Changes

==History==

==History==

While the roots of quantum mechanics can be traced to experiments performed in the 19th century, the theory began to emerge when [[Max Planck]] proposed a "quantum hypothesis" to explain the energy spectrum of [[black body]] radiation in 1900. He proposed that the energy of what we now call a photon is proportional to its frequency. In 1905, [[Albert Einstein]] also suggested that light is composed of discrete packets (''[[quanta]]'') in order to explain the [[photoelectric effect]]. A decade later, [[Niels Bohr]] proposed a model of the atom in which [[angular momentum]] is quantized. Eventually, the mathematical formalism that became known as quantum mechanics was developed in the 1920s and 1930s by [[John von Neumann]], [[Hermann Weyl]], and others, after [[Erwin Schrodinger]]'s discovery of wave mechanics and [[Werner Heisenberg]]'s discovery of matrix mechanics.

 

Until the early 1900's, scientists believed that [[electron]]s and [[proton]]s were small discrete lumps.  Thus, electrons would orbit the nucleus of an [[atom]] just as planets orbit the sun. The problem with this idea was that, according to classical [[electromagnetism]], the orbiting electron would emit energy as it orbited.  This would cause it to lose rotational kinetic energy and orbit closer and closer to the proton, until it collapses into the proton!  Since atoms are stable, this model could not be correct.

 

The idea of "quanta", or discrete units, of energy was proposed by [[Max Planck]] in 1900, to explain the energy spectrum of [[black body]] radiation. He proposed that the energy of what we now call a photon is proportional to its frequency. In 1905, [[Albert Einstein]] also suggested that light is composed of discrete packets (''[[quanta]]'') in order to explain the [[photoelectric effect]].

 

In 1915, [[Niels Bohr]] applied this to the electron problem by proposing that [[angular momentum]] is also quantized - electrons can only orbit at certain locations, so they cannot spiral into the nucleus.  While this model explained how atoms do not collapse, not even Bohr himself had any idea why.  As Sir James Jeans remarked, the only justification for Bohr's theory was "the very weighty one of success".<ref name="VA">http://galileo.phys.virginia.edu/classes/252/Bohr_to_Waves/Bohr_to_Waves.html</ref>

 

It was Prince [[Louis de Broglie]] who explained Bohr's theory in 1924 by describing the electron as a wave with wavelength λ=[[Planck's Constant|h]]/[[momentum|p]].  Therefore, it would be logical that it could only orbit in orbits whose circumference is equal to an integer number of wavelengths.  Thus, angular momentum is quantized as Bohr predicted, and atoms do not self-destruct. <ref name="VA" />

 

Eventually, the mathematical formalism that became known as quantum mechanics was developed in the 1920s and 1930s by [[John von Neumann]], [[Hermann Weyl]], and others, after [[Erwin Schrodinger]]'s discovery of wave mechanics and [[Werner Heisenberg]]'s discovery of matrix mechanics.

==Principles==

==Principles==

==External Links==

==External Links==

For an excellent discussion of quantum mechanics, see:

For an excellent discussion of quantum mechanics, see:

http://www.chemistry.ohio-state.edu/betha/qm/

http://www.chemistry.ohio-state.edu/betha/qm/