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Atomic Theory
Scientists and Terms Important to Atomic Theory
23
Chemistry
11th Grade
06/16/2009

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Term
Democritus
Definition
Ancient Greek philosopher who came up with the idea of the atom. Atom comes from the Greek atomos meaning indivisible.
Term
Aristotle
Definition
Ancient Greek philosopher who disagreed with Democritus and said that matter was infinitely divisible.
Term
John Dalton
Definition
English Schoolteacher and chemist who revived the idea of the atom in the early 1800's and made five important statements in his atomic theory.
Term
J.J. Thomson
Definition
English physicist who worked with cathode rays and discovered these rays had a negative charge and contained particles traveling at a high rate of speed. The particles in these cathode rays were later named electrons.
Term
Robert Millikan
Definition
American physicist who carried out an oil drop experiment to find the charge to mass ratio of an electron. The results of this experiment showed that the mass of an electron is approximately 2000 times smaller than a hydrogen atom. Despite its small size, the electron has a negative charge equal in magnitude to the positive charge on a proton.
Term
Ernest Rutherford
Definition
Physicist from New Zealand who performed the gold foil experiment in 1911. This experiment led to a new atomic model that included a densely packed positively charged nucleus, electrons orbiting the nucleus, and large amounts of empty space in between.
Term
Hantaro Nagaoka
Definition
Japanese physicist who suggests that an atom has a central nucleus. His idea is confirmed by the gold foil experiment performed by other scientists.
Term
Niels Bohr
Definition
Danish physicist who suggests that electrons move in circular orbits at fixed distances from the nucleus.
Term
Erwin Schrodinger
Definition
Austrian physicist who developed mathematical equations to describe the motion of electrons in atoms. His work led to the electron cloud model
Term
Louis de Broglie
Definition
French physicist who proposed that particles like electrons have some properties of waves.
Term
James Chadwick
Definition
English physicist who confirmed the existence of neutrons, which have no charge and are contained in the nucleus of atoms.
Term
Electron
Definition
Negatively charged particle that orbits the nucleus. Its mass is 1/1836 of an amu.
Term
Proton
Definition
Positively charged particle located in the nucleus, with a mass of approximately 1 amu.
Term
Neutron
Definition
Neutrally charged particle found in the nucleus with no charge.
Term
Atomic Number
Definition
The number of protons in the nucleus of a particular element.
Term
Mass Number
Definition
The number of protons and neutrons added together.
Term
Atomic Mass
Definition
The weighted average of the masses of all the isotopes of an element.
Term
Atomic Mass Unit
Definition
A unit of mass equal to one-twelfth of the mass of a carbon-12 atom.
Term
ion
Definition
an atom or group of atoms that has a positive of negative charge.
Term
Cation
Definition
An atom or group of atoms with a positive charge.
Term
Anion
Definition
An atom or group of atoms with a negative charge.
Term
Nucleus
Definition
The tiny, dense central portion of an atom, composed of protons and neutrons.
Term
Isotopes
Definition
Atom of the same element that have the same atomic number but different mass numbers due to a different number of neutrons.
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