All matter is composed of very small particles called atoms. All atoms of a given element are identical. Atoms cannot be created, destroyed, or subdivided. In chemical reactions, atoms combine with or separate from other atoms. In chemical reactions, atoms combine with each other in simple, whole-number ratios to form combined atoms.
JJ Thomson later discovered that atoms are not indivisible, and they can in fact be subdivided into parts (ex: protons and electrons)