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Spectroscopy
UV, Vis, Mass, PES, IR
Visible Light Spectroscopy
from AP Chemistry Crash Course, 2nd ed, by Adrian Dingle, 2014
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The technique is generally only useful for colored solutions.
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A colorimeter is used (or a Spec-20), and the wavelength of light chosen
for the experiment should be complementary to the wavelength of the
light reflected by the solution (complementary to the color of the solution).
This allows for maximum absorbance (an important aspect of collecting
reliable data over a range of concentrations).
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Linear plots of absorbance vs concentration are found.
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The equation used is A=abc (A is absorbance, a is an absorptive
constant, b is path length through the solution, c is concentration)
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Beer-Lambert law is used to assess the concentration of a solution by
relating the absorbance of the solution to the concentration of the solution
Mass Spectroscopy
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Often used as evidence for
isotopes
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Plot of charge/mass ratio on x
axis and relative abundance on
y axis
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The charges of most species at
most peaks is +1
The mass spectrometer: A sample of atoms is vaporized and
then bombarded with electrons to knock electrons free
creating positive ions. The positive ions are accelerated into a
beam using an electric field. The positive ions are deflected
in a magnetic field; lighter and more highly charged ions are
deflected more than heavier and lower charged ions. The
beam is detected and converted into a mass spectrum.
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The mass spectrum
of an element can be
used to calculate the
average atomic mass
of that element.
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This mass spectrum
show that there are 7
isotopes of Mo of
masses 92, 94, 95,
96, 97, 98, and 100.
These isotopes have
abundances of
14.84%, 9.25%,
15.92%, 16.68%,
9.55%, 24,13% and
9.63%, respectively.
Try to calculate the average
atomic mass of Mo.
The mass spectrum of neon shows three peaks at
mass/charge ratios of 20, 21, and 22 with abundances
of 90.48%, 0.27%, and 9.25%, respectively. Calculate
the average atomic mass of Ne based upon this data.
Which of the three peaks would be the tallest? Explain.
Photoelectron
Spectroscopy (PES)
Used as evidence for the shell model and orbitals
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High energy x-rays and UV photons are used to eject electrons
from atoms. This is called the photoelectric effect. The energy
applied can be calculated using E=hv where h=Planck’s constant
and v=frequency.
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Electrons close to the nucleus have a greater attraction and will
require larger energies to eject them.
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For example, the electron configuration of sodium is
1s22s22p63s1. We would expect four peaks on the PES plot
corresponding to the four different energies required to remove
electrons from the different sub-shells. Peaks have heights
relative to the number of electrons in each sub-shells, so for Na,
2:2:6:1.
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Note that the scale on the x-axis is not
linear. Electrons in different sub-shells
but the same shells tend to be close
together in terms of energy. Electrons
in different shells are often widely
separated in terms of energy.
2
2
6
2
1
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Aluminum has an electronic configuration of 1s 2s 2p 3s 3p
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How many unique peaks are expected in aluminum’s PES
spectrum? Explain.
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Which electrons correspond to the largest energies? Explain.
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Which peak in the spectrum will have the greatest intensity (be
the largest)? Explain.
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Explain why the peaks corresponding to the 3s and 3p electrons
are relatively close together, and why they are distinctly different
from the electrons in the 2s and 2p orbitals.
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An element produces a PES spectrum that has only three peaks,
in the ratio 2:2:1. Identify the element:
A) Aluminum B) Boron C) Carbon D) Sodium