DemocritusAtoms have their own size and shape. Atoms are "uncuttable", they are the smallest particles of matter.
John DaltonEach element has its own unique property. All matter are made up of tiny particles called atoms. All atoms of a given element are alike, but are different from the atoms of any other elements. Compounds are formed when atoms of different element combine in fixed proportions. A chemical reaction involves the rearrangement of atoms, not a change in atoms themselves. Atoms can't be created nor destroyed, only changed.