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442 BCE
Democritus
Born in Abdera, Greece around 460 B.C.; Died around 370 B.C. in Greece. He is known for being the first person to propose evidence based atomic theory.His formulation of the atomic theory of the universe concluded that everything is composed of atoms which were microscopic and indestructible. He took a seashell and kept breaking it in half until it was powder to test his theory. An accomplishment of Democritus was that he and another guy discovered the first atom theory in 492 B.C. -
John Dalton
Born in England on September 6, 1766; Died on July 26, 1844. Dalton's Atomic Theory proposed that
1. Matter is made up of atoms that are indivisible and indestructible 2. All atoms of an element are identical
3. Atoms of different elements have different weights and different chemical properties
4. Atoms of different elements combine in simple whole numbers to form compounds.
5. Atoms cannot be created or destroyed.
Created a model called Dalton's Model of the Atom -
J.J. Thomson
Born in Manchester, England on Dec. 18, 1856; Died on Aug. 30, 1940 in Cambridge, United Kingdom. In 1897, Thompson identified particles in cathode rays from a vacuum tube and correctly suggested that the rays were streams of particles that were contained in atoms.He discovered that these particles were electrons and that they were negative.He created a device that manipulated the path of electrons with electric and magnetic fields. Also created a model of the atom that looked like plum pudding. -
Robert Millikan
Born in Iowa on March 22, 1858; Died on December 19, 1953. In 1913, Robert Millikan discovered the charge of an electron by using an oil drop experiment. In the experiment, the force of the droplets passing through the electric field was measured, along with the electrical field between the electrodes. In measuring and calculating those quantities, he was then able to determine the charge on a single electron, which was negative. -
Ernest Rutherford
Born in Nelson, New Zealand on August 30, 1871; Died October 19, 1937 in Cambridge, United Kingdom. He conducted the Gold Foil experiment in 1911 and demonstrated that the atom has a tiny and heavy nucleus. He designed the experiment to use alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. He discovered the atomic nucleus and later became known as the father of nuclear physics. -
Neils Bohr
Born in Denmark on October 7, 1885; Died November 18, 1962. Bohr created an atomic model called the Bohr Model. He required that the electrons move in orbits of fixed size and energy. Bohr created an accurate formula for the energy levels of the hydrogen atom and assumed that otherwise electrons obey the laws of classical mechanics by traveling around the nucleus in circular orbits. -
Erwin Schrodinger
Born in Australia on August 12, 1887; Died January 4, 1961. In 1926, Schrodinger created the quantum mechanical model. This model used mathematical equations to determine the likelihood of finding an electron in a certain position. Unlike the Bohr model, the quantum mechanical model didn't not define the exact path of an electron, but rather, predicted the odds of the location of the electron.
Finding the location of an electron would create a much more accurate model of an atom. -
James Chadwick
Born in the Manchester, England on October 20, 1891; Died July 24, 1974 in Cambridge. He discovered neutrons in the atom in 1932, which are located in the nucleus with protons. Chadwick observed that beryllium, when exposed to bombardment by alpha rays, released an unknown radiation that ejected protons from the nuclei of many substances. Chadwick interpreted that radiation as being composed of particles of mass equal to that of the proton but without electrical charge—neutrons.
