SPECTRA EXPERIMENT 17
PART I
If available, observe flames from the following substances:
Sodium compounds:
NaCl,
NaNO3
Lithium compounds:
LiCl,
LiNO3
Strontium compounds:
SrCl2,
Sr(NO3)2
Calcium compounds:
CaCl2,
Ca(NO3)2
Copper compounds:
CuCl2,
Cu(NO3)2
Q1
Are there any elements in the above group that have characteristic colors that can be recognized from
the flame test? If so, name the element with the characteristic color and indicate the color.
Q2
In this group are there any elements that you cannot distinguish by the flame test? If so, name them.
GENERAL THEORY OF THE SPECTROSCOPE
As shown in figure 1, part of a wide beam of light originating from a source A passes through a narrow
slit B which defines a thin slice of light. The slice falls upon a diffraction grating C. The grating is a thin film
on which there are ruled many parallel lines very close together. When the lines are parallel to slit B part of the
light slice will be bent or diffracted at an angle  as it passes through the grating. The amount of bending
depends on two factors; d the distance between lines on the grating and the  (wavelength or color) of the light
hitting the grating. The exact relationship between  ,  , and d for first order diffraction is:
sin  = 
d
Thus as d increases,  increases. Longer wavelength (red) light is bent more than shorter wavelength (violet)
light.
INCIDENT BEAM
A
B
C

figure 1
BENT BEAM
If A is a source of white light (which contains all wavelengths) then an observer will see the white light
spread out into a continuous spectrum of colors, with red at the largest angle. If A is a source of
monochromatic light (which consists of a single wavelength) then an observer will see a single sharp line.
During the course of this experiment you will observe both continuous and bright line spectra, the former
characteristic of heated solid bodies, the latter isolated atoms in the gas phase being bombarded by electrons.
For general information the wavelength associated with colors of visible light are listed in Table II, page 6.
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PART II
Observe an incandescent light source with your spectroscope. This will be a spectrum from a glowing solid.
Q3
In your lab notebook draw a sketch of the spectrum making sure that you record the
scale reading and colors associated with your own scale and spectroscope. Use the
same spectroscope for the entire experiment. You should also note the position of
each line you observe on a diagram like the one below:
700
600
500
400
BE SURE TO DRAW THE DIAGRAM TO SCALE. (No smaller than the sample)
PART III
Calibrate your spectroscope by observing the light from a fluorescent arc.
All atoms and ions, when bombarded by enough electrons will emit light, which when passed through a
spectroscope appears to an observer as a series of colored lines characteristic of the differences between the
energy levels of the atoms or ions. In this experiment you will use a simple spectroscope capable of surprising
accuracy t o study the bright line spectra of some simple ions and relate your observations to electronic
transitions within the atoms.
Look through the diffraction grating at the narrow end of the spectroscope pointing the slit towards the
light source to be analyzed. The spectrum will appear on the right side of the slit below the scale where
the wavelengths of the absorption or or emission lines can be read. The visibility of the spectrum can be
improved by holding the hand around the narrow end of the spectroscope using the thumb and index
finger to keep stray light from around the eye.
The numbers on the scale represent 100 nm units. Looking through the spectroscope at a fluorescent
lamp, all colors of the spectrum will be visible with three brighter lines- one violet line at 436 nm and
one green line at 546 nm and one orange line at 605 nm.. If the lines are not exactly in place, the
difference must be added or subtracted, respectfully, in all further observations.
Q4
Sketch the spectrum of the fluorescent light as you did for the visible spectrum in part II.
Record the color of the lines and their scale reading.
Q5
Calculate the wavelength that corresponds to each of your scale readings for the fluorescent lines.
A sample table is given below. (Include units on your values) (Scale reading = nm )
Line in fluorescent
spectrum
1
2
color of line
scale reading
green
violet
???
???
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PART IV
Observe the spectrum of some gaseous elements. Around the laboratory you will find discharge tubes that
contain some gaseous elements. These tubes are connected to a high voltage supply so they will be energized to
give off light. DO NOT TOUCH THE DISCHARGE TUBES, DO NOT CHANGE THE DISCHARGE
TUBES. The reasons are they are very HOT, they have high electrical voltage, and if you break them they cost
around $45.00.
Q6
Observe each of the spectra and record the scale readings and the color of the various lines for each
element. (Try to find 4 lines in the spectra of H2 if possible, three are easily distinguished.)
PART V
Observe the spectra of the unknown substances (X & Y).
Q7
Record the scale readings of the unknown and diagram the spectra.
Q8
For the H2 and He lights determine the wavelength that corresponds to each scale reading.
Q9
Determine the percentage error for the experimental wavelength of He compared to literature values
given in Table I. Do this for all lines in the He spectrum.
Q10
What difference do you note in the spectra of tungsten as found in the incandescent lamp compared to
the other elements.
QUESTIONS 11-25 REFER TO THE HYDROGEN SPECTRA—All calculations from RED BLUE
Q11
Calculate the frequencies of the hydrogen lines that you saw in your spectroscope.
Convert nm to cm before determining frequency. (use page 5 of lab)
Q12
Calculate the energy associated with each frequency. This is the energy difference in a transition of an
electron from one energy level to another. Make sure to use Planck’s constant on pg 5 of lab.
Q13
These spectral lines correspond to energy levels given off as electrons drop from a higher energy level to
a lower energy level of -328 kj/mole. Calculate the value of the higher energy level from which the
transition occurred in dropping to a lower energy level. The visible spectra occurs when transitions are
made to the lower level of -328 kj/mole. (n=2)
Energy difference (from Q12) = Higher energy level - lower energy level
or
E difference = Higher energy level - (-328 Kj/mole)
Although the sign is negative it does not mean the energy is negative. This is merely convenience, a
convention that has been chosen.
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Review sections 4-1 and 4-2, pgs. 97-111 in your text.
Q14
We can assign an integer, n, to each energy level that can exist. The energy level we choose as -328
kj in question 13 will be called n = 2. We will name each higher energy level with the next highest
integer. Make a table relating the integer assigned to each energy with its energy level from Q13.
Include n, n2, 1/n2, and the energy level value.
n
n2
1/n2
2
Energy level
-328 Kj/mole_
3
4
5
Q15
Plot the value of the energy level versus the integer n on a full page graph.
Put n on the horizontal axis and E on the vertical axis with 0 being the top of the energy scale.
Q16
From the shape of the graph in Q15 what type of relationship exists between n and E ?
Q17
Plot the value of the energy level versus the value of n2.
Q18
What type of relationship exists in Q17 ?
Q19
Plot the value of the energy for the energy level versus the value of 1/n2.
Be sure to convert the value of 1/n2 to decimals before plotting the graph. (example : 1/4 = .25 )
Q20
What type of relationship exists in Q19 ?
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Q21
Draw a full page diagram like the one below. Note the value of n and the energy of the energy levels are
to be included. The scale should be set up to range from -1312 Kj/mole as a minimum energy and 0
as the maximum energy. Make the diagram to scale.
5
4
Energy ( kj/mole)
3
n
2
-328
1
-1312
Q 22
Specify what the code letters were for the unknowns and what substances you determined to be present.
Q23
What substance is present in fluorescent lights? (use your spectral data)
EQUATIONS AND CONSTANTS TO BE USED
E=h
h = 39.8 x 10-14 kj.sec/ mole
= c

c = 3.0 x 1010 cm/ sec
= wavelength
 = frequency in sec-1
nm x 1 x 10-7 cm = _______ cm
%error = accepted value – experimental value x 100
accepted value
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TABLE I --- REFERENCE SPECTRAL WAVELENGTHS
A.
Flame spectra (from the Handbook of Chemistry and Physics): in nm, (b) = position of band head, P =
most persistent
Barium: ------- 513 (b) ; 534 (b) ; 553 (b) P
Calcium: ------ 554 (b) ; 618 (b) P ; 620 (b) P
Lithium: ------ 610; 670 P
Potassium: ---- 404; 404; 766 P; 769 P
Sodium: ------ 589; 589
Strontium: ---- 606 (b) P ; 662 (b) ; 674 (b)
B.
Mercury arc: 623; 579; 577; 546; 496; 491; 435; 404
C.
RARE GAS: He:
400; 460; 500; 585; 680
Ar:
489; 537; 544; 621
Ne:
585; 602; 640
Kr:
583; 564
__________________________________________________________________________________________
TABLE II -----THE VISIBLE SPECTRUM
nm units
Visible Spectrum-----------------------------------------------------
400--700
Violet (representative 4100), limits--------------------------- 400--424
Blue (representative 4700), limits---------------------------
424--490
Green (representative 5200), limits--------------------------- 490--575
maximum visibility --------------------------------------------
556
Yellow (representative 5800), limits--------------------------- 575--585
Orange (representative 6000), limits--------------------------- 585--647
Red
(representative 6500), limits---------------------------
647--700
Infrared, greater than ----------------------------------------------
700
Ultraviolet, less than ----------------------------------------------
400
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