Evolution of Atomic Theory

Democritus

350

Building on the ideas of his teacher, Leucippus, Democritus developed the first true atomic model and theory, stating that all matter could be broken down into one singular indivisible piece, or as he called it "atomos", literally meaning uncuttable. He also believed that there were an infinite number of these indestructable atoms and that they varied in size, shape, and weight. Furthermore, he concluded that the space in between these atoms was something called "void" which is empty space.

Aristotle

400

To counter Democritus's atomic model, Aristotle supported the 4 elements model created by Empedocles. This theory stated that there were 4 elements (fire, water, air, and earth) which could be determined by 2 different qualities (hot, cold) (dry, wet). Although most of this was completely false, Aristotle's theory could be seen as a basis for the states of matter and the periodic elements.

John Dalton

Oct 21, 1803

Through his research with different types of gases in the early 19th century, Dalton revisited Democritus's atomic theory and created 5 main points which can be summarized as such:
1. All elements are made of atoms.
2. Atoms of the same elements have the same size, shape, and properties, and vice versa.
3. Atoms cannot be broken down, created, or destroyed.
4. Atoms chemically combine in a simple whole number ratio to form compounds.
5. Atoms are rearranged during a chemical reaction.

Eugen Goldstein

Oct 31, 1886

Using a cathode ray tube (CRT), Goldstein discovered rays with properties opposite of cathode rays which he called athode or canal rays. These "positively charged" rays would lead to the development of the proton.

J.J. Thomson

Apr 30, 1897

Through his experiments with the Cathode Ray Tube, JJ Thomson was able to determine negatively charged particles in the cathode rays which are now called electrons. Thus, he is credited with the discovery of the first subatomic particle and also proposed the plum pudding model, or a more modern analogy would be the cereal model where you have a positively charged container like milk in cereal holding all of these negatively charged "corpuscles" like the actual cereal inside the milk.

Max Planck

Oct 19, 1900

Planck is best known for being the pioneer of quantum theory. Through his experimentation on black body radiation, he was able to determine the equation E=hv with h being known as Planck's constant, representing the quantum energy of the photon, v being the frequency of radiation, and E being the energy produced.

Robert Millikan

Nov 2, 1909

By building upon Thomson's discovery of the electron and its mass to charge ratio in 1897, Millikan knew that by finding one, he would be able to find the other. This led him to team up with Harvey Fletcher in 1909 to create the famous Oil Drop experiment in order to find the charge of an electron and therefore its mass.

Ernest Rutherford

Nov 2, 1911

Through the gold foil experiment, Rutherford managed to disprove Thomson's "Plum Pudding" Model and create his own Rutherford model which consisted of a positively charged center (nucleus) randomly surrounded by electrons. By shooting alpha particles with a positive charge into the gold foil, Rutherford noticed that almost all of the particles went straight through, but some were reflected back. He theorized that since same charges repel, there must be a positively charged center in these atoms.

Niels Bohr

Nov 2, 1922

Bohr expanded upon Rutherford's model to create a new "planetary" model. Bohr believed that around this positively nucleus, electrons were in orbit like the planets around the Sun. Additionally, Bohr believed that with each ring came an electron "shell" which basically meant that each ring or shell could only hold a certain amount of electrons before needing a new ring. This helped to solve much confusion surrounding properties of certain elements in the Periodic Table.

Louis de Broglie

Nov 2, 1923

While there was no official experiment until 1927, deBroglie made a hypothesis in 1923 that electrons, just like light, acted as both a particle and a wave. This was a truly amazing discovery in the realm of quantum theory.

Werner Heisenberg

Nov 2, 1927

Heisenberg, the father of "Quantum Mechanics", proposed a law called the "Uncertainty Principle" where the greater the precision of the location of a particle, the less the precision of finding where the particle went and vice versa. Hence, it is impossible to know where the particle is and its velocity, you can only find one or the other. This is one of the most basic themes of Quantum Mechanics and is truly what lays down its foundation.

Erwin Schrodinger

Nov 2, 1930

Combining the ideas of Bohr's planetary model with Heisenberg's quantum mechanics and principle of uncertainty, Schrodinger proposed a radical new atomic model: The Electron Cloud. Although electron shells most likely went around the nucleus, Heisenberg showed that it was impossible to determine the exact location of an electron. However, Schrodinger reasoned that we can predict where the electrons will probably be. In essence, the greater the distance, the lower the probability and vice versa.

James Chadwick

Nov 2, 1932

By removing protons from paraffin wax, Chadwick proved that there was a third fundamental particle, the neutron. With neither positive nor negative charge, the neutron was versatile enough to form new elements through radioactive decay and not be repelled by protons in the nucleus.