Chemistry, Third edition

20 · LIGHT AND SPECTROSCOPY

Experimentally, the emission spectrum of the hydrogen atom is obtained by pass-

ing an electric spark through a quartz tube containing hydrogen gas. The energy from

the spark dissociates some of the hydrogen molecules into atoms. Some of the dissoci-

ated hydrogen atoms have so much energy that they are in an excited electronic state.

Although many of these excited atoms lose their excess energy by collisions with

other atoms or molecules, some lose the radiation as light. The emission spectrum is

obtained by passing the emitted light through a spectrometer. Measurements of the

Lyman series must be made in equipment from which air has been evacuated. This is

because the emission lines appear in the range 90–120 nm and both nitrogen and

oxygen gases absorb UV radiation at these wavelengths.

The spectrum of the Lyman series, obtained using a photosensitive emulsion as

detector, is shown at the bottom of Fig. 20.7. For clarity, all emission lines are shown

as being of equal intensity.

Transitions that cause the Lyman series

The hydrogen atom contains only one electron. The electronic energy of a hydrogen

atom depends upon the orbital in which the electron is found. If the electron lies in

the lowest energy orbital (n= 1), the atom is in its ground electronic energy level (its

ground state). The energy of 1 mol of hydrogen atoms in this level is 1310 kJ. (A neg-

ative energy seems strange, but this is just a convention. It means that a stationary

electron just outside the attraction of the hydrogen nucleus will have zero energy.) If

the electron lies in higher orbitals (n> 1), the atom is in an excited electronic state

and its energy becomes bigger (i.e. less negative). At very high values of nthe energy

of 1 mol of hydrogen atoms approaches zero (see Fig. 20.7).

The transitions involved in the Lyman series involve hydrogen atoms in higher

electronic energy levels losing energy by emitting light so that the hydrogen atom ends

up in its ground state. We can represent these transitions as follows:

H(n> 1)→H(n= 1)+h

electronically ground state emitted

excited light

state

wherenin the excited state takes the values 2, 3, 4, 5 etc.

These transitions are shown by downward arrows in Fig. 20.7.

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Fig. 20.7Schematic diagram

of the transitions and the

observed spectral lines in the

Lyman series of the hydrogen

atom.