-
500 BCE
Prehistory
Before Atomic Theory, people believed matter was infinitely divisible. Aristotle, among others, taught that everything was composed of varying amounts of four elements: fire, water, earth, and air. -
475 BCE
The birth of atomic theory
Along comes this fine fellow named Democritus, who proposes a new theory: everything is composed of tiny, indivisible particles, whose properties depend upon their shapes. Because he had no proof, this theory went unnoticed for millennia. -
John Dalton composes his theory
A brilliant chemist by the name of John Dalton discovered several rules about the behavior of atoms through careful experimentation and diligent research. He found that atoms could not be created or destroyed, in accordance to Lavoisier's rule of conservation, and they could not change between types of atom. -
J.J. Thomson discovers the electron
J.J. Thomson, in an experiment with a cathode ray tube, discovered the electron. He calculated it's charge by seeing how it was deflected by a magnet. He proposed the "plum pudding" model of the atom -
Henri Becquerel discovers radioactivity
A scientist named Henri Becquerel noticed that elements would develop photographic film. Note that he did not discover radiation, as x-rays were discovered by Wilhelm Röntgen earlier that year. Henri Becquerel, along with Marie and Pierre Curie, studied how heavy elements like Uranium and Thorium spontaneously emitted radiation. -
Max Planck theorizes the existence of quanta
Max Planck used the idea of quanta (discrete units of energy) to explain black-body radiation (the glowing of hot objects). This theory expanded far beyond it's original purpose, and laid the foundation for the quantum model of the atom. -
Ernest Rutherford develops a new model
After discovering that alpha particles wold occasionally bounce off of a sheet of gold foil instead of passing through, Ernest Rutherford realized that atoms are mostly empty space -
Neils Bohr therorizes that electrons orbit the atom in fixed orbitals
Developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of successive orbital shells of electrons. The works of Robert Millikan, who identified the charge of the electron, and Henry Mosley, who found the relationship between the charge of the atom and the atomic number, were instrumental in calculating the positions of these orbits -
Erwin Schrödinger develops a mathematical model of the atom
Erwin Schrödinger shows that electrons exist not in orbits, but in probabilistic clouds -
Werner Heisenberg describes the uncertainty principle
Werner Heisenberg described atoms by means of formula connected to the frequencies of spectral lines. Proposed Principle of Indeterminancy - you can not know both the position and velocity of a particle. -
James Chadwick discovers the neutron
Using alpha particles he, discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron.
