BOHR THEORY
4.2. BOHR THEORY
The first plausible theory of the electronic structure of the atom was proposed in 1914 by Niels Bohr (1885– 1962), a Danish physicist. To explain the hydrogen spectrum (Fig. 4-1), he suggested that in each hydrogen atom, the electron revolves about the nucleus in one of several possible circular orbits, each having a definite radius corresponding to a definite energy for the electron. An electron in the orbit closest to the nucleus has the lowest energy. With the electron in that orbit, the atom is said to be in its lowest energy state, or ground state. If a discrete quantity of additional energy were absorbed by the atom in some manner, the electron might be able to move into another orbit having a higher energy. The hydrogen atom would then be in an excited state. An atom in the excited state will return to the ground state and give off its excess energy as light in the process.
52 ELECTRONIC CONFIGURATION OF THE ATOM
[ CHAP. 4
In returning to the ground state, the energy may be emitted all at once, or it may be emitted in a stepwise manner (but not continuously) as the electron drops from a higher allowed orbit to allowed orbits of lower and lower energy. Since each orbit corresponds to a definite energy level, the energy of the light emitted will correspond to the definite differences in energy between levels. Therefore, the light emitted as the atom returns to its ground state will have a definite energy or a definite set of energies (Fig. 4-2). The discrete amounts of energy emitted or absorbed by an atom or molecule are called quanta (singular, quantum). A quantum of light energy is called
a photon.
Wavelength, nm (1 nm = 10 − 9 m) Fig. 4-1. Visible spectrum of hydrogen
Fig. 4-2. Possible return paths for electron in orbit 4 4→1
Only electron transitions down to the second orbit cause emission of visible light. Other transitions may involve infrared or ultraviolet light.
The wavelength of a photon—a quantum of light—is inversely proportional to the energy of the light, and when the light is observed through a spectroscope, lines of different colors, corresponding to different wavelengths, are seen. The origin of the visible portion of the hydrogen spectrum is shown schematically in Fig. 4-3.
2 Fig. 4-3. The origin of the visible spectrum of hydrogen (not drawn to scale)
Bohr’s original idea of orbits of discrete radii has been greatly modified, but the concept that the electron in the hydrogen atom occupies definite energy levels still applies. It can be calculated that an electron in a higher energy level is located on average farther away from the nucleus than one in a lower energy level. It is customary to refer to the successive energy levels as electron shells. The terms energy level and shell are
CHAP. 4]
ELECTRONIC CONFIGURATION OF THE ATOM
used interchangeably. The shells are sometimes designated by capital letters, with K denoting the lowest energy level, as follows:
Energy level
Shell notation
O ...
The electrons in atoms other than hydrogen also occupy various energy levels. With more than one electron in each atom, the question of how many electrons can occupy a given level becomes important. The maximum number of electrons that can occupy a given shell depends on the shell number. For example, in any atom, the first shell can hold a maximum of only 2 electrons, the second shell can hold a maximum of 8 electrons, the third shell can hold a maximum of 18 electrons, and so forth. The maximum number of electrons that can occupy any
particular shell is 2n 2 , where n is the shell number. EXAMPLE 4.1. What is the maximum number of electrons that can occupy the N shell?
Ans. The N shell in an atom corresponds to the fourth energy level (n = 4); hence the maximum number of electrons it can hold is
2n 2 = 2(4) 2 = 2 × 16 = 32