The History of the Atomic Theory

Democritus' Discovery

429 BCE

Democritus and his mentor Leucippus discovered the very first atomic theory.Their theory stated that all things were made up of atoms, space lies between atoms, and atoms were indivisible.Also stating that atoms are and will always be in motion and that there is an infinite amount of atoms with different shapes and structures. They also believed that the solidness of a material corresponded to the shape of the atom.

Democritus' Discovery

429 BCE

Democritus and his mentor, Leucippus, discovered the very fist atomic theory. Their main discoveries were that all things are made of atoms, space lies between atoms, atoms are indivisible, and that there is an infinite amount of atoms. Democritus was also one of the first to note the shape and connectivity of an atom.

John Dalton's Findings

Oct 21, 1803

Dalton's findings were very similar to that of Democritus'. He found that all matter is made of atoms, atoms are indivisible and indestructible. Dalton's major contributions were:
-Elements are made up of many atoms.
-Atoms of the same element are identical and atoms of different elements are different in shape and size.
-In chemical reactions, atoms are combined, separated, or rearranged.
-Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

J.J. Thomson

Apr 30, 1897

Thomson was the first to suggest that there was a subatomic particle 1,000 times smaller than an atom. He made his discovery through the study of cathode rays in which he found that the cathode rays traveled much faster than expected rays with particles the size of an atom. His main contribution was the finding that particles can be negatively or positively charged.

Ernest Rutherford

Aug 25, 1908

Approximate year, incorrect date. Rutherford, who worked with Thomson at one point, discovered the Rutherford model. Which was a model of the atom that proved Thomson's plum pudding model incorrect. Rutherford also made contributions to the discovery of two types of radiation: alpha ray and beta ray. Although, Rutherford's most famous work is the gold foil experiment, which showed the nuclear nature of atoms by deflecting alpha particles passing through a thin gold foil.

Robert Millikan

Aug 25, 1913

Approximate year, incorrect date.Millikan with the help of his then graduated student Harvey Fletcher discovered the charge and mass values of Thomson's charge to mass ratio of an electron theory. They also won a Nobel Prize for Physics in 1923 because of this discovery. Millikan also performed an oil drop experiment in which he measured the charge on a single electron which later on helped with the discovery above.

Neils Bohr

Aug 25, 1922

Approximate year, incorrect date. Bohr's greatest contribution is the development of the atomic model. Bohr was the first to discover that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element. Bohr also created the basis of the Quantum Theory, explaining that no matter how you look at an electron, all understanding of its properties must be rooted in empirical measurement.

Werner Heisenburg

Aug 25, 1925

Approximate year, unknown date. Heisenberg's major contribution is his uncertainty principle. Which says that electrons do not travel in neat orbits. He also discovered that all electrons contain photons. Heisenberg also won the Nobel prize for his uncertainty principle.

James Chadwick

Aug 25, 1932

Approximate year, unknown date. Chadwick's biggest discovery was the discovery of neutrons. Chadwick performed an experiment with Beryllium, which resulted in the release of a very energetic stream of radiation. Chadwick further investigated this and discovered Rutherford's particle. This discovery resulted in him receiving the Nobel Prize in 1935.

Erwin Schrodinger

Aug 25, 1933

Approximate year, unknown date.Schrodinger's major contribution was the making of the Quantum Theory. Quantum Theory is a theory of matter and energy based on the concept of quanta. In order to understand the Quantum Theory, Schrodinger made up a complete new math. Schrodinger also discovered the quantum mechanical model which is an equation that describes the likelihood of finding an electron in a certain position.