Experiment 5
Hydrogen
The Atomic Spectrum of
OBJECTIVES


To learn about atomic spectra
To become familiar with calcualtiong wavelengths, frequencies, and
energies
DISCUSSION
Electromagnetic radiation traveling through a vacuum has properties similar to those of
an ocean wave traveling through water, and we therefore speak of radiant energy as
having wavelike properties. Like an ocean wave, electromagnetic radiation is
characterized by a frequency, a wavelength, and an amplitude. Multiplying the
wavelength of a wave in meters by its frequency in reciprocal seconds gives the speed of
a wave in meters per second. The rate of travel of all electromagnetic radiation in a
vacuum is a constant, called the speed of light, c, with a numerical value of 3.00 x 10 8
m/s, written as
c = symbol 108 \f "Symbol" \s 11 symbol 110 \f
"Symbol" \s 11
Light is a form of energy that includes radio and television waves, radar rays,
microwaves, infrared rays, visible and ultraviolet light, x-rays, and gamma rays. Visible
light includes those forms of light that the human eye is capable of detecting. White
light is visible light that comes from the sun or an incandescent light bulb. When a
beam of white light is passed through a prism, the different wavelengths travel through
the glass at different rates. As a result, white light is diffracted into its components,
ranging from red to violet, or 400 nm to 700 nm.
Most substances will emit light energy when heated to high enough temperatures.
Individual atoms give of light when excited. Unlike white light, the light given off by an
energetically excited atom is not a continuous distribution of wavelengths, but rather a
line spectrum consisting of only a few wavelengths at discrete frequencies. Each
frequency, symbol 110 \f "Symbol" \s 11 , is related to a certain energy by
E = h symbol 110 \f "Symbol" \s 11
where h is the Planck constant, a physical constant with the value 6.63 x 10-34 Js.
Conversely, the energy of a photon of a given spectral line can be calculated using this
formula.
The fact that a given atom produces only certain fixed lines in its spectrum indicates
that the atom can undergo only certain fixed, definite energy changes. An atom can
emit only energy corresponding to definite, regular changes in the energies of its
component electrons.
Refer to your text (section 7.2) for a more thorough description of atomic spectra. Or
access
http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
MATERIALAS AND EQUIPMENT
Hand Spectroscope
Gas discharge tubes
CaCl2, BaCl2, NaCl, LiCl, KCl, CuCl2, SrCCl2 solutions
nichrome wire (or micro spatula)
Unknown
PRELABORATORY ASSIGNMENT: Atomic Spectra
Questions
1. The atomic spectrum of sodium consists of several lines. The most predominant one
is seen at 580 nm. Calculate its energy.
(Planck constant h = 6.626 x 10-34 J s; c = 2.998 x 108 m/s)
2. Compare the energy of an electron in the ground state to that in an excited state. In
which state does the electron have the higher energy? Please explain.
3. Two states differ in energy by 4.08 x 10-18 J. When an electron moves from higher to
the lower state, calculate symbol 108 \f "Symbol" \s 11 (a) in meters and (b) in
nanometers
4. Consider the following transitions
(1)
n = 1 to n = 3
(2)
n = 3 to n = 4
(3)
n = 3 to n = 2
(4)
n = 5 to n = 3
a. Which one involves a ground state?
b. Which absorbs the most energy?
c. Which emits the most energy?
EXPERIMENTAL PROCEDURE
A.
Flame Tests
In your notebook: Record the color and its dominant wavelength for each compound
as well as for the unknown. Determine the identity of your unknown.
1. Obtain a clean watch glass and carefully add 2-3 drops conc. HCl (Caution!). Set up
your Bunsen burner. Adjust the flame of the burner so that it is as hot as possible.
Dip the end of a nichrome wire (or micro spatula) into conc. HCl. Then heat the wire in
the flame until no visible color appears. Repeat this process as necessary.
2. Add the following chemicals onto separate clean watch glasses. CaCl2, BaCl2, NaCl,
LiCl, KCl, CuCl2, SrCl2.
Use your clean nichrome wire (or micro spatula) to pick up crystals (about the size of a
small pea), then hold the wire into directly the flame and record the color. View the flame
through a cobalt glass when using KC.
Correlate the color of the flame with it’s dominate wavelength.
Repeat the same procedure using your unknown.
B.
Emission Spectrum of Elements
In your notebook: Record and explain your observation.
1. Obtain a hand spectroscope and three gas discharge tubes (no Hydrogen please) that
you might like to examine.
2. Insert the first gas discharge tube into the power supply and observe the color of light
produced. Carefully examine the emitted light through the spectroscope. Record the
wavelengths that you observe.
3. Repeat the above procedure with two other gas discharge tubes. In each case, record
the wavelengths observed for that element.
C.
Hydrogen Emission Spectrum
Hydrogen is the simplest of the atoms, consisting of a single proton and a single electron.
The emission spectrum of hydrogen was the first to be completely explained by a theory of
atomic structure, by the Danish scientist Niels Bohr.
Insert a hydrogen gas discharge tube into the power supply and observe the color of light
produced through the hand spectroscope. Carefully examine the emitted light through
the spectroscope.
In your notebook:
1. Use the scale in the viewer to estimate the wavelengths of each line.
2. Now calculate the wavelengths using the chart below. After having done the
calculations, state the transition that accounts for each wavelength observed.
To complete the chart, you need to calculate E’s for all possible transitions for orbits n =
6 down. Round off energies to the nearest 0.01. Place this value in the upper portion of
the square. Convert the energy to wavelength in nanometers and place this value in the
lower portion of the square. Round off values to 4 significant figures.
The equations you will need to do these calculations are:
En = - 1312 kJ/mol
E = Ehi - Elo
n2
and
symbol 108 \f "Symbol" \s 11 = 1.196 x 105 kJ nm/mol
E
Calculate each of the first six energies in kJ/mol. Round off to the nearest 0.01.
Second, construct a table just like the one shown below and calculate the E’s and
wavelengths for all transitions. For example, the energy and the wavelength for the
transition from E5 to E3 have been calculated and filled in the appropriate box. Units are
omitted.

Elow
 Ehi
n=2
n=3
n=4
n=5
n=6
n=
n=1
n=2
n=3
93.30
1282
n=4
n=5
n=6
Question
What is the a) longest and b) shortest wavelength of the electromagnetic radiation
emitted by the hydrogen atom undergoing a transition from the n = 6 level.
WASTE DISPOSAL
Dispose of the salts in the designated container in the fume hood. Return the gas
discharge tubes to its proper place.