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ATOMIC SPECTRA
Objectives
1.
2.
3.
4.
Determine the emission spectrum of Hydrogen and other elements.
Calculate the expected wavelengths of H using the Rydberg equation.
Determine the composition of unknown solutions using flame tests.
Determine the absorption spectrum of colored solutions and solids.
Animation of the
dispersion of white
light as it travels
through a triangular
prism.
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”
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.
The Inquisition eventually forced Galileo
to publicly recant this conclusion.
Galileo Galilei
1564 - 1642
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”.
Sir Isaac Newton
1643 - 1727
Prior to this it was believed that
“white light” was equal to purity.
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
Weichelsberger.
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
by an atom or molecule.
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.
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.
Johann Jakob Balmer
(May 1, 1825 – March 12, 1898)
Swiss Mathematician &
Honorary Physicist
Wavelength (nm)
Color
656.2
red
486.1
blue
434.0
blue-violet
410.1
violet
Quantum Properties of Light
E = nh
E – the change
in Energy
n= 1, 2, 3, …
h – (Planck’s
constant)
h = 6.62610-34 Js
Max Karl Ernst Ludwig Planck
 - frequency
(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.
 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.
Calculations for Energy Levels
2
Z
E  2.178 1018 J ( 2 )
n
Where
Z = Atomic Number
of Element
n = Quantum Number
& the units are Joules.
Energy levels available to the electron in the hydrogen atom.
Calculating the Balmer & Lyman Series
As noted earlier, the four
bands of light calculated
by Balmer could be simply
calculated using the
Rydberg equation:
1
1 *
  R( 2  2 )
n1 n2
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.
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.
PART B: Emission spectrum of other compounds using
The STAR Spectrophotometer.
1. 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.
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.)
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?
Sample
Solution
1.2
Absorbance
1.0
0.8
0.6
0.4
0.2
0.0
400
440
480
520
560
600
640
680
720
760
Wavelengths (nm)
For the Solution first we’ll determine where the maximum absorption occurs.
Absorbance
Beer's law - the linear relationship between absorbance and
concentration of an absorbing species.
Second, we’ll 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.
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.
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.