Transcript Ch 38 Powerpoint

38 The Atom and the Quantum
Material particles and
light have both wave
properties and
particle properties.
38 The Atom and the Quantum
Atomic structure is
revealed by analyzing
light. Light has a dual
nature, which in turn
radically alters our
understanding of the
atomic world.
38 The Atom and the Quantum
38.1 Models
What are the two primary
models of light?
Through the centuries there have been two
primary models of light: the particle model and
the wave model.
38 The Atom and the Quantum
38.1 Models
Nobody knows what an atom’s
internal structure looks like, for there
is no way to see it with our eyes.
To visualize the processes that occur
in the subatomic realm, we construct
models.
The planetary model in which
electrons orbit the nucleus was
suggested by the Danish physicist
Niels Bohr in 1913. It is still useful
for understanding the emission of
light.
38 The Atom and the Quantum
38.1 Models
The planetary model has
been replaced by a more
complex model in which
the electrons are
represented as clouds.
Ernst Rutherford model.
38 The Atom and the Quantum
38.1 Models
Models help us to understand processes that are difficult to
visualize.
A useful model of the atom must be consistent with a model for
light.
Most of what we know about atoms we learn from the light and
other radiations they emit.
Most light comes from the motion of electrons within the atom.
There have been two primary models of light: the particle
model and the wave model.
• Isaac Newton believed light was composed of tiny
particles.
• Christian Huygens believed that light was a wave
phenomenon.
38 The Atom and the Quantum
38.1 Models
The wave model was reinforced when Thomas
Young demonstrated constructive and destructive
interference of light.
Later, James Clerk Maxwell proposed that light is
an electromagnetic wave.
The wave model gained further support when
Heinrich Hertz produced radio waves that behaved
as Maxwell had predicted.
In 1905, Albert Einstein resurrected the particle
theory of light.
38 The Atom and the Quantum
38.2 Light Quanta
How is the energy of a photon
related to its frequency?
The energy of a photon is directly proportional
to the photon’s frequency.
38 The Atom and the Quantum
38.2 Light Quanta
Albert Einstein visualized particles of light as
concentrated bundles of electromagnetic energy.
Max Planck had proposed that atoms do not emit and
absorb light continuously, but do so in little chunks.
Each chunk was considered a quantum, or a
fundamental unit.
Planck believed that light existed as continuous waves, but
that emission and absorption occurred in quantum chunks.
Einstein went further and proposed that light itself is
composed of quanta.
One quantum of light energy is now called a photon.
38 The Atom and the Quantum
38.2 Light Quanta
Matter is quantized, equal to some whole-number multiple
of the mass of a single atom.
Electric charge is quantized as a multiple of the charge of a
single electron.
Other quantities such as energy and angular momentum
are quantized.
The energy in a light beam is quantized and comes in packets,
or quanta; only a whole number of quanta can exist.
• The quanta of electromagnetic radiation are the photons.
• Photons have no rest energy.
• They move at the speed of light so the total energy of a
photon is the same as its kinetic energy.
38 The Atom and the Quantum
38.2 Light Quanta
The energy of a photon of light is proportional to its vibrational
frequency.
• When the energy E of a photon is divided by its frequency f, the
quantity that results is known as Planck’s constant, h.
• This quantity is always the same, no matter what the frequency.
• The energy of every photon is therefore E = hf.
• This equation gives the smallest amount of energy that can be
converted to light of frequency f. Planck's constant =
6.62606957 × 10-34 m2 kg / s
38 The Atom and the Quantum
38.3 The Photoelectric Effect
What does the photoelectric effect suggest
about the way light interacts with matter?
The photoelectric effect suggests that light interacts
with matter as a stream of particle-like photons.
Photoelectric effect video
Photoelectric effect Lonnie’s Lab video
38 The Atom and the Quantum
38.3 The Photoelectric Effect
Einstein found support for his quantum theory of light in the
photoelectric effect.
The photoelectric effect is the ejection of electrons from
certain metals when light falls upon them.
These metals are said to be photosensitive.
38 The Atom and the Quantum
38.3 The Photoelectric Effect
Explanation of the Photoelectric Effect
Energy from the light shining on a metal plate gives electrons bound in the metal
enough energy to escape.
• High-frequency light, even from a dim source, is capable of ejecting
electrons from a photosensitive metal surface.
• Low-frequency light, even from a very bright source, cannot dislodge
electrons.
• Since bright light carries more energy than dim light, it was puzzling that
dim blue light could dislodge electrons when bright red light could not.
38 The Atom and the Quantum
38.3 The Photoelectric Effect
Einstein explained the photoelectric effect in terms of
photons.
• The absorption of a photon by an atom in the metal
surface is an all-or-nothing process.
• Only one photon is absorbed by each electron ejected
from the metal.
• The number of photons that hit the metal has nothing to
do with whether a given electron will be ejected.
• If the energy in the photon is large enough, the electron
will be ejected from the metal.
38 The Atom and the Quantum
38.3 The Photoelectric Effect
The intensity of light does not matter. From E = hf, the critical
factor is the frequency, or color, of the light.
• Each blue or violet light photon carries enough energy
to free an electron from the metal.
• A few photons of blue or violet light can eject a few
electrons. Many red or orange photons cannot eject a
single electron.
• Only high-frequency photons have the energy needed
to pull loose an electron.
38 The Atom and the Quantum
38.3 The Photoelectric Effect
Support for the Particle Model of Light
The energy of a wave is spread out along a broad front.
For the energy of a light wave to be concentrated enough to
eject a single electron from a metal surface is unlikely.
The photoelectric effect suggests that light interacts with
matter as a stream of particle-like photons.
38 The Atom and the Quantum
38.3 The Photoelectric Effect
The number of photons in a light beam controls the brightness
of the whole beam.
The frequency of the light controls the energy of each
individual photon.
Experimental verification of Einstein’s explanation was made
11 years later by the American physicist Robert Millikan.
Every aspect of Einstein’s interpretation was confirmed,
including the direct proportionality of photon energy to
frequency.
38 The Atom and the Quantum
38.4 Waves as Particles
What causes light to behave like a
wave? Like a particle?
Light behaves like waves when it travels in
empty space, and like particles when it interacts
with solid matter.
Dr. Quantum Video
38 The Atom and the Quantum
38.4 Waves as Particles
A photograph taken with exceedingly feeble light provides a
striking example of the particle nature of light.
The image progresses photon by photon.
Photons seem to strike the film in an independent and
random manner.
38 The Atom and the Quantum
38.5 Particles as Waves
What did de Broglie suggest about
all matter?
De Broglie suggested that all matter could be
viewed as having wave properties.
Is light a particle or wave video
Verisatum video
38 The Atom and the Quantum
38.5 Particles as Waves
If waves can have particle properties, cannot particles have
wave properties?
This question was posed by the French physicist Louis de
Broglie and his answer later won the Nobel Prize in physics.
De Broglie suggested that all matter could be viewed as
having wave properties.
38 The Atom and the Quantum
38.5 Particles as Waves
All particles—electrons, protons, atoms, marbles, and even
humans—have a wavelength:
where h is Planck’s constant.
38 The Atom and the Quantum
38.5 Particles as Waves
The wavelength of a particle is called the
de Broglie wavelength.
• A particle of large mass and ordinary speed has
too small a wavelength to be detected by
conventional means.
• A tiny particle—such as an electron—moving at
typical speed has a detectable wavelength.
The wavelength of electrons is smaller than the wavelength
of visible light but large enough for noticeable diffraction.
A beam of electrons can be diffracted and undergoes wave
interference under the same conditions that light does.
38 The Atom and the Quantum
38.5 Particles as Waves
a. The diffraction of an electron beam produces an
interference pattern.
b. The fringes produced by a beam of light are very similar
to those produced by the beam of electrons.
38 The Atom and the Quantum
38.5 Particles as Waves
An electron microscope uses the
wave nature of electrons.
• The wavelength of electron
beams is typically
thousands of times shorter
than the wavelength of
visible light.
• The electron microscope is
able to distinguish details
thousands of times smaller
than is possible with optical
microscopes.
38 The Atom and the Quantum
38.6 Electron Waves
How did de Broglie’s theory of matter waves
describe electron orbits?
According to de Broglie’s theory of matter
waves, electron orbits exist only where an
electron wave closes in on itself in phase.
38 The Atom and the Quantum
38.6 Electron Waves
The planetary model of the atom was useful in explaining
the atomic spectra of the elements and why elements
emitted only certain frequencies of light.
• An electron has different amounts of energy when it is
in different orbits around a nucleus.
• An electron is in a different energy level when it is in a
different orbit.
• Electrons in an atom normally occupy the lowest
energy levels available.
38 The Atom and the Quantum
38.6 Electron Waves
In the Bohr model of the
atom, the electron orbits
correspond to different
energy levels.
38 The Atom and the Quantum
38.6 Electron Waves
Bohr Model Explanation of Atomic Spectra
An electron can be boosted to a higher energy level.
• This occurs in gas discharge tubes such as
neon signs.
• Electric current boosts electrons of the gas to
higher energy levels.
• As the electrons return to lower levels, photons
are emitted.
• The energy of a photon is exactly equal to the
difference in the energy levels in the atom.
38 The Atom and the Quantum
38.6 Electron Waves
The pattern of lines in the spectrum of an element
corresponds to electron transitions between the energy
levels of the atoms of that element.
By examining spectra, physicists were able to determine the
various energy levels in the atom.
Why were electrons at discrete distances from the atomic
nucleus?
This was resolved by thinking of the electron not as a
particle whirling around the nucleus but as a wave.
According to de Broglie’s theory of matter waves, electron
orbits exist only where an electron wave closes in on itself in
phase.
38 The Atom and the Quantum
38.6 Electron Waves
De Broglie’s Theory
The electron wave reinforces constructively in each
cycle, like the standing wave on a music string.
The electron is visualized not as a particle located at
some point in the atom.
• Its mass and charge are spread throughout a
standing wave surrounding the nucleus.
• The wavelength of the electron wave must fit
evenly into the circumferences of the orbits.
38 The Atom and the Quantum
38.6 Electron Waves
De Broglie suggested electrons have a wavelength.
a. Electron orbits exist only when the circumference of the
orbit is a whole-number multiple of the wavelength.
b. When the wave does not close in on itself in phase, it
undergoes destructive interference.
38 The Atom and the Quantum
38.6 Electron Waves
The circumference of the innermost orbit, according to this
model, is equal to one wavelength of the electron wave.
The second orbit has a circumference of two electron
wavelengths, the third three, and so on.
38 The Atom and the Quantum
38.6 Electron Waves
Orbit circumferences are wholenumber multiples of the electron
wavelengths, which differ for the
various elements.
This results in discrete energy levels,
which characterize each element.
Since the circumferences of electron
orbits are discrete, the radii of these
orbits, and hence the energy levels,
are also discrete.
38 The Atom and the Quantum
38.6 Electron Waves
In this simplified version of de Broglie’s theory of the atom,
the waves are shown only in circular paths around the
nucleus. In an actual atom, the standing waves make up
spherical and ellipsoidal shells rather than flat, circular ones.
38 The Atom and the Quantum
38.6 Electron Waves
This explains why electrons do not spiral closer and closer to
the nucleus when photons are emitted.
Since an orbit is described by a standing wave, the
circumference of the smallest orbit can be no smaller than one
wavelength.
In the modern wave model of the atom, electron waves also
move in and out, toward and away from the nucleus.
The electron wave is in three dimensions, an electron “cloud.”
38 The Atom and the Quantum
38.7 Relative Sizes of Atoms
What determines the radii of the electron
orbits in the Bohr model of the atom?
The radii of the electron orbits in the Bohr
model of the atom are determined by the
amount of electric charge in the nucleus.
Electrons, protons, neutrons video
38 The Atom and the Quantum
38.7 Relative Sizes of Atoms
The single proton in the hydrogen atom holds one negatively
charged electron in an orbit at a particular radius.
In helium, the orbiting electron would be pulled into a tighter
orbit with half its former radius since the electrical attraction is
doubled.
This doesn’t quite happen because the double-positive charge
in the nucleus attracts and holds a second electron.
The negative charge of the second electron diminishes the
effect of the positive nucleus.
38 The Atom and the Quantum
38.7 Relative Sizes of Atoms
This added electron makes the atom electrically neutral.
The two electrons assume an orbit characteristic of helium.
In a lithium atom, an additional proton pulls the electrons into
an even closer orbit and holds a third electron in a second
orbit.
38 The Atom and the Quantum
38.7 Relative Sizes of Atoms
As the nuclear charge increases, the inner orbits shrink because of the
stronger electrical attraction to the nucleus.
This means that the heavier elements are not much larger in diameter than
the lighter elements.
The diameter of the uranium atom, for example, is only about three
hydrogen diameters, even though it is 238 times more massive.
38 The Atom and the Quantum
38.7 Relative Sizes of Atoms
Each element has a unique arrangement of electron orbits.
The radii of orbits for the sodium atom are the same for all
sodium atoms, but different from the radii of orbits for other
kinds of atoms.
Each element has its own distinct orbits.
The Bohr model solved the mystery of the atomic spectra of
the elements.
It accounted for X-rays that were emitted when electrons made
transitions from outer orbits to innermost orbits.
Bohr was able to predict X-ray frequencies that were later
experimentally confirmed.
38 The Atom and the Quantum
38.7 Relative Sizes of Atoms
Bohr calculated the ionization energy of the hydrogen
atom—the energy needed to knock the electron out of
the atom completely.
This also was verified by experiment.
The model accounted for the chemical properties of the
elements and predicted properties of hafnium, which led
to its discovery.
38 The Atom and the Quantum
38.7 Relative Sizes of Atoms
Bohr was quick to point out that his model was to be
interpreted as a crude beginning.
The picture of electrons whirling like planets about the
sun was not to be taken literally.
His discrete orbits were conceptual representations of an
atom whose later description involved a wave
description.
38 The Atom and the Quantum
38.7 Relative Sizes of Atoms
a. In the Bohr model, the electrons orbit the nucleus like
planets going around the sun.
b. According to de Broglie’s idea, a wave follows along
an orbit.
c. The wave model—electrons are distributed in a
“cloud” throughout the volume of the atom.
38 The Atom and the Quantum
38.8 Quantum Physics
The subatomic interactions described by
quantum mechanics are governed by laws of
probability, not laws of certainty.
What is Quantum Physics? Video http://video.about.com/physics/What-isQuantum-Physics-.htm
38 The Atom and the Quantum
38.8 Quantum Physics
Physicists became convinced that the Newtonian laws
that work so well for large objects do not apply to the
microworld of the atom.
In the macroworld, the study of motion is called
mechanics, or sometimes classical mechanics.
The study of the motion of particles in the microworld of
atoms and nuclei is called quantum mechanics.
The branch of physics that is the general study of the
microworld of photons, atoms, and nuclei is simply called
quantum physics.
38 The Atom and the Quantum
38.8 Quantum Physics
There are fundamental uncertainties in the
measurements of the atomic domain.
For the measurement of macroscopic quantities, such as
the temperature of materials or the speeds of light and
sound, there is no limit to the accuracy with which the
experimenter can measure.
38 The Atom and the Quantum
38.8 Quantum Physics
Subatomic measurements, such as the momentum and
position of an electron or the mass of an extremely short-lived
particle, are entirely different.
In this domain, the uncertainties in many measurements are
comparable to the magnitudes of the quantities themselves.
The subatomic interactions described by quantum mechanics
are governed by laws of probability, not laws of certainty.
38 The Atom and the Quantum
38.9 Predictability and Chaos
What determines predictability in
orderly systems?
What laws govern the interactions described
by quantum mechanics?
Predictability in orderly systems, both
Newtonian and quantum, depends on
knowledge of initial conditions.
Chaos theory and the Butterfly effect video
Butterfly – the secret life of chaos BBC
38 The Atom and the Quantum
38.9 Predictability and Chaos
When we know the initial conditions of an orderly system we
can make predictions about it.
Knowing the initial conditions lets us state where a planet will
be after a certain time or where a launched rocket will land.
In the quantum microworld, we give odds where an electron is
likely to be. We calculate the probability that a radioactive
particle will decay in a given time interval.
Some systems, however, whether Newtonian or quantum, are
not orderly—they are inherently unpredictable.
These are called “chaotic systems.”
A feature of chaotic systems is that slight differences in initial
conditions result in wildly different outcomes later.
38 The Atom and the Quantum
38.9 Predictability and Chaos
Weather is chaotic. Small changes in one day’s weather
can produce big (and largely unpredictable) changes a
week later.
This barrier to good prediction first led the scientist
Edward Lorenz to ask, “Does the flap of a butterfly’s
wings in Brazil set off a tornado in Texas?”
Now we talk about the butterfly effect when dealing with
situations where very small effects can amplify into very
big effects.