The main points of Dalton’s atomic theory, an explanation of the structure of matter
in terms of different combinations of very small particles, are given by the following
1. All matter is composed of indivisible atoms. An atom is an extremely small
particle of matter that retains its identity during chemical reactions.
2. An element is a type of matter composed of only one kind of atom, each atom of
a given kind having the same properties. Mass is one such property. Thus, the
atoms of a given element have a characteristic mass.
3. A compound is a type of matter composed of atoms of two or more elements
chemically combined in fixed proportions. The relative numbers of any two kinds
of atoms in a compound occur in simple ratios. Water, for example, a compound
of the elements hydrogen and oxygen, consists of hydrogen and oxygen atoms in
the ratio of 2 to 1.
4. A chemical reaction consists of the rearrangement of the atoms present in the
reacting substances to give new chemical combinations present in the substances
formed by the reaction. Atoms are not created, destroyed, or broken into smaller
particles by any chemical reaction.
Today we know that atoms are not truly indivisible; they are themselves made up of
particles. Nevertheless, Dalton’s postulates are essentially correct.