Scroll down about half way in this article where wikipedia gives the four main parts of Dalton's theory of the atom.
http://en.wikipedia.org/wiki/John_Dalton
The model that describes an atom as the smallest piece of matter is Dalton's atomic model. This model was proposed by John Dalton in the early 19th century. According to Dalton's model, atoms are indivisible and indestructible particles that cannot be further divided into smaller pieces.
