John Dalton, an English schoolteacher, formulated these four postulates of atomic theory during 1803-1807:
- Each element is composed of extremely small particles called atoms.
- All atoms of a given element are identical; the atoms of different elements are different and have different properties.
- Atoms of an element are not changed into different types of atoms by chemical reactions; atoms are neither created nor destroyed in chemical reactions.
- Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.
These postulates then explain some more laws of atomic theory:
- The law of constant composition: In a given compound, the relative numbers and kinds of atoms are constant.
- The law of conservation of mass (matter): The total mass of the reactants in a chemical reaction is equal to the total mass of the products.
- The law of multiple proportions: If 2 elements combine to form different compounds, the ratio of the two elements in the two compounds will be a whole number.
This theory, however, is slightly flawed. Each atom, in fact, is composed of even smaller particles, protons, neutrons, and electrons. In turn, these are composed of quarks. However, Dalton's theory essentially remains correct and is used in one form by modern Chemists.
Source: Chemistry: the Central Science, by Brown, LeMay, and Bursten.