Democritus develops the concept of the atom. In about 442 BC Democritus, a Greek philosopher, develops the idea of the existence of the atom, stating that atoms are indivisible and indestructible.

John Dalton uses experimental evidence to form the atomic theory.
He suggests that the atom is like a billiard ball through his "billiard ball" model.

J. J. Thomson discovers the electron as he operates with cathode ray tubes. He proposes that the atom is like a plum pudding with his plum pudding model. He understood the atom to be like the plum pudding, with a positive charge and electrons (the plum in the pudding) distributed in it.

Ernest Rutherford discovers that the atom has a positively charged nucleus that makes up most of its mass, with electrons revolving around it. He supports his reasoning with his gold foil experiment. In this experiment, he throws alpha particles at gold foil and notices that although most of them pass through, some of them deflect. Thus he concludes that there must be a dense center in the atom (nucleus) that has a positive charge since it repels the alpha particles.

Niels Bohr expands on Rutherford's ideas. His model is known as the "Energy Level Model." His model also contains the nucleus. He suggests that electrons move in circular orbits (energy levels) that get bigger the farther they are around the nucleus.

Like Bohr's model, Schrodinger's model contains a nucleus. Unlike the Bohr Model though, the Quantum Mechanical Model explains that electrons don't have a fixed location around the nucleus. It shows the idea of the "uncertainty principle" (it's impossible to know the exact location and momentum of an atom simultaneously). The Quantum Mechanical Model has an electron cloud around the nucleus. The more dense areas of the cloud represent places where electrons are more likely to be found.