Democritus first developed the idea of invincible particles that made everything up. He named them "atomos", meaning indestructable. Him and Aristotle disagreed a lot on this subject, because Democritus' work contradicted Aristotle's.
Period: 400 BCE to
Theoretical idea of the Atom
This was when the idea of the atom was just a thought that was suggested by Democritus. There really was no picture of what the atom looked like at this time.
Aristotle held the belief that existence was made out of five basic things: earth, water, air, fire, and aether. This contributed to the major idea that everything was made out of fundamental objects.
Lavoisier performed several experiments and determined that there were several more fundamental substances and the specific recipes for different other substances(compounds).
Dalton did lots of work on that atoms had different compositions and masses. He determined everything could be broken down into these elements. He was the first one to expand on the Greek idea Democritus showed.
Dalton's model said that all atoms had different masses and all things were made of different combinations of these atoms. His idea was that atoms were just different sized spheres.
Henri Bequerel focused a lot of magnetivity and radioactivity and introduced the concept that all atoms shared some characteristics, with some having more of that characteristic than others. He introduced the concept of radioactivity to the scientific community
J.J. Thomson discovered the electron. This is very important because this was the first subatomic particle to ever be found. This brought the introduction of a new model into the chemical world and revolutionized what people thought the atom looked like.
The Thomsonian Model of the atom
The Thomsonian model of the atom said that the atom was comprised of a dense ball in the center with electrons flying around it. It survived until the Bohr model.
The Curies, Marie and Pierre made massive advances in discovering new elements, but they also made huge advances in the field of radioactivity. They discovered the nature of radioactivity and what it does. They were Nobel Prize winners.
Planck discovered a constant called "Planck's Constant" used in calculated information about electromagnetic energy. Plancks were named after him.
Bohr knew Rutherford and also did work on the atom. He made a model which emphasized the small size of a nucleus yet its density, and the existence of particles inside nucleus. There was no proved existence of neutrons yet.
The Bohr model is the model most likely to be found in a regular science textbook today. However, it does portray the electron clouds more simplistically than they actually are. It has been replaced with the Heisenberg model.
Henry Mosely worked on the justification of the organization of the periodic table: its families and atomic numbers. He did this and probably would have been awarded the Nobel Prize if he hadn't been killed in combat.
Ernest Rutherford developed (along with Heisenberg) the current model of the atom. He discovered the concept of the half-life, a concept in radioactivity on how long atoms exist. He also showed radioactivity causes one atom to turn into another.
Schrödinger worked a lot on quantum mechanics. His most famous contribution is his famous "Schrödinger's cat" experiment. He forumlated the expression of waves.
Robert Millikan studied about how the electron had a charge. He discovered electrons themselves had electric charges to them, the elementary forces
Heisenberg was a strong contributer to the ideas of quantum mechanics, but he also developed the idea of how electron paths existed. He said that we only know probabilities about trajectories and positions of electrons.
The Rutherford-Heisenberg Model
The Heisenberg Model is the model of the atom that portrays atoms as having disjointed and chaotic fields of electrons. It replaced the Bohr model and remains to this day.
Chadwick discovered the neutron, the final sub-atomic particle. This changed the way we saw atomic mass forever, and we use this concept in our understanding of atoms and why things are the mass they are. He was the most recent big development in the atomic model.