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400 BCE
Democritus
According to Democritus' atomic theory, the universe and all matter obey the following principles: Everything is composed of “atoms”, which are physically, but not geometrically, indivisible. Between atoms, there lies empty space. Atoms are indestructible. -
332 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. -
Antoine Lavoisier
His results led to one of the fundamental laws of chemical behavior: the law of conservation of matter, which states that matter is conserved in a chemical reaction. -
Joseph Louis Proust
He published his Law of Definite Proportions -
John Dalton
all matter was composed of atoms, indivisible and indestructible building blocks. -
Michael Faraday
Faraday was an English scientist who contributed to the fields of electromagnetism and electrochemistry. His main discoveries include those of electromagnetic induction, diamagnetism and electrolysis -
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 -
Marie and Pierre Curie
discovered two new elements radium and polonium -
Henri Becquerel
discovered radioactivity through his investigations of uranium and other substances. -
Albert Einstein
the theory of relativity, which laid the basis for the release of atomic energy. -
Robert Millikan
determined the unit charge of the electron with his oil drop experiment at the University of Chicago. Thus allowing for the calculation of the mass of the electron and the positively charged atoms. -
Ernest Rutherford
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. -
Niels Bohr
Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. -
Max Planck
originator of the quantum theory of energy -
James Chadwick
James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced.Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron.