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Atomic Structure 129

sun, which radiates very nearly like a blackbody heated to 5800 K, is surrounded by

an envelope of cooler gas that absorbs light of certain wavelengths only. Most other

stars have spectra of this kind.

The number, intensity, and exact wavelengths of the lines in the spectrum of an

element depend upon temperature, pressure, the presence of electric and magnetic

fields, and the motion of the source. It is possible to tell by examining its spectrum

not only what elements are present in a light source but much about their physical

state. An astronomer, for example, can establish from the spectrum of a star which

elements its atmosphere contains, whether they are ionized, and whether the star is

moving toward or away from the earth.

Spectral Series

A century ago the wavelengths in the spectrum of an element were found to fall into

sets called spectral series.The first such series was discovered by J. J. Balmer in 1885

in the course of a study of the visible part of the hydrogen spectrum. Figure 4.10 shows

the Balmer series.The line with the longest wavelength, 656.3 nm, is designated

H, the next, whose wavelength is 486.3 nm, is designated H, and so on. As the

wave-length decreases, the lines are found closer together and weaker in intensity until

the series limitat 364.6 nm is reached, beyond which there are no further separate

lines but only a faint continuous spectrum. Balmer’s formula for the wavelengths of

this series is

Balmer R   n3, 4, 5, (4.6)

The quantity R, known as the Rydberg constant,has the value

Rydberg constant R1.097 107 m^1 0.01097 nm^1

The Hline corresponds to n3, the Hline to n4, and so on. The series limit

corresponds to n, so that it occurs at a wavelength of 4R, in agreement with

experiment.

The Balmer series contains wavelengths in the visible portion of the hydrogen spec-

trum. The spectral lines of hydrogen in the ultraviolet and infrared regions fall into

several other series. In the ultraviolet the Lyman seriescontains the wavelengths given

by the formula

1



n^2

1



22

1





Figure 4.10The Balmer series of hydrogen. The Hline is red, the Hline is blue, the H and H

lines are violet, and the other lines are in the near ultraviolet.

Hα Hβ Hγ Hδ H∞

656.3 nm 364.6 nm

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