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Quantum Mechanics - Old Quantum Theory
Jeanette Cain
The fundamental theory of the motions of particles is called quantum mechanics. This theory describes systems which are atomic sized. Until the
20th century, scientists considered Newton's classical mechanics as giving a complete description of the behavior of matter. This description included all extended objects and particles, but scientists soon
realized Newton's mechanics was too simple for the world of quantum mechanics.
J. C. Maxwell and H. A. Lorentz provided the classical mechanics and electrodynamics of the old quantum theory. Its lack was first discovered by Planck in 1900, when he formulated the quantum existence of
radiant or electromagnetic energy, and has now assumed the status of a physical law. Planck's hypothesis stated that the electromagnetic waves of a given frequency can give off and take in
energy, which appears or disappears in units called quanta. A quanta is equal to h times the frequency. The constant h has a magnitude of 6.624 X 10-27 erg-second.
This is a fundamental constant of nature.
Einstein used the quantum idea in 1904. He used it to account for the energies of electrons, which are given off by metal and metallic surface with the photoelectric effect. In other words, for the
energies of electrons affected by an action of light; Einstein also used this application in 1907. In 1912 Debye used the quantum theory to explain the low temperature with specific heats in
solids. It explained the measure by observation that a solid will require less heat for raising its temperature by a given number of degrees, if it is at a low temperature rather than a high
temperature.
In 1908 J.R. Rydberg and W. Ritz used the study of sharp spectral lines given off by atoms to develop the combination principle. Their principle says that the frequency of a spectral line can
be expressed as a difference between the members of the set of frequency terms. Rutherford, in 1911, observed that an atom was made of small, bulky and positively charged electrons. This made
it electrically neutral. With this discovery in 1913, Bohr showed that Planck's hypothesis and the combination principle could be given for a correct model of the hydrogen atom.
Bohr's theory claimed that frequency terms, which when multiplied by h, would give distinct energy levels that would describe motions of the electrons around the nucleus. This meant
that these were the only possible states in which an atom could exist. A change between these two states would have a giving off or taking in of quanta of the electromagnetic waves. Bohr gave a
specific set of rules for this quantization, which explained the energy levels of hydrogen. Wilson and Sommerfeld extended its usage to apply to a large variety of atomic systems in 1915
and 1916. Atomic energy level existence was observed, independently, by J. Franck and G. Hertz in 1913. They used evidence from a spectroscopic observation with a transferring of energy to an
atom when an electron collides with it.
Source:
Editors. The World Book Encyclopedia. World Book-Childcraft International, Inc: Chicago. 1990
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