of the same charge are brought side-by-side and so the crystal repels itself to pieces!

Sodium chloride is soluble in water

Many ionic solids are soluble in water - although not all. It depends on whether there are big enough attractions between the water molecules and the ions to overcome the attractions between the ions themselves.

Molecular Structures

This page describes how the physical properties of substances having molecular structures varies with the type of intermolecular attractions - van der Waals forces. I wish to focus on ice, iodine and polthene.

Iodine, I2

Iodine is a dark grey crystalline solid. Iodine has a low melting point solid. The crystalline suggests a regular packing of the molecules.

The structure is a cube of iodine molecules with another molecule at the centre of each face.

Notice that as you look down on the cube, all the molecules on the left and right hand sides are aligned the same way. The ones in the middle are aligned in the opposite way.The iodine molecules are, of course, touching each other.

Ice

There are lots of different ways that the water molecules can be arranged in ice. The one below is based on the water molecules arranged in a diamond structure.

This is just a small part of a structure which extends over huge numbers of molecules in three dimensions. In the diagram, the lines represent hydrogen bonds. When ice melts, the structure breaks down and the molecules tend to fill up this wasted space.This means that the water formed takes up less space than the original ice. Ice is a very unusual solid in this respect - most solids show an increase in volume on melting.When water freezes, the opposite happens - there is an expansion as the hydrogen bonded structure establishes. Most liquids contract on freezing.

Polymers

Polymers like poly(ethene) - commonly called polythene - consist of very long molecules. Poly(ethene) molecules are made by joining up lots of ethene molecules into chains of covalently bound carbon atoms with hydrogens attached. There are two main types of polthene which consist of high density and low density polthene:

High density polythene

Molecules lie close together in a regular way which is almost crystalline.Because the molecules lie close together, and so the plastic is relatively strong and has a somewhat higher melting point than low density polythene. High density polythene is used for containers for household chemicals like washing-up liquid, for example, or for bowls or buckets.

Low density polythene

Low density polythene has lots of short branches along the chain. These branches prevent the chains from lying close together in a tidy arrangement. As a result dispersion forces are less and the plastic is weaker and has a lower melting point. Its density is lower, of course, because of the wasted space within the unevenly packed structure.Low density polythene is used for things like plastic bags.

Abison Logeswaran.