 Bohr summarized his quantum theory by means of two assumptions or postulates. According to the first postulate, electrons cannot travel in any path around the nucleus; rather, atomic systems can only exist in one of a series of discrete “stationary states,” in which the electron is in a particular allowed stable periodic orbit and is not emitting radiation. Intuitively, these stationary states can be thought of as a series of concentric circular orbits around the nucleus, along which the electron travels; these stationary states are labeled by means of the principal quantum number \(n\), with the lowest allowed orbit (the “normal” or ground state) labeled \(n = 1\), the next stationary state of higher energy \(n = 2\), and so on. Bohr postulates that when the electron is in one of these stationary states, its motions can be adequately described by means of classical mechanics; however, when the electron makes a transition from one stationary state to another, the classical theory no longer applies. The second postulate of Bohr’s old quantum theory is that, when there is a transition between different stationary states, \(n'\) and \(n''\), the emitted radiation is of a single frequency, \(\nu\), that is given by the difference in the energy of the two states, \((E_{n'} - E_{n''})\), divided by Planck’s constant.