1
Lab #14
EMISSION SPECTROSCOPY
INTRODUCTION:
The emission spectrum is the set of light frequencies emitted by substances after they
have been excited with various forms of energy, most commonly heat or electrical. Since the
frequency of light emitted under these conditions depends on the energies of the excited and
ground states of electrons in the atoms, the spectrum serves as a very sensitive āfingerprintā of
the atoms present. For example, by studying emission spectra of the stars, we can determine
their chemical composition. Also, emission spectra are used to identify poisons in food,
pesticides in the environment, and numerous substances in forensic samples.
Although emission spectroscopy has many practical uses, it is equally interesting because
it provided the first quantitative information about the energy levels in atoms, and allowed
chemists to calculate values for the allowable energies of electrons in atoms.
The Bohr equation (named after Danish physicist, Niels Bohr):
(Equation 1)
Ī½2
i
2
f
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n1
-
n1
)(s10 x 3.289 = )(s 1-151-
can be used to calculate the frequency of light emitted (Ī½
Ī½Ī½
Ī½) when an electron falls from an upper
level (ni) to a lower level (nf). The constant, 3.289 x 1015 s-1, is known as the Rydberg Constant.
For the visible lines of the hydrogen spectrum, nf is always 2, and the equation can be rearranged
to the form y = mx + b (the equation of a straight line, where m is the slope and b is the y-
intercept):
(equation 2) 410 x 3.289
+
n1
10 x 3.289- = )(s 15
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2
i
Ī½
A plot of Ī½
Ī½Ī½
Ī½ vs. 1
ni
2 should give a slope equal to the Rydberg Constant (3.289 x 1015), and an
intercept equal to the Rydberg Constant divided by 4, if the electronic transitions all terminate on
the second level.
In todayās experiment we will observe emission spectra of electrically excited gases and,
if your instructor chooses, thermally excited salts. A spectroscope will be calibrated by
observing the helium spectrum. The calibrated spectroscope will be used to determine the
wavelengths of the visible light spectrum (called the Balmer Series) in the hydrogen spectrum,
and these wavelengths will be used to determine an experimental value for the Rydberg Constant.
In addition an unknown salt or gas may be identified by its spectrum.
