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400 BCE
Democritus
The Greek philosopher Democritus suggested that all matter was formed of different types of tiny discrete particles and that the properties of these particles also determined the properties of matter. His mentor, Leucippus, originally came up with the atomic theory, but it was then adopted by Democritus. The atomic theory stated that “The universe is composed of two elements: the atoms and the move.” -
384 BCE
Aristotle
Aristotle did not believe in the atomic theory and he taught so otherwise. He thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. He believed all substances were made of small amounts of these four elements of matter. -
Joseph Louis Proust
This law states that a compound is composed of exact proportions of elements by mass regardless of how the compound was created. -
Antoine Lavoisier
Lavoisier found that mass is conserved in a chemical reaction. ... But the number of hydrogen and oxygen atoms before and after the reaction is the same. -
John Dalton,
All matter is made of atoms. Atoms are indivisible and indestructible.All atoms of a given element are identical in mass and properties -
Michael Faraday
His work contributed to the fields of electromagnetism and electrochemistry. -
Henri Becquerel
He contribution to our understanding of atomic theory when he discovered the existence of radioactivity. -
J.J. Thomson
J.J. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He demonstrated that cathode rays were negatively charged. In addition, he also studied positively charged particles in neon gas. -
Marie and Pierre Curie
Curie began the systematic study of other elements to see if there were others that also emitted this strange energy. Within days she discovered that thorium also emitted radiation, and further, that the amount of radiation depended upon the amount of element present in the compound. -
Neils Bohr
Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. -
Ernest Rutherford
Rutherford overturned Thomson's model in 1911 with his well-known gold foil experiment in which he demonstrated that the atom has a tiny, heavy nucleus.Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. -
Max Planck
The theory of energy for which he was awarded the Nobel Prize in 1918. His work contributed significantly to the understanding of atomic and subatomic processes. -
Albert Einstein
It laid the basis for the release of atomic energy. In 1905 Albert Einstein formulates Special Theory of Relativity. Einstein calculates how the movement of molecules in a liquid can cause the Brownian motion. -
Robert Millikan
His oil drop experiment helped to quantify the charge of an electron, which contributed greatly to our understanding of the structure of the atom and atomic theory. -
James Chadwick
James Chadwick played a vital role in the atomic theory, as he discovered the Neutron in atoms. Neutrons are located in the center of an atom, in the nucleus along with the protons. They have neither a positive nor negative charge, but contribute the the atomic weight with the same effect as a proton.