The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter was composed of small, finite particles that they called atomos, meaning “indivisible.” Later, Aristotle and others came to the conclusion that matter consisted of various combinations of the four “elements” — fire, earth, air, and water — and could be infinitely divided. Interestingly, these philosophers thought about atoms and “elements” as philosophical concepts, but apparently never considered performing experiments to test their ideas. 

The Aristotelian view of the composition of matter held sway for over two thousand years until English scientist John Dalton revolutionized chemistry with his hypothesis that the behavior of matter could be explained using an atomic theory. First published in 1807, many of Dalton’s hypotheses about the microscopic features of matter are still valid in modern atomic theory. 

Here are the postulates of Dalton’s atomic theory:

1. Matter is composed of exceedingly small particles called atoms. An atom is the smallest unit of an element that can participate in a chemical change.
2. An element consists of only one type of atom, which has a mass that is characteristic of the element and is the same for all atoms of that element. A macroscopic sample of an element contains an incredibly large number of atoms, all of which have identical chemical properties.
3. Atoms of one element differ in properties from atoms of all other elements.
4. A compound consists of atoms of two or more elements combined in a small, whole-number ratio. In a given compound, the numbers of atoms of each of its elements are always present in the same ratio
5. Atoms are neither created nor destroyed during a chemical change but are instead rearranged to yield substances that are different from those present before the change.

Dalton used three laws of chemical reactions as a basis for his theory: (1) The Law of Conservation of Mass, (2) The Law of Definite Proportions, and (3) The Law of Multiple Proportions. Dalton’s atomic theory provides a microscopic explanation of the many macroscopic properties of matter. 

The law of conservation of mass was discovered by the French chemist, Antoine Lavoisier. It states that atoms are neither created nor destroyed during a chemical change; the total mass of matter present when matter changes from one type to another remains constant.

Dalton also knew of the experiments of French chemist Joseph Proust, who demonstrated that all samples of a pure compound contain the same elements in the same proportion by mass. This statement is known as the law of definite proportions or the law of constant composition. The suggestion that the numbers of atoms of the elements in a given compound always exist in the same ratio is consistent with these observations. 

Dalton also used data from Proust, as well as results from his own experiments, to formulate another interesting law. The law of multiple proportions states that when two elements react to form more than one compound, a fixed mass of one element will react with masses of the other elements in a ratio of small, whole numbers.

Dalton’s theory provided a framework that was later expanded to demonstrate that the atom is composed of subatomic particles and that atoms of the same element can differ in mass, known as isotopes.

This text is adapted from Openstax, Chemistry 2e, Section 2.1: Early Ideas in Atomic Theory.