The Foundations of Chemistry

Figure 5-17 (a) The energy levels that the electron can occupy in a hydrogen atom and

a few of the transitions that cause the emission spectrum of hydrogen. The numbers on

the vertical lines show the wavelengths of light emitted when the electron falls to a lower

energy level. (Light of the same wavelength is absorbed when the electron is promoted to

the higher energy level.) The difference in energy between two given levels is exactly the

same for all hydrogen atoms, so it corresponds to a specific wavelength and to a specific

line in the emission spectrum of hydrogen. In a given sample, some hydrogen atoms

could have their electrons excited to the n2 level. Some of these electrons could then

fall to the n1 energy level, giving off the differencein energy in the form of light (the

1216-Å transition). Other hydrogen atoms might have their electrons excited to the n 3

level; subsequently some could fall to the n1 level (the 1026-Å transition). Because

higher energy levels become closer and closer in energy, differencesin energy between

successive transitions become smaller and smaller. The corresponding lines in the

emission spectrum become closer together and eventually result in a continuum, a series

of lines so close together that they are indistinguishable. (b) The emission spectrum of

hydrogen. The series of lines produced by the electron falling to the n1 level is known

as the Lyman series;it is in the ultraviolet region. A transition in which the electron falls

to the n2 level gives rise to a similar set of lines in the visible region of the spectrum,

known as the Balmer series.Not shown are series involving transitions to energy levels

with higher values of n. (c) The Balmer series shown on an expanded scale. The line at

6563 Å (the n 3 nn2 transition) is much more intense than the line at 4861 Å (the

n 4 nn2 transition) because the first transition occurs much more frequently than

the second. Successive lines in the spectrum become less intense as the series limit is

approached because the transitions that correspond to these lines are less probable.

202 CHAPTER 5: The Structure of Atoms

n ∞ 5 4 3 2 1

Ground

state

6563 A

°

4861 A

°

4340 A

°

1216 A

°

1026 A

°

973 A

°

950 A

°

°

Balmer

series

Balmer series

Excited states

Lyman series

Lyman series

red Visible region violet Ultraviolet

Series limit

6563 4861 4340 4102 3646 (A)

(A)

(a) °

(b)

(c)

5000 2500 2000 1500 1250 1000

Increasing energy

(Enrichment, continued)

(c)

(b)

(a)