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400 BCE
Democritus
Democritus thought that if you take a piece of matter and continue to divide until you cannot anymore, which is known as an atom. -
322 BCE
Aristotle
Aristotle disagreed with Democritus and denied the existence of atoms. He believed that four elements made up all matter. -
Antoine Lavoisier
Lavoisier was one of the first to use an analytical balance to monitor chemical reactions. He studied the thermal decomposition of mercury. -
John Dalton
He was the first to adapt Democritus’ theory into the first modern atomic model and developed many new ideas in his book, "New System of Chemical Philosophy." -
J. Thomson
He showed that electrons have a negative electric charge and come from ordinary matter. For matter to be electrically neutral, there must also be positive charges lurking somewhere. -
Robert Millikin
Millikin concluded that the mass of an electron must be very small for such a large charge. -
Ernest Rutherford
He conducted an experiment intended to determine the angles through which a beam of alpha particles (helium nuclei) would scatter after passing through a thin foil of gold. He stated that electrons orbit the nucleus similar to the orbit of planets about the sun. -
Niels Bohr
He created a new model in which there are particular orbits in which the electrons do not lose energy and therefore do not spiral into the nucleus. This model was the beginning of quantum mechanics, which successfully explains many properties of atoms. -
Louis DeBroglie
De Broglie applied his theory of wave, the particle duality to the Bohr model to explain why only certain orbits are allowed for the electron. He argued that only certain orbits allow the electron to satisfy both its particle and wave properties at the same time. -
Werner Heisenberg
Heisenberg showed that it is impossible to take any measurement of an object without disturbing the object. -
Erwin Schrodinger
Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. -
James Chadwick
Chadwick bombarded beryllium atoms with alpha particles and that eventually led him to the discovery of the neutron.
