Dalton’s atomic theory
Democritus first suggested the existence of the atom but it took almost two millennia before the atom was placed on a solid foothold as a fundamental chemical object by John Dalton (1766-1844). Although two centuries old, Dalton's atomic theory remains valid in modern chemical thought.
Dalton's Atomic Theory
1) All matter is made of atoms. Atoms are indivisible and indestructible.
2) All atoms of a given element are identical in mass and properties
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) A chemical reaction is arearrangement of atoms.
Modern atomic theory is, of course, a little more involved than Dalton's theory but the essence of Dalton's theory remains valid. Today we know that atoms can be destroyed via nuclear reactions but not by chemical reactions. Also, there are different kinds of atoms (differing by their masses) within an element that are known as "isotopes", but isotopes of an element have the same chemical properties. Many heretofore unexplained chemical phenomena were quickly explained by Dalton with his theory. Dalton's theory quickly became the theoretical foundation in chemistry.
Drawbacks of Dalton's atomic theory
Dalton's atomic theory failed to explain the dissimilarities in the properties of different allotropes of an element. This theory states that elements must combine in simple, whole-number ratios to form compounds, which is not necessarily true.
The English physicist J. J. Thomson (1856–1940) disproved Dalton's idea that atoms are indivisible. When elements were excited by an electrical current, atoms break down into two parts.
In both cases, the silicon to oxygen ratio is the same, thus conforming to the law of definite proportions.