Atomic model timelines

Democritus

400

Please note that the year is B.C.
Democritus is a Greek philosopher who is often credited for being the developer of the atomic theory. His atomic theory stated that all matter is made up of tiny particles he named atoms. He said that these tiny part were indestructible. As he is a philosopher, he didn't use evidence to back up his claim. His theory was 'raw' and was based on thought rather than evidence, He didn't mention the structure of the atom.

Joseph Proust

Oct 10, 1799

Joseph Proust was the scientist who came up with The Law of Definite Proportions which stated that every substance has the same amount of elements combined in the same amount of mass. His discovery really paved the way and acted as the basis for every scientists' atomic theory. He experimented this by combining two elements which would react finding that the third element would always have the same composition.

John Dalton

Oct 10, 1803

Two thousand years after Democritus' idea, a man named John Dalton backed Democritus' idea with evidence and even contributed to the atomic model on his own. He said that all atoms are indivisible and indestructible. He also said that all atoms from an element have the same properties and have the same mass. He also said that in a chemical reaction, atoms are rearranged. He experimented these theories by using chemical analysis. Other chemists and he analyzed substances to give evidence.

J.J Thomson

Oct 10, 1898

J.J Thompson was the scientist who first discovered the electron. His discovery really impacted the atomic theory and paved the way for other theories and ideas. He believes that the electrons serve to make the atom electrically neutral to balance with the protons. He backed his theory by doing the Cathode Ray experiment. He found the electrons by finding out the the ratio of charge and mass did not match.

Albert Einstein

Oct 9, 1906

Although he is more credited with his theory of relativity, Alber Einstein actually made a huge contribution to the Atomic Theory. A theory that he had was that all atoms can be converted to energy. This discovery was what essentially led to the atomic bomb. He tested his theory by devising a formula which calculated the size and mass of atoms. He tested it on a sugar molecule dissolved in water. He also noticed that the movement of molecules can be observed even though the molecules can not.

Robert Millikan

Oct 10, 1909

Along with his science lab partner Harvey Fletcher, Robert Millikan discovered that the charge of electrons is negative. He discovered this by conducting the oil drop experiment. He did this by creating a tube where the top and bottom is electrically charged plates. He then attached x rays to the oil droplets to test if their charge. He found out that electrons are negative.

Niels Bohr

Oct 10, 1913

Niels Bohr's was the scientist who discovered that energy levels have a specific amount of electrons. He also stated that an atom emits electromagnetic radiation when it exchanges electrons with another atom. His theory was based on the work of scientist Ernest Rutherford. This is why his model is called the Bohr-Rutherford model rather than the Bohr model.

Arnold Sommerfeld

Oct 10, 1915

Arnold Sommerfield was a physicist who extended Bohr's atomic theory and stated that the electrons which surrounded the nucleus moved in circular or eliptical motions. He found this after he studied and experimented on the atomic spectra

James Chadwick

Oct 10, 1923

Even though the atom was discovered ages ago, the neutron was not discovered until recently, by a scientist named James Chadwick. He discovered the neutron by taking a radioactive material and hounded the alpha rays. The radioactive element emitted rays which showed particles that have the same mass as protons. Chadwick named them neutrons.

Ernest Rutherford

Oct 10, 1932

Ernest Rutherford was the scientist who discovered that every atom has a nucleus. He also discovered that atoms are made up of mostly empty space. He discovered that by shooting alpha particles at thin gold foil. He noticed that many particles just went through but most deflected and went back or went at a certain angle. He discovered that the alpha particles were hitting the nucleus.

Lise Meitner

Oct 10, 1938

Lise Meitner is credited as being the mother of the atomic bomb. She was the first scientist to discover nuclear fission and that an atom can be split. She discovered this when she split uranium atoms into two or more parts. Although she is not widely credited for coming up with this idea as she is a woman and at the time it was not socially acceptable for a woman to be a scientist so German scientist Otto Han is credited with this discovery.

Murray Gell-Mann

Oct 10, 1964

Murray Gell-Mann is an American Physicist who discovered quarks. He believed that quarks are tiny particles which make up subatomic particles such as protons and electrons. His experiment was a very complicated experiment where he needed to find hadrons to make sure of the existence of quarks.