The existence of isotopes violates one of the original ideas of dalton's atomic theory. which one? matter is composed of exceedingly small particles called atoms. an atom is the smallest unit of an element that can participate in a chemical change. an element consists of only one type of atom, which has a mass that is characteristic of the element and is the same for all atoms of that element. atoms of one element differ in properties from atoms of all other elements. a compound consists of atoms of two or more elements combined in a small, whole-number ratio. in a given compound, the numbers of atoms of each of its elements are always present in the same ratio. atoms are neither created nor destroyed during a chemical change, but are instead rearranged to yield substances that are different from those present before the change.