Lesson 24 Emission and Absorption Spectra!
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Part 1: Types of Light Spectra !
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The field of spectroscopy involves looking at light from various sources separated through a diffraction
grating or a prism and analyzing the colours that are seen. !
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There are three types of Spectra that can be observed:!
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Continuous Spectrum!
Emission Spectrum !
Absorption Spectrum!
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Each element has a unique emission and absorption spectrum.!
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Since the emission and absorption spectrum for each element is
unique, scientists can study the light from far away galaxies to learn
about their chemical composition.!
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Part 2: Bohr’s explanation for elemental spectra!
Neils Bohr came up with a model for the atom in 1913 to help to
explain the observed emission and absorption spectra for each
element. !
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Bohr’s model of the atom:!
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Electron Energy Levels:!
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Emission of photons:!
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Absorption of photons:!
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Ground state:!
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Ionization:!
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Excited states:!
Example: Hydrogen!
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Electron transition from n=2 to n=5!
Electron transition from n=2 to n=4!
Electron transition from n=2 to n=3!
Example: What wavelength of photon would you require to
completely ionize a hydrogen atom with its electron in the first
excited state?!
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Absorption Spectra:!
Bohr’s model for the atom explained the
absorption spectra of different elements.
Only photons with energies exactly
matching the transitions of electrons
between energy levels could be absorbed. !
Emission Spectra:!
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Bohr’s model also explained the unique
emission spectra of elements and why the
emission and absorption spectra match. If
electrons of a gas are in an excited state
(either by heating the gas or exposing it to a
high voltage), they will emit photons
matching their energy level transitions as
they fall back to lower states. !
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Bohr’s model also explained why emission
spectra of elements have a few more lines
than their absorption spectra. This is
because electrons can transition between
states as they fall. !
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Two kinds of energy level diagrams:!
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1) Ionization Energy is zero!
2) Ground state is zero!
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Both types of diagrams tell you the same thing, the relative difference between energy levels, from
different perspectives.!
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Part 3: The Franck-Hertz Experiment!
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The Franck-Hertz Experiment was conducted by James Franck and
Gustav Hertz in 1914. This experiment help to confirm the Bohr
energy levels model. In the experiment, high energy electrons very
fired through mercury vapour. They measured the kinetic energy of
each electron before it entered the vapour and after it exited.!
Mercury
Vapour
Electron
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Electron Kinetic
Energy Before
Electron Kinetic Energy
After
2.0 eV
2.0 eV
3.0 eV
3.0 eV
4.0 eV
4.0 eV
5.0 eV
0.1 eV
6.0 eV
1.1 eV
7.0 eV
2.1 eV or 0.3 eV
8.0 eV
3.11 eV or 1.33 eV
9.0 eV
4.1 eV or 2.3 eV or 0.2 eV
Energy Absorbed?
Example: Mercury Energy Levels!
Electron!
Electron!
Ek = 7.0 eV
Ek = 7.0 eV
Summary:!
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Emission of Photons!
Absorption of Photons!
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Absorption of electron energy
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Practice Problems:!
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1.
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2.
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3.
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4.
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When a solid, liquid or very dense gas is heated until it gives off light and the light is
passed through a prism, a(n) _______________ spectrum is produced.
When a rarefied gas is excited with electrical energy until it gives off light which is passed
through a prism, a(n) _______________ spectrum is produced.
Why can an emission spectrum be used to positively identify different elements or
molecules in their gaseous states?
When white light is passed through a gas and then allowed to go through a prism, it will
produce a(n) _______________ spectrum.
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A Frank-Hertz experiment was carried out with on Lichtium vapour in a chamber. The energies
of the electrons sent into the chamber (Einput) and those coming out of the chamber (Eouput) were
measured and the data is given below. What can be inferred about the possible energy levels
within this fictitious atom? Assume that the ground state energy is zero.
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6.
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7.
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Einput (eV)
Eoutput (eV)
4.0
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
4.0
0.0
0.5
1.0
1.5
0.0 or 2.0
0.5 or 2.5
0.0 or 1.0 or 3.0
0.5 or 1.5 or 3.5
a) Draw an energy level diagram for Lichtium.
b) What wavelengths of light would you expect Lichtium to absorb? (248 nm, 177 nm,
155 nm)
c) What wavelengths of light would you expect Lichtium to emit? (total of six
wavelengths)
d) If an atom was excited to the fourth excitation state, how many possible wavelengths
of light would be emitted when the atom fell to its ground state? (10)
An electron of energy 4.7 eV collided with an atom called Chekleyium and was reflected
with an energy of 1.4 eV. Immediately after the atom emitted a photon. What is the
frequency of the photon? (8.0 x 1014 Hz)
A mercury atom has stationary energy states of 4.9 eV, 6.7 eV and 8.8 eV above the
ground state.
A. An electron with an energy of 3.6 eV collides with an unexcited atom. Estimate
the energy of the reflected electron.
B. Another electron with an energy of 6.8 eV is incident on the atom. What is (are)
the possible energy(ies) of the reflected electron?
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The emission spectrum of an unknown substance contains lines with the following
wavelengths: 172 nm, 194 nm, and 258 nm. If these all represent transitions to the
ground state (a) calculate the energies of the first three excitation states, and (b)
determine the wavelengths of three other lines in the substance’s emission spectrum.
(4.82 eV, 6.41 eV, 7.23 eV; 516 nm, 782 nm, 1517 nm)
9.
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10.
What is the energy difference between the two energy levels in a sodium atom that give
rise to the emission of a 589 nm photon? (2.1 eV)
A Frank-Hertz experiment was carried out with a fictitious gas in a chamber. The
energies of the electrons sent into the chamber (E1) and those coming out of the chamber
(E2) were measured and the data is given below. What can be inferred about the
possible energy levels within this fictitious atom? Assume that the ground state energy is
zero.
E
input
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11.
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12.
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13.
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14.
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6.4
6.7
6.8
7.8
8.7
8.9
9.1
9.3
9.5
(eV)
E
output
(eV)
6.4
6.7
0.0
1.0
0.0 or 1.9
0.2 or 2.1
0.4 or 2.3
0.0 or 0.6 or 2.5
0.2 or 0.8 or 2.7
What happens to a mercury atom that has been raised to its first excitation level by a
collision with an electron?
An electron whose kinetic energy is 3.0 eV collides with a free mercury atom. What will
be the kinetic energy of the electron after the collision? (3.0 eV)
Electrons are accelerated in a Franck-Hertz experiment through mercury vapour with a
potential difference of 7.0 V. What are the energies of all the photons that may be
emitted by the mercury vapour? (4.9 eV, 6.7 eV, 1.8 eV)
If an atom emits a photon of wavelength 684 nm, how much energy does the atom lose?
(1.82 eV)