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364
Aristotle
Aristotle believed in the four elements of earth, wind, water, and fire. He also believed that a portion of matter could be cut in half infinitely many times. (364 B.C.) -
420
Democritus
Democritus was the first person to propose the existence of an ultimate particle. He described this particle using the word "atomos". He made this proposition in 420 B.C. -
Antoine Lavoisier
Antoine Lavoisier (1743-1794) is called the Father of Modern Chemistry, having discovered that water is made of hydrogen and oxygen. He also invented the analytical balance and showed that chemical elements were neither created nor destroyed, just combined into different compounds in chemical reactions. From this work follows one of the most fundamental principles of physics, the conservation of mass -
Joseph Proust
Joseph Proust proposed the law of Constant Composition which states that samples of a pure compond always contain the same elements in the same mass proportion -
John Dalton
John Dalton proposed the Law of Multiple Proportions. This law led to the development of the atomic theory. He pictured the atom as a small indestuctible particle with no internal structure. -
J.J Thomson
Thomson identified the negatively charged electron. He concluded that the electron was a component of all matter. He also proposed the "plum pudding" model of the atom. -
Robert Millikan
Millikan dtermined the unit charge of the electron. His discovery allowed for the calculation of the electron and all positively charged atoms. -
Ernest Rutherford
Ernest Rutherford published an atomic theory in which the atom has a central positive nucleus surrounded by negative orbiting electrons. -
Neils Bohr
Bohr applied the quantum theory to Rutherford's atomic structure and discovered a method of calculating the possible energy levels of stationary electrons. He also discovered that an emission of light occurs when an electron moves into a differnet energy level orbit.
