Page 63 - Theory and Problems of BEGINNING CHEMISTRY

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.

656.3 486.1 434.0 410.1

700 600 500 400
Wavelength, nm (1 nm = 10 −9 m)
Fig. 4-1. Visible spectrum of hydrogen

4

3

2

1
Fig. 4-2. Possible return paths for electron in orbit 4
4 → 1 4 → 2 → 1 4 → 3 → 2 → 1 4 → 3 → 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.

6
5
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