
Bohr atom
Fig. 1.22 . In this model, proposed in the early 20th century by the Danish physicist Niels Bohr (18851962), a hydrogen atom’s one electron revolves around the hydrogen nucleus, a single proton, in welldefined orbits described by the integer n = 1, 2, 3, 4, 5, … An electron absorbs or emits radiation when it makes a transition between these allowed orbits. The electron can jump upward, to orbits with larger n, by absorption of a photon of exactly the right energy, equal to the energy difference between the orbits; the electron can jump down to lower orbits, of smaller n, with the emission of radiation of that same energy and wavelength. Transitions that begin or end on the n = 2 orbit define the Balmer series that is observed at visible wavelengths. They are designated by Hα, Hβ, Hγ…. The Lyman series, with transitions from the first orbit at n = 1, is detected at ultraviolet wavelengths. The orbits are not drawn to scale for the size of the radius increases with the square of the integer n.
Copyright 2010, Professor Kenneth R. Lang, Tufts University

