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John Dalton (1766 - 1844 C.E.)
John Dalton believed in Democritus' work. His theory consisted of four basic ideas:
1) All matter is composed of atoms that are indivisible.
2) All atoms are identical in properties, such as size and mass. 3) Atoms are rearranged as a chemical reaction occurs.
4) Compounds are formed by combination of multiple atoms.
5) They cannot be created or destroyed. -
J.J. Thomson (1856 – 1940 C.E.)
He discovered the electron. His work proved one of Dalton's ideas wrong, the atom was not indivisible, but in fact made up of multiple components. He conducted an experiment with a cathode ray tube, when an electrical current was passed through, glowing material could be seen. When he put two electrically charged plated on opposite ends of the tube, the light bent on the positive plate. He thought that the atom looked like raisin pudding, a ball with negative electrons clumped on it. -
Ernest Rutherford (1871 – 1937 C.E.)
In 1908, Ernest Rutherford proved Thomson's atomic model to be incorrect. He is famous for his gold foil experiment which stated the existence of protons. He shot alpha particles at the gold foil, and observed where they hit. Most were undeflected, while the rest bounced off in different directions. -
Neils Bohr (1885 - 1962 C.E.)
In 1913, Neils Bohr created a new model of the atom. It consisted of electrons that orbited in "shells" or layers around the nucleus. As the energy level changed, so did the location of the electron. Neils Bohr was also the first to introduce the idea of the quantum mechanical model, where a cloud of electrons surrounds the nucleus. -
Erwin Schrödinger (1887 - 1961 C.E.)
Erwin Schrödinger built upon the quantum mechanical model. He used a mathematical equation to describe the odds of finding the location of an electron, he called the layers, orbitals. These orbitals could be described as "electron density clouds". -
James Chadwick (1891 - 1974 C.E.)
In 1932, James Chadwick proved the existence of neutrons. He shot alpha particles (two protons and two neutrons bound) at a sheet of beryllium, that filtered out the electrons. The neutrons hit a sheet of paraffin wax. The ones that filtered through the paraffin wax were the protons. Chadwick discovered that neutrons help reduce the repulsion between protons to stabilize the atom's nucleus. Neutrons always reside in the nucleus of atoms, and they are about the same size as protons.
