Failures
There was one problem with Dalton's atomic theory. It was that it claimed that all matter is composed of tiny indivisible atoms. Now, in 2013, atoms are divisible and are composed of smaller, subatomic particles called electrons, protons, and neutrons. He stated that atoms of an element are identical in mass. This is not correct because all elements have isotopes -- atoms with this same proton number but different numbers of neutrons. This means that atoms of an element do not have to have the same mass.