Classification of Matter

As humans, we like to try and make sense of the world around us. The world is full of very complex ideas that cannot be studied at face value and need to be broken down in some sort of logical order. That's where classification comes in.

Classification

To make patterns easier to see we often classify things into similar groups. Into each group we put items that have similar properties. For example, you should be familiar with the concepts of birds. Immediately on reading the word bird, you should be thinking of animal which has feathers, a beak and lays eggs.

If I were to say the word fish, you would now be thinking about an animal which has fins, scales and lives in the water. The natural world did not define these groups, or classifications. We saw that certain animals have similar characteristics, and categorised them to make it easier for us to talk about them.

The same is true for study of other subjects, and chemistry is no different. One of the most basic classification we use is for matter.

Pure substances

Elements

First of all, let's look at something like an iron bar. If we exam the iron bar closely enough we would find it is made of only iron atoms. All of the iron atoms would look identical and have the same properties as the larger iron bar. There are no other atoms of any other type in this bar. When we have a substance made of only type of atom, then we call this substance an element. It is worth noting that even though there are no other atom types present, the atoms of this element can and do bond to one another, with the exception of the noble gases.

Compounds

Now chemistry would not be very interesting if it were just composed of elements, and in fact you and I would not exist! Elements are not often found in nature. A compound is formed when atoms from 2 of more elements are joined together in a tight bond. It is usually pretty difficult to break these atoms apart from one another, and it is not possible by physical means. The way that these atoms are bonded together will vary their chemical and physical properties often quite considerably. For example table salt, which you eat, is composed of sodium and chlorine. The element sodium is a highly reactive metal that reacts with water and even catches fire. The element chlorine is a poisonous green gas that was used during World War I to kill enemy troops. Combine those two deadly elements together and you get the compound sodium chloride, which is not only non-toxic, but is essential to life (meaning that you can die from not having enough of it).

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