In 1803, John Dalton developed a hypothesis that atoms are a type of solid sphere. He has four primary statements in his theory, including that atoms are responsible for making up all matter, that atoms join to form compounds, that within the same mass, all atoms are identical and that atoms are indivisible.
While Dalton's theory is quite revolutionary for its time, it does have a few issues. It does not take into consideration the subatomic particles, including protons, electrons and neutrons. It only provides a conceptual definition of what an atom is, and it does not provide a scientific view of an atom.