THE ATOMIC SPECTRUM OF HYDROGEN

Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 7 – Atomic Structure and Periodicity LAB 5
THE ATOMIC SPECTRUM OF HYDROGEN
Introduction:
In your chemical studies, you learned that after electrons are excited from the ground
state (by an external source of energy), they naturally fall back to lower energy levels emitting
energy in the process. It has been determined that electrons that fall to the first energy level
emit energy in the range of ultraviolet light [Lyman Series], visible light is emitted if they fall to
the second energy level [Balmer Series], and infrared light emitted if they fall to the third
energy level [Paschen Series]. If one uses appropriate equipment, it is possible to view the
visible emission lines of substances and make appropriate measurements. Using these
measurements, one can determine the principal quantum number of the orbital from which the
electron fell.
A diffraction grating acts similar to a prism. It spreads a light into its component parts.
When one looks through a diffraction grating off to the side, visible emission lines from the light
source can be seen. Multiple sets of lines, like double rainbows, are visible as you look further
out.
Safety:
Use caution when operating high voltage electrical equipment. DO NOT TOUCH the gas
spectrum tube if it is plugged in and/or turned on.
Equipment and Supplies:
meter sticks, optical bench, hydrogen spectrum tube, diffraction grating, power supply
Procedure:
In this experiment, the light source is a
hydrogen spectrum tube which is energized by a
power supply. You will measure the distances (a) &
(b) and determine the angle of diffraction, θ, for each
line in the spectra. Maximize your measured
distances to minimize possible error. Before coming
to lab, you should determine how to find θ from
distances (a) and (b).
emission
spectra
light source
| ||
b
θ
a
optical
bench
diffraction
grating
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Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 7 – Atomic Structure and Periodicity LAB 5
Calculations:
Calculate the wavelength of the bright first order violet, blue and red lines in the
hydrogen spectrum, and one of the 2 nd order lines, using the formula: nλ = A sin θ, where λ is
the wavelength in meters; A = 1/N with N being the number of lines/unit length of the
diffraction grating; and n is the order of the spectrum. From this, one can proceed to calculate
the number of the principal energy level from which the electrons fell to produce each line
from the equations: c = λν, E = hν, ∆ .
, where n 2 = 2 for visible
light (see introduction). When considering the sign for ∆E, remember that the photons you
observe have the energy that is released electrons.
In the calculations section of your write-up, a sample calculation should be shown for
the determination of the wavelength of one emitted line, the energy emitted, and the energy
level from which the electrons fell. Present the results of calculations for all emitted lines
(including the decimal equivalent before rounding to the integer energy level) in tabular form.
Include the theoretical values for each of the calculations in your table as well (see your
textbook or notes for the expected visible emission spectrum of hydrogen).
Questions:
1. Discuss the relationships among the wavelength of light emitted, the energy emitted,
and the energy level from which the electrons fell.
2. What limitations of this experimental procedure prevented you from calculating the
energies emitted for electrons that fell to the 1 st principal energy level? the 3 rd principal
energy level?
3. Explain your experimental results as they pertain to Bohr’s model of the atom?
Error Analysis:
Be sure to evaluate the error for the decimal equivalent before rounding to the integer
energy level compared to the theoretical values. Discuss possible sources for that error AND
explain how those sources of error affected the calculations. Please do not discuss “calculation
errors,” “human errors,” or “measurement errors” because these are not specific enough for
error analysis.
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Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 7 – Atomic Structure and Periodicity LAB 5
THE ATOMIC SPECTRUM OF HYDROGEN
Grading Rubric
HEADINGS ON EACH PAGE
Title of Experiment
Date Experiment Started
Successive Page Numbers
Name & Partner
Item
Points
DATA SHEET
Appropriate Purpose /2
Clear & Complete Procedure /1
Procedure Initialed by Instructor /1
Clear & Complete Data and Observations /3
Data and Observations Initialed by Instructor /1
WRITE UP
Clear & Complete Pertinent Data /1
Clear & Complete Sample Calculations Present /11
Clear & Complete Answers to Questions /9
Clear & Complete Error Analysis
Sources of Error
/4
Effects of Errors
Clear & Complete Conclusion /2
TOTAL /35
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