Physical Chemistry , 1st ed.

its component colors and pass this colored light through a chemical sample.

The sample absorbed some wavelengths of light, not others, resulting in a dark

line superimposed on a continuous spectrum of colors. Heated samples that

gave off light would have this light analyzed through the spectroscope, show-

ing only lines of light that appeared in the same relative positions as the dark

lines. Bunsen and Kirchhoff eventually noticed that each element absorbed or

emitted only characteristic wavelengths of light, and proposed that this might

be a technique to identify the chemical elements. Figure 9.5 shows several char-

acteristic spectra of some vapors of elements. Note that they are all different.

In 1860, the proposal was put to the test by an analysis of a mineral whose

spectrum showed new lines never before measured. Bunsen and Kirchhoff an-

nounced that the novel spectrum must be due to an undiscovered element. In

this way, the element cesium was discovered, and its discovery was eventually

confirmed by chemical analysis. In less than a year, rubidium was also discov-

ered the same way.

Each element, then, had its characteristic spectrum, whether absorption (if

light was passed through a gaseous sample of the element) or emission (if the

9.4 Atomic Spectra 249

B

F

D

E

C

G

A

Figure 9.4 An example of an early spectroscope, like that invented by Bunsen and Kirchhoff.

The two discovered several elements (cesium and rubidium among them) by detecting their char-

acteristic light with a spectroscope. A. Spectrometer box. B. Input optics. C. Observing optics.

D. Excitation source (Bunsen burner). E. Sample holder. F. Prism. G. Armature to rotate prism.

Figure 9.5 Line spectra of several elements. Note the relatively simple spectra for H and He.

Hydrogen (H)

400Å 700Å

Helium (He)

Zinc (Zn)

Mercury (Hg)

500Å 600Å