ATOMIC%20SPECTRA

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ATOMIC SPECTRA
Objectives 1. Determine the emission spectrum
of Hydrogen and other elements. 2. Calculate
the expected wavelengths of H using the Rydberg
equation. 3. Determine the composition of
unknown solutions using flame tests. 4.
Determine the absorption spectrum of colored
solutions and solids.
Animation of the dispersion of white light as it
travels through a triangular prism.
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History of Optics Light Studies
Ibn Alhazen is considered the Father of
Optics. He wrote the Book of Optics, which
correctly explained and proved the modern theory
of vision. His experiments on optics greatly
influenced later scientists. His
experiments included ones on lenses, mirrors,
refraction, reflection, and the dispersion of
light into its constituent colors. He studied
the electromagnetic aspects of light, and argued
that rays of light are streams of energy
particles traveling in straight lines.
Ibn Alhazen (965 1039) Arab Muslim
Scientist Father of Optics
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Historical Background of Spectroscopy
In 1608, Galileo Galilei is credited as the first
to turn his telescope to the heavens. He soon
discovered craters on our Moon, sun spots, the
moons of Jupiter, and that Venus has phases like
our Moon. Galileo claimed that his observations
only made sense if all the planets revolved
around the Sun (as proposed by Aristarchus and
Copernicus) rather than the Earth.
Galileo Galilei 1564 - 1642
The Inquisition eventually forced Galileo to
publicly recant this conclusion.
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A Quantitative Study of Light
Sir Isaac Newton was one of the first people to
study light scientifically. In 1672, Newton
directed a beam of white light through a
triangular bar of glass, called a prism. He
discovered that the light coming out of the prism
was separated into bands of colors. The
arrangement of colors produced by a prism is
called a spectrum. Prior to this it was
believed that white light was equal to purity.
Sir Isaac Newton 1643 - 1727
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Original Studies Of Light Used Only One Prism
.

When a narrow band of light from a white light
source is sent through a prism, a continuous
spectrum containing all wavelengths of visible
light is formed.
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Newtons Contribution to Spectroscopy
Newton contributed more to spectroscopy than
scientifically proving that sunlight traveling
through a prism was always broken down into the
components of the rainbow.
In fact, his main contribution was to show that
after the sunlight had been broken down into its
components by one prism, if a narrow ray of the
light from the first prism was passed through
another prism there would be no further breakdown.
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Classification of Electromagnetic Radiation
                                                                     

The color components of light are separated along
the visible range of light. The visible range of
light (400-700 nm) is merely a small portion of
the entire electromagnetic spectrum.
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Advancements in the Study of Light
Joseph von Fraunhofer is best known for his
discovery of the dark absorption lines known as
Fraunhofer lines in the Sun's spectrum, and for
designing achromatic telescope objectives. At
age 11, he was orphaned and forced to apprentice
for no pay for a harsh glassmaker named Philipp
Anton Weichelsberger. In 1801, the glass shop
collapsed and Fraunhofer was buried alive.
When Fraunhofer survived the collapse, the
court-councilor von Utzschneider, gave him books
on mathematics and optics. King Max Joseph also
took an interest in him and gave him a present of
eighteen ducats. With this money Joseph acquired
his own glass grinding machine and bought his
release from Weichelsberger.
Joseph von Fraunhofer (March 6, 1787 June 7,
1826) German Optician
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Development of the Spectroscope
Joseph von Fraunhofers initial desire was to
create a glass lens that did not produce an image
that was fringed with a rainbow of colors. He
realized the problem was that the glass lens bent
some colors more than others. He began searching
for a source of light of a single color. In
1814, he developed a spectroscope to study the
spectrum of the light given off by the sun. He
was amazed to discover that in the midst of the
rainbow of colors was a series of black lines.
These dark lines were later determined to be
the result of the absorption of selected
frequencies of the electromagnetic radiation by
an atom or molecule.
Joseph von Fraunhofer (March 6, 1787 June 7,
1826)
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Development of Diffraction Gratings
Fraunhofer also completed an important
theoretical work on diffraction and established
the laws of diffraction. One important
innovation that Fraunhofer made was to place a
diffraction slit in front of the objective of a
measuring telescope in order to study the solar
spectrum. He later made and used diffraction
gratings with up to 10,000 parallel lines per
inch. By means of these gratings he was able to
measure the minute wavelengths of the different
colors of light. (Diffraction gratings will be
discussed more later.)
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1855-1860 - Gustav Kirchhoff and Robert Bunsen
Gustav Robert Kirchhoff (March 12, 1824 October
17, 1887) German Physicist
Robert Wilhelm Eberhard Bunsen (March 31, 1811
August 16,1899) German Chemist
Bunsen and Kirchhoff further developed the
spectroscope by incorporating the Bunsen burner
as a source to heat the elements. In 1861,
experiments by Kirchhoff and Bunsen demonstrated
that each element, when heated to incandescence,
gave off a characteristic color of light. When
the light was separated into its constituent
wavelengths by a prism, each element displayed a
unique pattern or emission spectrum.
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Emission Spectra Complement Absorption Spectra
The emission spectrum seemed to be the
complement to the mysterious dark lines
(Fraunhofer lines) in the sun's spectrum. This
meant that it was now possible to identify the
chemical composition of distant objects like the
sun and other stars. They concluded that the
Fraunhofer lines in the solar spectrum were due
to the absorption of light by the atoms of
various elements in the sun's atmosphere.
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Atomic Spectra Experiment

• PART A Hydrogen emission spectrum.
• PART B Emission spectrum of other elements.
• PART C Flame Tests (organic inorganic).
• PART D Absorption spectrum of colored solutions
  
• and solids.

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PART A Record Hydrogen line spectrum with a
Scanning Spectrophotometer.
The hydrogen line spectrum contains only a few
discrete wavelengths. In the visible region,
there are only four wavelengths.
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Scanning Spectrophotometer (side view)
A light beam enters the spectrophotometer.
The focal point of the beam is brought to the
slit of the spectrophotometer. The light passing
through the slit is reflected off of a
collimating mirror and sent to the diffraction
grating. The diffraction grating disperses the
parallel beams of light into their component
wavelengths. Each different wavelength comes off
of the grating at a slightly different angle. So
the image of the slit is spread out by color
similar to a rainbow.
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Scanning Spectrophotometer (top view)
A hydrogen light source will be viewed using
a scanning spectrophotometer. The wavelengths
will be calculated for the Balmer and Lyman
series and then compared to those generated by
the computer attached to the scanning
spectrophotometer.
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Computer Output from a Scanning Spectrophotometer
The peaks on the spectrograph correspond to the
energy changes of the electrons for the Hydrogen
atom.
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Hydrogen Spectrum The Balmer Series
In 1885, Johann Jakob Balmer analyzed the
hydrogen spectrum and found that hydrogen emitted
four bands of light within the visible spectrum.
His empirical formula for the visible spectral
lines of the hydrogen atom was later found to be
a special case of the Rydberg formula, devised by
Johannes Rydberg.
Wavelength (nm) Color
656.2 red
486.1 blue
434.0 blue-violet
410.1 violet

Johann Jakob Balmer (May 1, 1825 March 12,
1898) Swiss Mathematician Honorary Physicist
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Quantum Properties of Light
?E nh? ?E the change in Energy n
1, 2, 3, h (Plancks constant) h
6.626?10-34 Js ? - frequency
Max Karl Ernst Ludwig Planck (April 23, 1858
October 4, 1947) German Physicist
The Nobel Prize in Physics 1918 for The discovery
of energy quanta.
The profile of radiation emitted from a black
body
In 1900, Planck hypothesized that energy was
quantized (i.e., energy can be gained or lost
only in whole-number multiples of the quantity
h?.) This hypothesis of quantum properties was
later extended by Albert Einstein to include
light. Einstein envisioned light as small
discrete particles of energy called photons.
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• In 1913, Bohr developed a quantum model
• for the hydrogen atom.
• Proposed a model that the electron
• in a hydrogen atom moves around
• the nucleus only in certain allowed
• circular orbits.

Niels Henrik David Bohr Oct. 7, 1885 Nov. 18,
1962 Danish Physicist
The Nobel Prize in Physics 1922 for the
investigation of the structure of atoms and of
the radiation emanating from them.
Solar System Model has electrons moving around
the nucleus.
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Calculations for Energy Levels
Where Z Atomic Number of
Element n Quantum Number the units
are Joules.
Energy levels available to the electron in the
hydrogen atom.
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Calculating the Balmer Lyman Series
As noted earlier, the four bands of light
calculated by Balmer could be simply calculated
using the Rydberg equation

Where v frequency n the quantum number R
(Rydberg constant) R 3.29 ?1015 Hz 1 Hz 1
s-1
The permitted energy levels of a hydrogen atom.
Write this equation on top of page 9-7.
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Recall that Frequency and Wavelength are related
where frequency times wavelength equals the speed
of light.
Wavelength (?) Distance between two consecutive
peaks unit nm Frequency (?) Number of waves
per second that pass a given point in space
unit s-1 (Hertz)


? ? c
Where C is the speed of light C 2.9979?108
m/s
Write this equation on top of page 9-7.
Since the speed of light is a constant, as
wavelength decreases, then frequency must
increase.
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PART B Emission spectrum of other compounds
using The STAR Spectrophotometer.

• View the line spectrum through the STAR
  Spectrophotometer
• - point arrow towards the light and view to the
  left.
• 2. Verify that the scale is lined-up accurately
  by looking at the fluorescent light. In addition
  to other lines, you should see a green doublet
  for mercury at 570 nm (the scale on the bottom).

3. Measure the line spectrum of the gas tubes set
up in Room 201. 4. Compare your results with
literature values.
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Atomic Spectra of Noble Gases
Argon
Neon
Helium
The Atomic Spectra will be determined for
Hydrogen the Noble Gases by looking at the gas
discharge tubes.
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PART C Flame Test (Organic Compounds)
Beilstein Test
If a clean copper wire is coated with a
halogen-containing compound and placed in a
flame, the presence of the halogen is revealed by
a green to blue color.
It is often possible to distinguish between
chlorine, bromine and iodine based on the color
of the flame.
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PART C Flame tests and identification of an
unknown metal.
Observe and record the color of the flame for
each known sample. Then determine the unknown
compound based on the comparison between its
flame color and those of the known samples.
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Flame Tests

• Flame Test A test used in the identification of
  certain elements.
• It is based on the observation that light
  emitted by any element gives a unique spectrum
  when passed through a spectroscope.

Flame spectrum for lithium. (Notice the faint
bands of color in the spectra.)
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PART D Absorption spectrum of colored solutions
and solids.
Sample Solution
Which color is being transmitted by this sample?
Which color is being absorbed by this sample?
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Sample Solution
For the Solution first well determine where the
maximum absorption occurs.
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Absorbance
Beer's law - the linear relationship between
absorbance and concentration of an absorbing
species.
Second, well produce A Working Curve by
plotting the Absorbance vs. the Concentration.
From this we can determine the concentration of
an unknown sample by knowing the absorption.
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Checkout (All items checked out should be
returned) 1-STAR Spectroscope 1-nichrome flame
test loop 1-copper Beilstein test loop 1-pc
blue cobalt glass (to block yellow Na
emission) 8-cuvettes in a test tube
rack 2-beral pipettes In Lab Flame test knowns
in hoods Flame test unknowns vials at bench
by blackboard 12M HCl for cleaning loops in
Beilstein hood Gas discharge tubes (for
viewing by STAR spectroscope) in
201 Computerized spectrophotometer 1 setup in
201 Cobalt Chloride solution at bench by
blackboard Colored solids to be viewed at
bench by blackboard Color spectrum vs.
wavelength charts Color Wheels on
desks Spec-20s on desks
All students View Scanning Spectrophotometer
for Part A in Room 201. View Gas Discharge tubes
for Part B in Room 201.
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Hazards 12M HCl strong acid, corrosive
(use solid NaHCO3 on spills) CoCl2 solution -
heavy metal, irritant, oxidizer CH2Cl2 -
halogenated volatile organic solvent Bunsen
Burner open flames Waste Liquid waste in
container marked Atomic Spectra. Next
Week Turn In Colorimetry Formal Report
Atomic Spectra Worksheets. Read Gas
Chromatography in the Green Book.