He stated that all objects are made up of little indestructible and indivisible things called atoms.
Indivisible, Solid Sphere Model
Aristotle said that all things are made up of form and matter which can only be changed with growth, decay, or motion, and also they are made up of the four main elements, water, earth, air, fire.
John Dalton's atom theory
Dalton created a scientific theory based off of and including Democritus' atom idea: All elements are created from indestructible particles called atoms. His theory was that when atoms of different elements are joined, separated, or rearranged chemical reactions result.
J.J Thompson's studies
When high energy was moved from the negative cathode plate through a vacuum sealed container to a positive plate, it created a beam of electrons. This resulted in the Thompson/Plum Pudding atomic model.
Plum (raisin) Pudding model
While conducting his gold foil alpha particle scattering experiment, Rutherford observed a flash of light on different places of the phosphorus screen because instead of the alpha particles going straight through the foil with little to no deflections, the beam deflected in multiple directions leading him to determine that atoms had a nucleus.
Radiation experimentation with black body (object that absorbs all electromagnetic radiation) led to his creation of the Planck Postulate that described the stability of electrons.
By using the very famous Oil Drop Experiment, Millikan found the charge of an electron to be negative. It's approximately described as 1/1840 the mass of a Hydrogen atom.
Using Planck's Quantum Theory and Rutherford's study of the nucleus in atoms, Bohr came to the conclusion that atoms have a heavily charged nucleus with the electrons.
A French physicist, Louis studied the stability of the atom. He determined that all forms of energy exist in both particles and wavelengths, establishing the wave-particle duality of nature.
Based off of Louis de Brogile's work, he came up with the quantum mechanical atomic model which shows the orbitals around the nucleus in constant motion.
He realized that there are many things about atoms which cannot be exactly determined or calculated. Mainly determining an electrons location and velocity at the same time. This is called the Uncertainty Principle.