Chemistry Reference
In-Depth Information
Solution
Notice the units of h and
joules times seconds for h and
the reciprocal of seconds for
n.
:
E
h
10
34
J
#
s)
10
16
J
7.50
(6.63
10
18
/s
10
18
/s
1.13
1
1.13
10
8
m/s)/ (1.13
1
10
18
/s)
10
18
m.
l
c
/
(3.00
2.65
This photon is in the X-ray portion of the spectrum.
Practice Problem 4.1
Calculate the energy of the photons in green
light having wavelength
10
7
5.00
m.
A most important concept that we will learn is that distinct wavelengths
of light emitted from gaseous atoms result from electronic transitions between
definite energy states within the atoms (Sections 4.2 and 4.4).
Snapshot Review
❒
Light has both wave and particle properties. Its wavelength
(l)
is
inversely proportional to its frequency
(
)
and also to the energy of
its photons:
E
h
hc
/l.
ChemSkill Builder 9.2
10
8
A. Calculate the energy of a photon of light if m.
B. If the frequency of light decreases from violet light to red light, what
happens to the (a) energy of the photons? (b) wavelength?
l
1.73
4.2
Bohr Theory
When gaseous atoms of a given element are heated, they emit light of only spe-
cific energies. When gaseous atoms of that same element absorb light, they
absorb those same energies (see Figure 3.6). To explain these phenomena of
light emission
and
light absorption,
Niels Bohr (1885-1962) (Figure 4.3) pos-
tulated that the electrons in atoms are arranged in
orbits,
each with a definite
energy. The
Bohr theory
was the first to include the explanation that electrons
in atoms have
discrete energy levels;
that is, electrons may be found only in
orbits with specific energies.
When an atom absorbs energy, an electron is “promoted” to a higher
energy level. Because each orbit has a discrete energy level, the
difference
in
energy between the orbits is also definite. After an electron has been promoted
to a higher energy level, it falls back to a lower energy level (Figure 4.4).
When it falls back, light of energy equal to the difference in energy between
the orbits is emitted from the atom. In a different experiment, when light is
absorbed by the atom, the electron is raised from one orbit to another one.
Because there is the same energy difference between the orbits, the same
energy of light is absorbed. An example of these effects is shown in Fig-
ure 4.5. Some of the possible electron transitions in a hydrogen atom are
diagrammed in Figure 4.6.
Figure 4.3
Niels Bohr
