Atoms and the Atomic Theory

In 1808, an English scientist and schoolteacher, John Dalton, developed the atomic model of matter that underlies modern chemistry.  Three of the main postulates of modern atomic theory, all of which Dalton suggested in a somewhat different form, are stated below.

1. An element is composed of tiny particles called atoms.  All atoms of a given element show the same chemical properties.  Atoms of different elements show different properties.
2. In an ordinary chemical reaction, atoms move from one substance to another, but no atom of any element disappears or is charged into an atom of another element.
3. Compounds are formed when atoms of two or more elements combine.  In a given compound, the relative numbers of atoms of each kind are definite and constant.  In general, these relative numbers can be expressed as integers or simple fractions.

On the basis of Dalton's theory, the atom can be defined as the smallest particle of an element that can enter into a chemical reaction.