Interest in and study of the atom dates back to Ancient Greece, but it was not until the nineteenth century that the first theories began to develop. The main ones are:
The theory by English chemist John Dalton, which established that matter was made up of indivisible, equal elementary particles.
The atomic model by English scientist J.J. Thomson, where he suggests the existence of electrons and refutes the theory of his predecessor regarding the indivisibility of the atom. His model is known as the plum pudding model, and it explains that atoms are masses with positive and negative charges.
The theory of the nuclear atom developed by New Zealand scientist Ernest Rutherford, who discovered that most of the mass of an atom is located in its nucleus, with negative charges orbiting around it.
The atomic model by Danish physicist Niels Henrik David Bohr, proposing that an atom’s electrons are in orbit at some distance from the nucleus and that they can jump from one orbit to another with just the right amount of energy.
Quantum mechanical model of the atom, developed by physicists Werner Heisenberg, Louis de Broglie, and Erwin Schrödinger. It states that electrons behave like standing waves orbiting in an electron cloud.
Finally, in 1932, scientist James Chadwick discovered the neutron, thus completing the model of the atom known today.