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400 BCE
Democritus
Democritus had discovered in 400 B.C. that everything within an atom is indivisible and indestructible. There is an empty space that lies between atoms and those atoms are always moving. Democritus had also found that there are an infinite number of atoms and that they are shaped in many different ways. -
350 BCE
Aristotle
Aristotle had created the idea that all matter were made up of four elements. These four elements are fire, air, water, and earth. He believed that all matter was infinitely divisible. He completely disagreed with Democritus. -
Lavoisier
Lavoisier discovered oxygen's role in combustion. He had recognized and named hydrogen and oxygen in the late 1700's. Lavoisier is also named the "Father of Chemistry" because he discovered hydrogen and founded the metric system. -
John Dalton
Dalton had a theory of his own. Dalton's theory had said that all elements were made of atoms. He said that atoms of the same element are the same and that atoms of different elements can be combined to make compounds. He also said that a chemical reaction occurs when atoms split, combine or rearrange. His last thing was that atoms have different weights when they are from a different elements. He developed the modern atomic theory. -
Avogadro
Avogadro had created the Avogadro law. This law said that equal volumes of different gases have the same number of molecules is they have the same pressure and temperature. His hypothesis was only accepted after his death. -
Berzelius
Berzelius had composed a table of relative atomic weights in 1818. The table included all the known elements at that time and that oxygen was set to 100. This gave evidence to prove Dalton's theory. -
Mendeleev
In 1869, Dmitri Mendeleev had published his periodic table of elements. His table consisted of the known elements at the time and he sorted them by chemical properties and in the order of their relative atomic mass. He arranged them into 7 groups, each group consisted of elements that had the same properties. -
Becquerel
Becquerel, in 1896, had been studying the effect of x-rays on a photographic film using uranium. While he was studying that, he had discovered that some chemicals decompose spontaneously and these chemicals give off penetrating rays. He had discovered what is called radioactivity. -
J.J. Thomson
J.J. Thomson in 1897 had discovered the electron. He also discovered than an atom can be divided. He found out that an atom has a positively charged core and negatively charged particles that lie within it. Before the atomic nucleus was discovered, he had created the Plum Pudding Model. He was the first to say that the smallest unit was actually 1,000 times smaller than an atom and that it was a subatomic particle which is now the electron. -
Marie Curie
Marie had studied uranium and thorium. She had found their spontaneous decay process "radioactivity". She had discovered polonium and radium which are both radioactive elements with her husband. -
Planck
Max Planck had established the Quantum Theory where energy can only be absorbed and or radiated in quanta or discreet values. He is considered the father of the Quantum Theory. He originated the Quantum Theory. -
Robert Millikan
Robert was an American physicist who had discovered an electrons charge. He had used the Oil drop experiment or the falling drop method to discover it. He had quantified the charge of an electron which had helped people after him know more about the structure of an atom. He also determined that there was a smallest unit charge, or that it is quantized. -
Rutherford
Rutherford had found that the nucleus of an atom is very dense and small. He also found that it was positively charged. Rutherford soon came up with the Rutherford model. This model is a model of the atom. His model consists of a positively charged center and negatively charged electrons orbiting that center. He also suggested the idea of neutrons present in the nucleus. With this theory he had overturned Thomson's model. -
Moseley
Moseley studied under Rutherford. He had developed that using X-ray spectra you could study the atomic structure, mainly the amount of protons an atom has. His discovery had made a more accurate periodic table with better positioning by closer determination of atomic numbers. He arranged them by the number of their protons instead of the weights. This had made the problems with Mendeleev's periodic table disappear. His periodic table is now the modern day periodic table. -
Bohr
Niels Bohr had worked with Rutherford and created the Bohr model which showed that electrons moved in specific orbits. This was similar to the Solar System. He found that the outer orbit hold more electrons than the inner orbit and that electrons travel successfully in certain larger orbits. He found that when an electron jumps from one orbit to another with lower energy, a light quantum is emitted. He won a Nobel Piece prize for this. -
Schrodinger
Erwin Schrodinger had viewed the electrons as continuous waves. He took Bohr's Model a little further and had created the Quantum Mechanical Model. He used mathematical equations to explain the probability and likelihood of an electron to be in a certain position. This was a powerful model of the atom because it combined equations of the behavior of waves. He used the de Broglie equation to generate this model. He also created the Electron Cloud Model. -
Heisenberg
Werner used quantum mechanics to add to the atomic theory. He had created an Uncertainty principle. This said the more precisely the position of an electron is determined, the less precisely it's momentum is known and vice versa. Basically he said it is impossible to know the position and velocity of an electron at the same time. He said said you can't know the future position of any particle and had found a fault in Bohr's atom- there isn't a way to measure the orbit of an electron. -
Chadwick
James Chadwick had discovered and proved that neutrons existed. He used alpha particles to find a neutral particle with a mass close to the proton. Using the alpha particle, he had found an unknown radiation. He had found that this radiation was composed of a neutral electrical charge. -
Meitner
Meitner along with a few others had been the first to find out the Uranium atom, when attacked with neutrons, actually splits. She had found nuclear fission. This split creates huge amounts of energy. She used heavy elements and found out that they capture neutrons and form unstable products that undergo fission. This ejects neutrons which helps this chain keep going. This information helped build the atomic bomb, even though Lisa Meitner had refused to help build the atomic bomb.
