One problem with his atomic theory was that it claimed that all matter is composed of tiny indivisible atoms. We now know that atoms are divisible and are composed of smaller, subatomic particles (electrons, protons, and neutrons). He also stated that atoms of an element are identical in mass- this is incorrect because all elements have isotopes (atoms with this same proton number but different numbers of neutrons) which means that atoms of an element do not have to have the same mass.