Some Basic Terms in Chemistry

Atoms

An atom is the smallest particle of matter that you can get by chemical means (although it is possible to split an atom using nuclear forces). It is the basic unit of matter from which all things are made. The Greeks first proposed that everything is made of atoms ('atom' is a Greek word). In the 18th century, John Dalton put forward his theory of atoms to account for elements and compounds. He believed that atoms were hard microscopic spheres, like tiny billiard balls. We now know that atomic structure is much more complicated.

Molecules

A molecule is two or more atoms joined together chemically. If you think of an atom as being like a plastic Lego brick, then a molecule would be two or more bricks joined together to form a model.

Element

Any substance made from only one kind of atom is called an element. The atoms may be separate (as in the element Argon) or joined in molecules (as in the element Carbon), but there must be only one type of atom present. There are just over 100 elements known to science. They are listed in the Periodic Table.

The example on the right shows a gas consisting of molecules of an element. There is only one type of atom present (hence it is an element), and the atoms are chemically joined together (in pairs in this case - hence it consists of molecules).

Compound

Any substance made from more than one kind of atom chemically joined together is a compound. They are often dissimilar to the elements that make them up e.g. hydrogen gas burns in oxygen gas to form water, a liquid at room temperature.

Mixture

Two or more types of atom mixed together but not chemically joined e.g. air which consists mainly of nitrogen gas and oxygen gas mixed together. It is also possible to have a mixture of compounds, i.e. when one compound is mixed with another but does not chemically react with it. An example is a mixture of salt and sand - salt is a compound (sodium chloride) and sand is itself a mixture of compounds of silicon.

Solids, Liquids and Gases

These are the three states of matter. We are all familiar with solids - most of the things that we see around us are solid. When you heat a solid sufficiently, it either decomposes in some way (burns or breaks up), or it can melt, which is the process of turning into a liquid. Liquids can flow freely, and adopt the same shape as any container in which they are placed. When a liquid is heated, provided it doesn't undergo a chemical reaction of some sort, then it will boil, when it turns into a gas.

The process happens in reverse when you cool a gas. Firstly, it condenses into a liquid, and then, if you carry on cooling the liquid, it freezes into a solid. Here are the processes summarised in a diagram:

	Melting		Boiling
Solid		Liquid		Gas
	Freezing		Condensing

Of course, not all substances follow this simple pattern. For instance, if you heat wood, it will burn rather than melt - there's no such thing as molten wood! Even if you heated it without any oxygen present, so that it couldn't burn, then it would eventually decompose.

Liquids and gases are together known as fluids, which means that they are capable of flowing.

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