Chemistry, Third edition

20 · LIGHT AND SPECTROSCOPY

Electronic spectra of other atoms

The emission spectrum of sodium atoms is produced by passing an electric dis-

charge through sodium vapour (sodium vapour is easily produced by heating

sodium metal). Figure 20.8(a) shows how the two main emission lines in the sodium

atom spectrum would appear when recorded on a photographic emulsion. When

the human eye is used as a detector (as in a hand-held spectroscope), both lines are

seen to be an intense yellow colour. The lines are responsible for the yellow colour

of sodium street lights and the yellow colour observed when sodium compounds

(such as salt) are sprinkled into flames. Although the lines are separated only by

0.6 nm (0.6 nanometres), they can be distinguished (resolved) even with cheap spec-

troscopes.

The absorption spectrum (Fig. 20.8(b)) of sodium atoms is produced by measur-

ing the light wavelengths absorbed when white light is passed through sodium vapour.

The absorption spectrum as recorded on a photographic emulsion contains lines at

the same frequency as those found in the emission spectrum, but they are light (not

dark) because the lines represent a reductionin the intensity of the white light source at

those wavelengths.

The absorption and emission spectra of metals such as copper, lithium, caesium,

calcium and potassium involve lines at different wavelengths to those observed in the

sodium spectrum. As with sodium, a few lines in each spectrum dominate the colours

of compounds of these metals in the gas flame. This is the basis of the flame testswhich

are used to indicate the presence of compounds of these metals (see page 195).

Electronic spectra of molecules

Changes of electronic energy in simple molecules, such as H 2 O, HCl or H 2 , generally

give rise to absorption and emission spectra in the UV region. Many larger organic

molecules (including dyes) and some inorganic ions display part of their electronic

spectra in the visible region. Figure 20.9 shows the electronic spectrum of potassium

manganate(VII) (potassium permanganate, KMnO 4 ), in aqueous solution between

400 and 600 nm. Water and potassium ions do not absorb in this wavelength range –

the absorption is solely due to the MnO 4 – ion.

Fluorescence

Some molecules absorb ultraviolet light and immediately emit light at a slightly

longer wavelength. This is called fluorescence. For example, quinine (which gives

tonic water its bitter taste) is colourless, but upon irradiation with UV light it emits a

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Fig. 20.8The two strongest lines in the

visible region in (a) the emission and (b)

the absorption spectra of sodium atoms.