ATOMIC SPECTRA
Objectives
1. Determine the emission spectrum of Hydrogen and other elements.
2. Determine the composition of unknown solutions using flame tests.
3. Determine the absorption spectrum of colored solutions and solids.
Animation of the
dispersion of white
light as it travels
through a triangular
prism.
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.
Sir Isaac Newton
1643 - 1727
The arrangement of colors
produced by a prism is called a
“spectrum”.
Prior to this it was believed that
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.
Newton’s 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.
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.
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.
Joseph von Fraunhofer
(March 6, 1787 – June 7, 1826)
German Optician
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
Development of the Spectroscope
Joseph von Fraunhofer’s 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.
Joseph von Fraunhofer
(March 6, 1787 – June 7, 1826)
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
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.)
1855-1860 - Gustav Kirchhoff and Robert Bunsen
Gustav Robert Kirchhoff
Robert Wilhelm Eberhard Bunsen
(March 12, 1824 – October 17, 1887)
German Physicist
(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.
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.
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.
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.
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.
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.
Computer Output from a Scanning Spectrophotometer
The peaks on the spectrograph correspond to the energy changes
of the electrons for the Hydrogen atom.
Atomic Spectra of Noble Gases
Helium
Neon
Argon
The Atomic Spectra will be determined for Hydrogen &
the Noble Gases by looking at the gas discharge tubes.
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.
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.
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.)