In atomic physics, the Rutherford–Bohr model or Bohr model, introduced by Niels Bohr in 1913, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus. I do not know if you appreciate the fact that long papers have a way of frightening readers, who feel that they have not time to dip into them. Ernest Rutherford, letter to Niels Bohr, March 29, 1913, on Bohr's paper "On the Constitution of Atoms and Molecules". Bohr proposed in the first place that the energies of atoms are quantized, in the sense that the atom exists in only a discrete set of states, with energies (in increasing order) E1, E2, . . . . The frequency of a photon emitted in a transition m → n or absorbed in a transition n → m is given by Einstein’s formula E = hν and energy conservation by ν = ( E m − E n ) / h {\displaystyle \nu =(E_{m}-E_{n})/h} . ... Bohr’s formulas could be used not only for single-electron atoms, like hydrogen or singly ionized helium, but also roughly for the innermost orbits in heavier atoms, where the charge of the nucleus is not screened by electrons, and we can take Ze as the actual charge of the nucleus. Steven Weinberg, Lectures on Quantum Mechanics (2015), Chap. 1 : Historical Introduction Wikipedia has an article about: Bohr model
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