The Foundations of Chemistry

ATOMIC SPECTRA AND THE BOHR ATOM

Incandescent (“red hot” or “white hot”) solids, liquids, and high-pressure gases give contin-

uous spectra. When an electric current is passed through a gas in a vacuum tube at very

low pressures, however, the light that the gas emits can be dispersed by a prism into distinct

lines (Figure 5-14a). Such an emission spectrumis described as a bright line spectrum.

The lines can be recorded photographically, and the wavelength of light that produced

each line can be calculated from the position of that line on the photograph.

Similarly, we can shine a beam of white light (containing a continuous distribution of

wavelengths) through a gas and analyze the beam that emerges. We find that only certain

wavelengths have been absorbed (Figure 5-14b). The wavelengths that are absorbed in

this absorption spectrumare also given off in the emission experiment. Each spectral

line corresponds to a specific wavelength of light and thus to a specific amount of energy

that is either absorbed or emitted. An atom of each element displays its own character-

istic set of lines in its emission or absorption spectrum (Figure 5-15). These spectra can

serve as “fingerprints” that allow us to identify different elements present in a sample,

even in trace amounts.

5-12

Figure 5-14 (a) Atomic emission.The light emitted by a sample of excited hydrogen atoms

(or any other element) can be passed through a prism and separated into certain discrete

wavelengths. Thus, an emission spectrum, which is a photographic recording of the

separated wavelengths, is called a line spectrum. Any sample of reasonable size contains an

enormous number of atoms. Although a single atom can be in only one excited state at a

time, the collection of atoms contains all possible excited states. The light emitted as these

atoms fall to lower energy states is responsible for the spectrum. (b) Atomic absorption.When

white light is passed through unexcited hydrogen and then through a slit and a prism, the

transmitted light is lacking in intensity at the same wavelengths as are emitted in part (a).

The recorded absorption spectrum is also a line spectrum and the photographic negative of

the emission spectrum.

198 CHAPTER 5: The Structure of Atoms

Film or detector

Film or detector

Prism

Prism

White

light

source

Absorbing sample

Excited

sample

(a)

(b)

Increasing wavelength

Absorption spectrum

Increasing wavelength

Emission spectrum