Democritus’ atomic theory stated that all matter is made up small indestructible units he called atoms.

Isaac Newton theorized a mechanical universe with small, solid masses in motion.

John Dalton proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together.

J.J. Thomson determined the charge to mass ratio of electrons.

discovered alpha, beta, and gamma rays in radiation.

proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.

proposed that electrons have a wave/particle duality.

Werner Heisenberg contributed to the atomic theory by including quantum mechanics, the branch of mechanics, based on quantum theory, used for interpretating the behavior of elementary particles and atoms.

used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom.

discovered neutrons, particles whose mass was close to that of a proton.