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100
Aristotle
384 BC
Aristotle believed in the four elements which consisted of Air, Earth, Water, and Fire. He did not believe in atoms because he thought it restricted the gods. He did althought believe that regardless of the number of times you had to cut a piece of matter in half, you would always have a smaller piece. This was one of the basic ideas that scientists would use in later years. -
100
Democratus
Around 400 BC
He thought that matter could not be divided into smaller and smaller pieces infinitely. He believed that eventually there would be a single indivisable piece. He named this the "Atomos".
This would be another basis for what the atom would be thought as. -
Antoine Lavoisier
1774
Antoine used an experiement of turning HgO into Hg+O. He used the experiment to help him come up with the law of conservation of mass. This states that matter cannot be made or destroyed. He also hints at the rearrangment of matter in reactions. Matter could be rearranged but would never disappear. This began conversation on what an atom really was. Antoine was also influenced by Aristotle. -
John Dalton
His theory stated that all elements are made up of small indivisible parts called atoms. Atoms of the same element are identicle in size, weight, and properties. Atoms of different elements are different from each other and had different weights. Atoms can neither be divided into smaller pieces or be destroyed. Chemical reactions occur due to rearrangement, seperation, or combination of the atoms. Atoms combine in whole number ratios. Atoms of two or more different elements combine to form rxns. -
Henri Becquerel
1896
Henri Becquerel was mostly known for the discovery of radioactivity. The discovery of radioactivity broke the ideas of indivisibility of atoms. It also helped us better understand their structures. J.J. Thompson would later use this discovery to make his own theory. -
Max Planck
1900
Max proposed the quantum of action which explained the pattern of light intensity emitted from a black body at any given frequency. His paper, On the Theory of the Law of Energy Distribution in the Continuous Spectrum, formed a field of physics called quantum mechnaics. Max explained that energy is not a continuous, flowing entity, but instead is carried in tiny, discontinuous units, which he called quanta. His theory would be used by Bohr to create his atomic model. -
Marie & Pierre Curie
1903
Marie and Pierre were the pioneers of the discovery of radioactivity. They discovered how some elements were much more radioactive then they thought it would be. This led to them discovering two new elements. Marie and Pierre used Henri's discoveries to help themselves. -
J.J. Thompson
1904
Thompson suggested that an atom would look like plum pudding. This model would be known as the "plum pudding" model. He also discovered the electron using discharge in a high-vacuum cathode-ray tube. He suggested that atoms are spheres of positive matter in which electrons are positioned by electrostatic forces. -
Ernest Rutherford
1907
Ernest published his theory saying that atoms had a centeral positive nucleus surrounded by negative orbiting electrons. The model suggested that most of the mass in the atom is contained in the nucleus and that the rest of the atom was mostly empty space. He found this by conducting the Gold Foil experiment. Where he shot alpha particles at a small sheet of gold foil. -
Robert Millikan
1909
J,J. Thompson had hypothosized that the mass of an atom was at least 1000 times smaller than the smallest atom. Robert wanted to to measure the charge of an electron with his oil drop experiment. An atomizer was sprayed from a purfume bottle into a small chamber. Some droplets fell into the hole in an area between the two plates. The middle chamber was ionized with x-rays. Particles that did not capture electrons fell to the bottom. -
Niels Bohr
1912-1922
Bohr was the first to discover that electrons did not move in the same orbit but in seperate orbits. Also found that number of electrons in the outer orbit determines the properties of the element. He combined Rutherford's description of the nucleus and Planck's theory about quanta. -
Henry Mosely
1913
Henry published results of his measurements of wavelengths of X-ray spectral lines of elements which showed that the ordering of wavelengths of the X-ray emissions was the same with ordering the elements by atomic number. With the discovery of isotopes, you could see that atomic weight was not the key point in the periodic law but instead the properties varied periodically with atomic number. -
Werner Heisenberg
1925
One of Werner's most important discoveries was the Uncertainty Principle. This meant that electrons did not travel in neat orbits. It also said that all electrons that contain photons will change momentum and physics. He also calculated the behavior of electrons and subatomic particles that make up an atom. This showed that the electrons did not orbit in set orbits but in an area called the electron cloud. -
Erwin Schrodinger
1926
Erwin took Bohr's model of the atom and took it a step further. He used math to describe the likelyhood of finding an electron in an exact position. He called this model the Quantum mechanical model. This does not define the exact path of electrons, but instead predicts the odds of a location of an electron. It is portrayed as a nucleus surrounded by a cloud of electrons. -
James Chadwick
1932
James predicted that atoms would have a neutron. He was the one who established that the atomic number would be the number of protons in an element. He used alpha rays which would become repelled from the electrical forces in the nuclei.
