1. Elements are composed of atoms that are indestructible
2. All atoms of a given element are identical; same size/mass/chemical properties
3. Atoms of 1 element are different from the atoms of other elements
4. Compounds are composed of atoms with more than 1 element. The relative number of atoms for each element are of a given compound are always going to be the same.
(Extra one) 5. Chemical reactions are only ever going involve the rearrangement of the atoms. Atoms are not created/destroyed during the chemical reactions. (Law of Conservation of Mass: nothing can ever be created or destroyed.)