The atomic molecular theory  (Page 15/4)

Foundation

There are over 18 million known substances in our world. We will begin by assuming that all materials aremade from elements , materials which cannot be decomposed into simpler substances. We will assume that wehave identified all of these elements, and that there a very small number of them. All other pure substances, which we call compounds , are made up from these elements and can be decomposed into these elements. For example, metalliciron and gaseous oxygen are both elements and cannot be reduced into simpler substances, but iron rust, or ferrousoxide, is a compound which can be reduced to elemental iron and oxygen. The elements are not transmutable: one elementcannot be converted into another. Finally, we will assume that we have demonstrated the Law of Conservation of Mass .

Law of conservation of mass

Goals

The statements above, though correct, are actually more vague than they might first appear. For example, exactly what do we mean when we say that all materials are made from elements? Why is it that the elements cannot be decomposed? What does it mean to combine elements into a compound? We want to understand more about the nature of elements and compounds so we can describe the processes by which elements combine to form compounds, by which compounds are decomposed into elements, and by which compounds are converted from one to another during chemical reactions.

One possibility for answering these questions is to assume that a compound is formed when indestructible elements are simply mixed together, as for example, if we imagine stirring together a mixture of sugar and sand. Neither the sand nor the sugar is decomposed in the process. And the mixture can be decomposed back into the original components. In this case, though, the resultant mixture exhibits the properties of both components: for example, the mixture would taste sweet, owing to the sugar component, but gritty, characteristic of the sand component.

In contrast, the compound we call iron rust bears little resemblance to elemental iron: iron rust does not exhibit elemental iron's color, density, hardness, magnetism, etc. Since the properties of the elements are not maintained by the compound, then the compound must not be a simple mixture of the elements.

We could, of course, jump directly to the answers to these questions by stating that the elements themselves are comprised of atoms: indivisible, identical particles distinctive of that element. Then a compound is formed by combining the atoms of the composite elements. Certainly, the Law of Conservation of Mass would be easily explained by the existence of immutable atoms of fixed mass.