Each of the elements has been given a one or two letter symbol to make them easy to write. The element helium is given the symbol He. Clearly there are more than 118 substances in the universe. Dalton knew that in addition to the 'pure' elements, there were many substances that were formed by the chemical combination of two or more elements, these substances were called compounds. Most of the substances we come into contact with in everyday life are compounds.
Water is a good example.
If we can imagine a single particle of water, it might look like the picture to the right. Each of the colored circles represents an atom, and the entire picture represents a single molecule of water. The idea that compounds have defined chemical formulas led to the law of definite proportions . This law states that when elements react to form specific compounds, the reaction always takes place in a defined whole number ratio. For example, hydrogen reacts with oxygen to form water.