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100
Democritus (Suppose to be 400 BC)
Democritus was a Greek philosopher. He was the first to suggest that there was an ultimate pariticle. This meant that if you broke something in half over and over again, eventually you would reach a particle so tiny, it could not be broken in half. He named this ultimate particle "atomos". This became atom theory. Although Democritus's theory was correct, he had a very basic understanding of atoms. At his time period, he did not have the technology to advance further into his study of atoms. -
116
Aritotle (Suppose to be 384 BC)
Aristotle believed that when your break things down multiple times, you would eventually form the four elements: wind, earth, fire, and water. He completely dismissed the atom theory, and since he was so famous, many others also dissmissed the theory created by Democritus Aristotle had a negative effect on the history of atoms, because he caused a lot of people to completely ignore the theory. -
Antoine Lavoisier
Lavoisier started by studying the burning process. After doing so, he proposed Combustion theory, named oxygen, and discovered oxygen's role in combusition. Later, Lavoisier would propse the Law of Conservation of Mass and create a list of 33 elements. Lavosoisier also helped construct the metric system. Unforunately, he was beheaded by a guillotine. Lavoisier's discoveries led to the beginning of modern chemistry, as many more elements were discovered shortly after his premature death. -
John Dalton
John Dalton came up with the idea of relative atomic weights. Through this, he created the Law of Multiple Proportions. This led straight to his most famous work: the Atomic Theory which was proposed later in 1803. Atomic Theory originated from Democritus's Atom Theory. Atomic Theory states that everything is composed of many small units called atoms. Dalton also constructed a system of symbols to represent the names of the atoms of different elements. -
J.J. Thomson
JJ Thomson was the first to discover and identify the electron. Using cathode rays, he determined that all atoms have electrons. JJ Thomson was also credited with the first evidence of isotopes of stable elements. JJ Thomson created the "plum-pudding model of the atom. In this version of the atom, the nucleus of the atom is very large and the atom is the nucleus itself surrounded by electrons. In this model, the nucleus is the larger portion and the electrons within and around it are raisins. -
Max Planck
Max Planck was a German theoretical physicist who originated quantum theory. This theory changed the way we looked at atomic and subatomic processes. He created the quantum mechanical model, which says that it is impossible no the exact location of an electron at a single time. He said that there are places where the electrons are more likely to be. These places are called electron clouds. -
Robert Millikan
The single most important thing Robert Millikan did was determine the charges of the subatomic particles. Through extensive research and experimentation, he found that electrons had a negative charge. Based off that, he assumed that the rest of atom (the nucleus) had to be relatively positive. Robert Millikan is also well known for his oil-drop experiment. Through this experiment, he proved JJ Thomson's previous hypthesis: electrons are much smaller than the rest of the atom. -
Ernest Rutherford
In his early works, he created the concept of half-lifes, differentiated alpha and beta radiation, and also discovered that radiation involved transmutation of two elements. Rutherford's most famous experiment is the gold foil experiment. In this experiment, Rutherford shot alpha particles at gold foil.Based off of this experiment, he discovered that the nucleus of the atom is actually very small, and that electrons revolve around the nucleus. Lastly, he found and named the proton. -
Niels Bohr
Neils Bohr was a Danish physicist that came up with today's most well-known model of the atom. This model was called Bohr's model, named after himself. The Bohr model illustrated that electrons revolve in seperate orbits. He stated that certain orbits can only contain a certain amount of electrons. He also said that on the outer most orbit, there are valence electron, and these electrons determine the properties of the element. -
Erwin Schrodinger
Schrodinger developed the fundamental results of quantum theory which led to the creation of wave mechanics. Schrodinger used mathematical equations to more enhance the Bohr Model. Using math, he predicted the likelihood of an electron being at a certain area. His model consisted of a nucleus and an electron cloud. Wherever the cloud is most dense, there is a higher likelihood that an electron will be there. Unlike Bohr's model, however, did not illustrate the exact path of the electron. -
Henry Moseley
Moseley invented the concept of the atomic number. Before Moseley had done this, the atomic number was just a spot on the periodic table of elements. Moseley created Moseley's rule, in which certain elements were placed, in a more logical way, on the periodic table based off of similar characteristics. Henry Moseley did not create a new model of the atom, but instead strengthened Niels Bohr's model, which is the most commonly used model as of today. -
James Chadwick
Chadwick's most important contribution was the discovery of the neutron. Chadwick determined that the neutron had no charge and that it made up approximately half of the atom. He created a new version of the model of the atom. This version was more elaborate, as it included all three of the subatomic particles: proton, neutron, and electron. -
Werner Heisenberg
Heisenberg was credited with the creation of quantum theory. He is most well-known for the uncertainty principle. The uncertainty principle states that you cannot tell where an electron will be. This principle is based on probability rather than certainty. It states that an electron can most likely be in an "electron cloud" or an orbit. -
Henri Becquerel,Marie Curie, and Pierre Curie
All three of these scientists worked together to find radioactivity. When Becquerel was working with Uranium, he found that the Uranium made images on a photographic plate without any source of energy. After this discovery, he worked with the Curies to explain this phenomenon. In the end, all three of them were credited with the discovery of radioactivity, and all three won Nobel Prizes. Later Marie Curie would be the first to win two Nobel Prizes for the discovery of polonium and radium.
