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400
Democritus
400 B.C. Democritus was the first scientist to create a model of the atom. He was the first one to discover that all matter is made up of invisible particles called atoms. He created the name "atom" from the Greek word "atomos", which means uncuttable. He also discovered that atoms are solid, insdestructable, and unique. -
Antoine Lavoiser
1780 Lavoisier was a French nobleman that founded several elements and put the first table of elements together. He used Aristotle's ideas of fire, earth, air, and water to create experiments invesigating combustion and oxidation. He discovered important elements like oxygen, hydrogen, and sulfur by using his previous knowledge of atomic bonding. He discovered that water was made of oxygen and hydrogen, and air included nitrogen. Lavoisier created the first chemistry textbooks and table. -
John Dalton
1803 John Dalton was an English chemist that created the Atomic Theory of Matter, a composition of previous findings by Democritus and his own findings. This theory states that all matter is made of atoms, that atoms cannot be created nor destroyed and also, atoms of different elements combine in whole ratios to form chemical compunds. -
Dmitri Mendeleev
1869 Arranged elements into 7 groups with similar chemical and physical properties. He discovered that the properties of elements "were periodic functions of the their atomic weights". This became known as the Periodic Law. -
J.J. Thomson
1897 J.J. Thomson was a very important scientist when it came to the atomic model. Up until his time, all models of the atom looked like a big solild ball. J.J. Thomson discovered the electron, which led him to create the "plum pudding" atomic model. In this model, he thought that the atom was mostly positive, and negative electrons wandered around the atom. The "plum pudding" model influenced other scientists to make better models. -
Robert Millikan
1908 Robert Millikan was an American scientist that was interested in J.J. Thomson's finding of the electron. J.J. Thomson predicted that the electron was 1000 time smaller than the atom. Millikan wanted to prove this hypothesis, so he preformed an "oil-drop experiment" in which he found that J.J. Thomson was correct. Millikan was also involved in the Quantum Theory after he was inspired by Max Planck. -
Ernest Rutherford
1911 Using alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm) gold foil . He established that the nucleus was dense, small and positively charged. He assumed that the electrons were located outside the nucleus. -
Henry Moseley
1913 Henry Mosely was an English scientist who worked with Niels Bohr in order to create the real atomic number. Mosely used X-rays to find the frequencies of elements on the periodic table. Before his discovery, the atomic number was just an assigned number to a random element. Mosely used these frequencies to find that the number of protons in the nucleus have to do with the atomic number. This created Mosley's Law. -
Neils Bohr
1913 Niels Bohr was a Danish scientist that decided to make a new model based off of Rutherford's model, but changed the orbit of the electrons. Also, he created energy levels in the atom, where only a certain amount of electrons could fit on one energy level of the atom. Bohr also used Planck's ideas in order to create quantum mechanics, his new concept regarding energy. -
Erwin Schrodinger
1926 Erwin Scrhodinger was an Austrian scientist that worked with the Quantum model of the atom. He didn't agree with Bohr's theory, so he created his own. He thought that the only way to find the location and energy of an electron in an atom was to calculate its probability of being a certain distance from the nucleus. -
James Chadwick
1932 Jame Chadwick was an English scientist that discovered the neutron. Before this discovery, Rutherford had said that the nucleus was made of positive matter. It made sense that the atom was neutral because the negative electrons and the positive protons cancelled out. But, Chadwick started to question why there was a difference between the atomic mass and the number of protons. Chadwick then found that the missing component was a neutral part: the neutron. -
Gold Foil Experiment
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Cathode Ray Tube
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Plum Pudding Atomic Model
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Bohr Planetary Model
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Law of Conservation of Mass
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Dalton's Atomic Theory
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Rutherford Model
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Quantum Mechanical Model
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Electron Cloud Model
