Transcript AtomicPhysicsPres

Inside the Modern Atom
Review of Waves
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Properties:
 Wavelength
(l): distance between two successive crests
 Frequency (f): number of wave crests passing per second
 Speed of wave: v=fl
 Amplitude (A): maximum displacement
 Energy: E  A2
Interference
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Only waves experience
interference  this is
just adding up the parts
of a wave:
Where two crests (or
troughs) meet, they add
Where a crest and
trough meet, they
cancel
Light: Particles vs. Waves
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Light was originally thought to be a particle
 E.g.,
look at sharp shadows, photoelectric effect, etc.
 also, Newton endorsed the view that light was
made up of particles
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Particles behave completely differently
when they encounter slits...
E.g., suppose you fired bullets at one slit?
What does that look like? Just a simple curve...
What if you fired bullets at two slits? Just two
separate curves
So what happens when you fire light at it?
Interference like water waves!
The Photoelectric Effect
• When light hits certain metals, e- are
ejected
•Only the frequency (color) of the light
affected the energy of the ejected e• Higher frequency light ejected e- with
higher energy
The Atomic Spectra
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Light from bulbs, stars, etc. show a
continuous spectrum when seen
through a prism
A hot gas, however, has an
emission line spectrum made of a
few, discrete lines of color
Why don't hot gases show
continuous spectra?
Absorption
Emission
Quantum Hypothesis: Energy is quantized
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What is quantization? It only comes in discrete chunks
instead of a continuous range of energies
Planck suggested Energy is quantized in units of h and was
proportional to the oscillators frequency: E = hf
Continuous
Discrete
Quantum Hypothesis: Light is quantized
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Einstein proposed that light is also quantized and its
energy is also determined by its frequency via E = hf
Each individual packet of light energy is called a photon and
an EM wave is made of these individual "particles"
 Brighter light → more photons strike metal each second →
more e- ejected/sec (but it does not increase the energy of
each e-)
 Higher frequency light ejects e- with more energy because
each photon has more energy to give
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E  hf
h  6.626 1034 J  s
K max  hf  
Structural Models of the Atom
Aristotle’s
“Point” Model
e
Thompson
“Plumb Pudding”
Model
r
Rutherford “Point
Nucleus” Model
 200
2
r  2a0
Ze
Bohr “Planetary” Model
QM “Probability” Model
Quantum Hypothesis: Orbits are quantized
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Bohr suggested that the orbits of electrons are also quantized
An electron can go from one level to another by absorbing or emitting a
photon of light
If light energy is quantized and electron orbits are also quantized, that
would explain why atomic spectra are discrete (since atoms/electrons
only absorb or emit a single photon at a time)
The Bohr Model of the Hydrogen Atom
Bohr’s Quantum
Conditions
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I. There are discrete
stable “tracks” for the
electrons. Along these
tracks, the electrons
move without energy
loss.
II. The electrons are able
to “jump” between the
tracks.
Ei-Ef=hf
In the Bohr model, a photon is
emitted when the electron drops
from a higher orbit (Ei) to a lower
energy orbit (Ef).
Predicted Energy Levels
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Instead of looking at orbits, we now look at energy
levels, which are the certain, allowed energy states
 Lowest energy level (corresponding to innermost
orbit in Bohr theory) is called the ground state and
higher energy states are excited states
The structure of the atom is shown schematically on an
energy-level diagram labeled with a quantum number
n
As quantum number ↑, Energy associated with that
state ↑
Transition of the electron from one orbit to another is
now represented as the atom going from one energy
level to another
Transition achieved by absorption or emission of a
photon with an energy corresponding to the difference
in energy between the two levels, or states
 When white light hits an atom, only photons with the
right energy are absorbed!
Energy, Light, & Orbits are quantized!
 This is why it's called quantum mechanics:
everything is quantized (comes in discrete chunks
instead of a continuous range of values) -- matter
(the Bohr "orbitals"), light, and even energy are
ALL quantized!
 DeBroglie further hypothesized that since electrons
also behave as waves, they must also have a
electron
wavelength: λ = h/mv
wave
de Broglie Waves in the
Hydrogen Atom
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In this example, three complete
wavelengths are contained in the
circumference of the orbit
In general, the circumference
must equal some integer number
of wavelengths
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2  r = n λ ; n = 1, 2, …
Heisenberg Uncertainty Principle
 Heisenberg proposed that the wave aspect of an electron
makes it impossible to know both the position and
momentum to arbitrary precision
 Heisenberg Uncertainty Principle (HUP):
Δx • Δ(mv) ≥ h/4π
 E.g., if you have a periodic wave (or a standing wave) you
can't really tell what its position is (it's spread out over the
whole string, e.g.). But you can tell exactly what its
wavelength is. Now if you send a wave pulse down the
string, you can't tell what its wavelength is (doesn't make
sense for a pulse) but you can tell exactly what its position
is.
The Atomic Structure
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So we can't say where exactly the electron is (it's not like a billiard ball,
or like a wave, or like a puffy cloud, or like anything else we know from
ordinary experience)
"Now we know how the electrons and light behave. But what can I call
it? If I say they behave like particles I give the wrong impression; also if
I say they behave like waves. They behave in their own inimitable way,
which technically could be called a quantum mechanical way. They
behave in a way that is like nothing that you have ever seen before.
Your experience with things that you have seen before is incomplete.
The behavior of things on a very tiny scale is simply different. An atom
does not behave like a weight hanging on a spring and oscillating. Nor
does it behave like a miniature representation of the solar system with
little planets going around in orbits. Nor does it appear to be somewhat
like a cloud or fog of some sort surrounding the nucleus. It behaves like
nothing you have ever seen before." -- Richard P. Feynman, The
Character of Physical Law
Since we can't talk about its exact location, it's more useful to
concentrate on the electron's energy
Some Atomic Physics
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Atom can gain or lose energy by absorption or
emission of photons or by collisions
Pauli Exclusion Principle: two electrons cannot occupy
the same quantum state at the same time
Number of quantum states in a given energy level
given by 2n2
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If even one electron is in a higher energy level, the atom
is said to be in an excited state
Properties of each element determined by the groundstate configuration of its atoms (e.g., valence electrons,
etc.)
Four Known Forces
 Two familiar kinds of interactions:
gravity (masses attract one another) and
electromagnetism (same-sign charges repel,
opposite-sign charges attract)
 What causes radioactive decays of nuclei ?
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Must be a force weak enough to allow most atoms
to be stable.
 What binds protons together into nuclei ?
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Must be a force strong enough to overcome
repulsion due to protons’ electric charge
Previously, we peered inside the atom
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We recalled that electrons orbit
the atom’s massive nucleus and
determine an element’s chemical
behavior.
We explored the proton and
neutron content of nuclei and the
phenomena of radioactivity,
fission, and fusion they make
possible.
 Today we’ll look inside the
nucleons themselves.
Fundamental particles in the
Standard Model are:
 Leptons
 Quarks
 Intermediate Gauge Bosons
Anti-matter
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Each kind of elementary particle has a counterpart
with the same mass, but the opposite electric charge,
called its “anti-particle”.
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Electron: m= .0005 GeV, charge = +1, symbol e-
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Positron: m = .0005 GeV, charge = -1, symbol e+
The anti-particle has a bar over its symbol:
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Anti-proton is written
p , anti-neutrino is
v
Anti-matter is rare in the explored universe
It’s created in cosmic rays and particle accelerators
and some radioactive decays.
When a particle
and its anti-particle collide, they
“annihilate” one another in a flash of
energy.
Stability diagram
Heavy elements can fission
into lighter elements.
Elements from helium to iron were
manufactured in the cores of stars by fusion.
Heavier elements are metastable and were
made during supernovae explosions.
Light elements can undergo
fusion into heavier elements.
Chain reaction
For reaction to be self-sustaining, must have
CRITICAL MASS.
Fusion
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Light nuclei are more stable when
combined
Tremendous energy released
Hydrogen bombs and Fusion
power?