Niels Bohr

Ernest Rutherford's model of the atom, developed at the turn of the century, pictured negatively charged electrons moving in circular orbits about a positively charged nucleus. Contradictory to electrodynamic theory the electrons did not emit electromagnetic radiation. Niels Bohr provided the explanation by incorporating Max Planck's quantum theory into Rutherford's atomic model. He envisioned specific discrete energy levels (i.e., shells) for the electrons within which they could move yet not emit radiation. Only if the electrons dropped to a lower energy level, or were raised to a higher level, would they emit or absorb electromagnetic radiation. That the energy of the emitted or absorbed radiation must equal the difference between the original and final energy levels of the electrons explained why atoms only absorb certain wavelengths of radiation. To Albert Einstein, Bohr's achievements were "the highest form of musicality in the sphere of thought". In recognition, Bohr received the Nobel Prize in physics in 1922. Later, Louis de Broglie and Erwin Schrödinger described the electron as a standing wave rather than a particle, which "explained" how Bohr's electrons could move about within a defined energy level without emitting radiation. This led Bohr to his famous principle of complementarity, whereby the electron could be interpreted in two mutually exclusive yet equally valid ways: by either the particle or wave models. Later, Bohr hypothesized how an incoming particle could strike a nucleus and create an excited "compound" nucleus. This idea formed the basis for his "liquid drop" model of the nucleus which would provide Lise Meitner and Otto Frisch with the theoretical basis for their explanation of fission.

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