Atomic theory (by Dalton) summarized:
1. All matter is composed of extremely small particles called atoms
2. Atoms of a given element are identical in size, mass, and other properties.
3. Atoms cannot be subdivided, created, or destroyed in chemical reactions.
4. Atoms of different elements can combine in simple, whole number ratios to form chemical compounds.
5. In chemical reactions, atoms are combined, seperated, or rearranged.
Hope this helps (attachment is all about the theory that I found).