Molecules:

Let us take a piece of matter, for example, a drop of water and see what happens when it is sub-divided into smaller and smaller portions.

The drop is first cut in half, each half drop-let halved and so on indefinitely. The resulting smaller and smaller droplets will soon become invisible to the naked eye, but it is known what happens if the process could be carried far enough; a point would eventually be reached where the particles of water are of such a size that further sub-division would split them into the hydrogen and oxygen of which they are composed. These last minute particles of water are known as molecules and are the smallest particles of water which can exist alone and still behave chemically as water.

Every material is built-up from molecules and there are as many different molecules as there are different substances in existence.

Molecules. The molecule of an element or compound is the smallest particle of it which can normally exist separately. It consists of one or more atoms, of the same or different types joined together. The term ‘molecular structure' is used when compounds are discussed.