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Atomic Physics
Preview
Section 1 Quantization of Energy
Section 2 Models of the Atom
Section 3 Quantum Mechanics
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Section 1
Atomic Physics
TEKS
The student is expected to:
8A describe the photoelectric effect and the
dual nature of light
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Section 1
Atomic Physics
Section 1
What do you think?
• Albert Einstein, while working as a Swiss patent clerk in
1905, submitted three papers for publication. The topics
for the three papers were:
• (1) the special theory of relativity, including E = mc2
• (2) an explanation of Brownian motion
• (3) a quantum theory of light used to explain the photoelectric
effect
• Sixteen years later he was awarded the Nobel Prize in
Physics for one of these papers.
• Which paper earned him this award?
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Atomic Physics
Section 1
Blackbody Radiation
• A blackbody is a perfect
absorber and perfect
emitter of radiation.
– All radiation passing
through the hole is trapped
by multiple reflections and
very little is emitted.
– Called a blackbody
– When you look into the hole,
it is black because no light
escapes.
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Atomic Physics
Blackbody Radiation
• The radiation that escapes a
blackbody has a wide range of
wavelengths.
• At higher temperatures, the
shorter wavelengths such as
visible and UV light become
more prominent.
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Section 1
Atomic Physics
Failure of Classical Wave Theory
• The wavelength
distribution predicted by
classical wave theory was
wrong.
• Max Planck proposed a
theory that matched the
radiation curve.
– Energy is given off in
discrete amounts of
energy.
– Each unit of light energy is
called a quanta.
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Section 1
Atomic Physics
Section 1
Planck’s Quantum Theory
• Energy can only be absorbed or released in multiples of
a specific value:
E = nhf
– h is Planck’s constant (6.63  10-34 J•s)
– f is the frequency
– n represents the multiple (1,2,3,…….)
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Atomic Physics
Section 1
Planck’s Quantum Theory
• The energy absorbed or released must always
be a whole number multiple of hf.
• A unit of energy used for extremely small values
is the electron volt (eV).
– The amount of energy an electron gains when
changing its potential difference by 1.0 V
– 1 eV = 1.60  10-19 J
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Atomic Physics
Section 1
Blackbody Radiation and the Ultraviolet
Catastrophe
Click below to watch the Visual Concept.
Visual Concept
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Atomic Physics
Section 1
Classroom Practice Problem
• Find the energy of a photon of green light with a
frequency of 5.50  1014 Hz . Give your answer
in joules and in electron volts.
• Answer:
– 3.65  10-19 J or 2.28 eV
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Atomic Physics
The Photoelectric Effect
• When light strikes a metal
surface, electrons may be
ejected.
• To escape from the
binding forces of the
atoms, electrons must
absorb enough energy
from the light.
• Classical wave theory did
not predict the
experimental results.
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Section 1
Atomic Physics
Photoelectric Effect
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Section 1
Atomic Physics
Section 1
Photoelectric Effect
• Einstein theorized that light was quantized.
– Photons, not waves
– E = hf
• Based on this theory, either the frequency (or
energy) of the photons was high enough to eject
the electrons, or it wasn’t.
– If energy (hf) was not high enough, no electrons
would escape no matter how intense the light or how
much time the light shined on the metal.
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Atomic Physics
Section 1
Photoelectric Effect
Click below to watch the Visual Concept.
Visual Concept
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Atomic Physics
Section 1
Photoelectric Effect
• Work function is the minimum
energy needed to escape the
metal’s surface.
– Some electrons require more than this
minimum.
– Any additional energy above hft is the
KE the electron has after ejection.
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Atomic Physics
Section 1
Photoelectric Effect
• Einstein’s photon theory explains the
photoelectric effect.
– No matter how bright the light (more photons), no
electrons are emitted unless the photons have enough
energy (hft).
– A brighter light produces more electrons because
more photons strike the surface, but the maximum KE
of the electrons does not change.
– Electrons are emitted almost instantaneously
because each photon ejects a single electron.
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Atomic Physics
Section 1
The Dual Nature of Light
Click below to watch the Visual Concept.
Visual Concept
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Atomic Physics
Section 1
Compton Scattering
• If light has particle-like
properties, it could collide
with electrons and lose
some energy.
– Like billiard balls
• The scattered photon has
less energy (E = hf), so
the wavelength is greater.
– The wavelength change is
called the Compton shift.
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Atomic Physics
Section 1
Compton Shift
Click below to watch the Visual Concept.
Visual Concept
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Atomic Physics
Section 1
Now what do you think?
• In science, theories are developed and tested.
When a theory fails to match the results of
experiments, the theory must be modified.
• In what ways did the wave theory fail to explain the
ejection of electrons from a metal surface when it was
illuminated with light of various wavelengths?
• How did Einstein’s explanation of the photoelectric
effect change our ideas about light and also explain
the experimental results?
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Atomic Physics
TEKS
Section 2
The student is expected to:
2C know that scientific theories are based on
natural and physical phenomena and are capable
of being tested by multiple independent
researchers. Unlike hypotheses, scientific theories
are well-established and highly-reliable
explanations, but may be subject to change as new
areas of science and new technologies are
developed
3D explain the impacts of the scientific contributions
of a variety of historical and contemporary scientists
on scientific thought and society
8B compare and explain the emission spectra
produced by various atoms
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Atomic Physics
Section 2
What do you think?
• Describe an atom. Consider the following in your
explanation:
• What are the component parts of an atom?
• How do they compare in size?
• How does the charge on each compare?
• Where are these parts located in the atom?
• Do the component parts of an atom move about or
remain in fixed positions?
• If they move, describe the motion.
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Atomic Physics
Models of the Atom
• The earliest models described
atoms as tiny, indestructible,
neutral spheres.
• J.J. Thomson discovered
electrons in 1897.
– Since atoms had negatively
charged electrons, they must have
an equal positive charge of some
sort.
• Thomson’s model is like seeds
(electrons) in watermelon
(positive charge).
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Section 2
Atomic Physics
Section 2
Ernest Rutherford
• Rutherford fired positive alpha particles at a thin sheet of
gold foil.
– Most passed straight through the foil.
– Some were deflected as shown.
– Thomson’s spheres of + charge would not cause this much
deflection.
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Atomic Physics
Section 2
Rutherford's Gold Foil Experiment
Click below to watch the Visual Concept.
Visual Concept
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Atomic Physics
Section 2
Rutherford’s Atom
• A small, positively-charged core called the
nucleus
• Mostly empty space outside the nucleus
• Electrons circle the nucleus like planets around
the sun.
– There was a problem with the electron motion in this
model:
• A circling charged particle is accelerating and would radiate
energy, and would spiral into the nucleus as it lost energy.
• Atoms could not exist in this model for more than onebillionth of a second.
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Atomic Physics
Section 2
Atomic Spectra
• The specific wavelengths of light given off when an
electric current passes through a gas
– Every element produces different spectral lines.
– Rutherford’s model could not explain this phenomenon.
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Atomic Physics
Atomic Spectra
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Section 2
Atomic Physics
Section 2
Atomic Spectra
• Emission spectra are the colors given off by the gas.
• When white light passes through hydrogen, it absorbs
the same frequencies.
– Absorption spectra are used to study gases surrounding the
stars.
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Atomic Physics
Niels Bohr’s Model of the Atom
• Bohr’s model assumed that the
orbits of electrons were
quantized.
– Electrons existed only in certain
energy levels.
– Ground state is the lowest level.
– Electrons absorb energy and move
to excited states.
– They release the energy when
returning to ground state.
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Section 2
Atomic Physics
Section 2
Hydrogen’s Spectrum Explained
• The visible lines in
hydrogen’s spectrum result
from jumps to level 2 from
levels 6, 5, 4 and 3.
• Would drops to level 1
produce photons with more
or less energy?
• Where would these spectral
lines appear in the
spectrum?
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Atomic Physics
Section 2
Bohr Model of the Atom
Click below to watch the Visual Concept.
Visual Concept
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Atomic Physics
Section 2
Classroom Practice Problems
• The five lowest energy levels
for Hg vapor are shown above.
Assume an electron falls from
E5 to E2.
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• What amount of
energy is given off in
eV and in joules?
• What is the frequency
of this photon?
• Is this photon in the
visible spectrum?
• Answers:
– 2.01 eV, 3.22  10-19 J
– 4.86  1014 Hz, visible
Atomic Physics
Section 2
Bohr’s Model of the Atom
• Bohr’s model was the first to explain spectral
lines, but it raised some questions as well.
– It could not be extended well to multi-electron atoms.
– It did not explain why electrons did not radiate energy
as they circled the nucleus.
– It did not explain why electrons had only certain
stable orbits and other orbits did not exist.
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Atomic Physics
Section 2
Now what do you think?
• Describe an atom. Consider the following in your
explanation.
• What are the component parts of an atom?
• How do they compare in size?
• How does the charge on each compare?
• Where are these parts located in the atom?
• Do the component parts of an atom move about or
remain in fixed positions?
• If they move, describe the motion.
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Atomic Physics
TEKS
Section 3
The student is expected to:
3D explain the impacts of the scientific
contributions of a variety of historical and
contemporary scientists on scientific thought
and society
8A describe the photoelectric effect and the
dual nature of light
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Atomic Physics
Section 3
What do you think?
• The photoelectric effect shows that electromagnetic
waves can act like particles. Does it seem possible that
particles such as bullets or electrons can show wavelike
behavior?
• If hundreds of baseballs pass through a doorway, will
they spread out like a sound wave?
• Will a beam of electrons going through a small
opening diffract?
• After splitting a beam of electrons, could an electron
from one beam interfere constructively with one from
the other beam?
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Atomic Physics
Section 3
Wave-Particle Duality
• Low-frequency photons (radio waves) have very little
energy.
– Energy values (E = hf) are about 10-27 J.
• Too small to detect single photons
• A large number of photons/second are needed
• Behaves more like a wave of photons
• Visible light photons have enough energy to be
detected in some experiments, like the photoelectric
effect.
• High-frequency photons such as gamma rays behave
more like particles.
– Wave properties (diffraction, interference) are hard to observe.
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Atomic Physics
Section 3
Matter Waves
• Louis de Broglie hypothesized that, if light could behave
like particles, then particles could behave like waves.
– All forms of matter have wavelike properties.
– He derived an equation for the wavelength ().
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Atomic Physics
Section 3
Matter Waves
• De Broglie postulated that Planck’s equation could be
used to find the frequency of matter waves.
• De Broglie wavelengths and frequencies for matter
waves depend on momentum (mv) and energy (E).
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Atomic Physics
Section 3
Matter Waves
• Light waves passing through an opening exhibit
interference.
• Interference, a wave property, was demonstrated with
electrons passing through a crystal of nickel years after
De Broglie’s prediction.
– The spacing between nickel atoms was about the same as the
wavelength of the electrons.
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Atomic Physics
Section 3
Classroom Practice Problems
• Calculate the De Broglie wavelength of an
electron moving at a speed of 8.0  106 m/s.
– Answer: 9.1  10-11 m
• Calculate the De Broglie wavelength for a
0.145 kg fastball thrown at 40.0 m/s.
– Answer: 1.14  10-34 m
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Atomic Physics
Section 3
De Broglie Waves in Atoms
• Electrons in atomic orbits behave like waves and set up
standing waves around each orbital.
• Only certain multiples are possible.
– Just like standing waves on a string
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Atomic Physics
Section 3
De Broglie and the Wave-Particle Nature of
Electrons
Click below to watch the Visual Concept.
Visual Concept
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Atomic Physics
Section 3
The Uncertainty Principle
• Heisenberg’s uncertainty principle states that it
is impossible to simultaneously determine a
particle’s position and momentum with infinite
accuracy.
– Measuring one of the quantities alters the other one.
– For example, measuring the position of an electron
requires that photons bounce off the electron so we
can see it. However, the photon striking the electron
will change the electron’s momentum.
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Atomic Physics
Section 3
Electron Probability
• Since it is impossible to measure the exact
position of an electron, we can only calculate the
probability of finding an electron at various
locations.
• Erwin Schrödinger developed a mathematical
model of the electron using a wave function to
describe it.
– The wave function is used to calculate the probability
of finding electrons at various distances from the
nucleus.
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Atomic Physics
Section 3
Electron Cloud
• The most likely position of
the electron in the ground
state, according to the wave
function, is the same as the
Bohr radius.
– However, the electron may be
closer or farther away as
shown by other probabilities.
• An electron cloud is a
picture of the probabilities,
where denser means more
probable.
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Atomic Physics
Section 3
Heisenberg Uncertainty Principle
Click below to watch the Visual Concept.
Visual Concept
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Atomic Physics
Section 3
Now what do you think?
• The photoelectric effect shows that electromagnetic
waves can act like particles. Does it seem possible that
particles such as bullets or electrons can show wavelike
behavior?
– If hundreds of baseballs pass through a doorway, will
they spread out like a sound wave?
– Will a beam of electrons going through a small
opening diffract?
– After splitting a beam of electrons, could an electron
from one beam interfere constructively with one from
the other beam?
© Houghton Mifflin Harcourt Publishing Company