Changes

Until the early 1900s, 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.

Until the early 1900s, 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]].

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>

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>