We seem obliged to think of atoms as very minute material particles, which either normally are, or under definite conditions may be, associated with electrically charged particles very much lighter than themselves, all of which are identical, whatever be the atoms with which they are associated or from which they are produced.
In their study of different kinds of matter, chemists have found it very helpful to place in one class those substances which they have not been able to separate into unlike parts.

They have distinguished this class of substances from other substances, and have named them _elements_.

The expression _chemical elements_ is merely a summary of certain observed facts.

For many centuries chemists have worked with a conceptual machinery based on the notion that matter has a grained structure.

For more than a hundred years they have been accustomed to think of atoms as the ultimate particles with which they have had to deal.