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100
Democritus of Adbera (450BC)
Democritus created the first atomic theory in human history circa 450BC. Although modern science has disproved several aspects of the theory, his theory claimed that all atoms of an element are identical and that the shape of atoms determined their properties. -
100
Aristotle ( 384 BC – 322 BC )
Aristotle proposed a theory that claims all matter consists of only earth, air, water, and fire, and that each of these elements could have qualities of dryness, hotness, coldness, and wetness. Although modern science disproves this theory, Aristotle’s theory was believed for about 2,000 years. -
Apr 1, 1400
Alchemists
The Alchemists during the Late Middle Ages determined that there are only three elements: mercury, salt, and sulfur. Additionally, Alchemists attempted to use equipment to experiment. -
Benjamin Franklin
Benjamin Franklin proved that there were positive and negative charges. He proposed that “electrical fluid” could travel to and from differently charged substances. -
Antoine Lavoisier
Antoine Lavoisier introduced the Law of Conservation of Mass to the scientific community in 1772. He discovered this by burning a diamond in a jar, which he proved was the same weight before and after the diamond’s combustion. -
Joseph Proust
Joseph Proust proposed the Law of Constant Composition in 1799. His theory claimed that a chemical must always have the same ratio of elements by mass. -
John Dalton
In 1803, John Dalton proposed a theory that claimed five ideas: All matter is made up of atoms, atoms of the same element are identical, atoms cannot be broken down any further, atoms can combine in whole number ratios to synthesize new chemicals, and atoms are rearranged in reaction. -
Michael Faraday
Faraday discovered that the structure of an atom contributed to its electrical charge. He discovered this component of the atom by placing a negatively and positively charged electrodes in water. He observed that some elements went to the positive electrode while other elements to the negative end. -
Dmitri Mendeleev
Dmitri Mendeleev published the first periodic table in 1869. He grouped all of the elements into 7 groups, based on their properties. -
William Crookes
William Crookes that cathode rays only travel in straight lines away from the cathode, can be repelled by magnets, and can make pinwheels spin. He realized that this ability to move pinwheels demonstrated that cathode rays have mass. -
Henry Becquerel
Henry Becquerel discovered radioactivity in 1896 after observing that uranium emits radiation that is not X-ray radiation. -
Joseph John Thomson
Thomson used cathode rays to study the electrical charge of subatomic particles. Through these experiments, he found that electrons are negatively charged. -
Marie and Pierre Curie
Marie and Pierre Curie discovered polonium and radium in 1898 after years of researching the emission of radiation from various samples of uranium. It was through the study of a sample of uranium called pitchblende, which was actually a mix of uranium and other radioactive elements, that they discovered the existence of polonium and radium. -
Max Planck
In 1899, Max Planck discovered that light and electromagnetic radiation travels in quanta. He related the energy of the quanta and the frequency of the wave in which it travels, and defined this relationship with the Planck Constant. -
Ernest Rutherford
With the help Hans Geiger, Rutherford discovered that atoms are mostly free space with a dense and massive nucleus in the middle. He discovered this idea by observing the Gold Foil Experiment, in which he fired alpha particles at a sheet of a gold foil, only to have 1 in 20,000 particles bounce off the foil. -
Joseph John Thomson
He created the “Plum Pudding Model”, which claimed that atoms are made up of negatively charged electrons within a lot of positive matter. -
Robert Millikan
While performing the oil drop experiment in 1909, Millikan discovered that the charge of a single electron is 1.60 x 10^-19 coulomb and that a single electron has a mass of 9.11 x 10^-28 grams. -
James Chadwick
He helped Rutherford change the nuclei of atoms by bombarding them with alpha particles. -
Neils Bohr
Bohr introduced the Bohr Planetary Model to the scientific community. His model claims that the nucleus is positively charged, and electrons orbit the nucleus in several orbitals. He also found that the properties of an atom are derived from its valence electrons. -
Louis deBroglie
He proposed in his 1924 Doctoral thesis that electrons must travel in waves. This theory built on Einstein’s previous theory of waves of light. -
Werner Heisenberg
In 1925, Werner Heisenberg created matrix mechanics to explain quantum mechanics -
Werner Heisenberg
In 1926, Werner Heisenberg proposed the Uncertainty Principle. He claimed with this theory that not all variables in quantum physics can be certain. -
Erwin Schrodinger
Erwin Schrodinger built upon the Bohr model by developing a model that attempts to predict the probability of an electron’s presence in a particular position around the atom’s nucleus. He also introduced wave mechanics to the study of quantum mechanics. -
James Chadwick
In 1932, James Chadwick that neutrons make up the nuclei of atoms. He claimed that neutrons are particles in the nuclei of atoms that have no electrical charge. -
Otto Hahn
Otto Hahn forced uranium and thorium to undergo fission in 1938, along with several other atoms with heavy nuclei. -
Lise Meitner
Lise Meitner was the first scientist to discover the fission of uranium with the addition of a single neutron. The United States used Meitner's science to create the first nuclear bomb. -
Albert Einstein
He explained the random motion of particles in liquid with molecular motion. During this year, he also claimed that E=mc^2 with E meaning energy, m meaning mass, and c meaning the speed of light.
