Atomic Theories

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Atomic Theories
Democritus (300 B. C.)
Matter can be cut into smaller and smaller
pieces, but eventually you will end up with tiny
particles (called atoms) that are indivisible.
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Daltons Atomic Model (1808)
1. All matter is made up tiny particles called
atoms.
2. Atoms cannot be created, destroyed, or
subdivided in chemical changes. (Law of
Conservation of Mass)
3. All the atoms of one element have the same
properties, such as mass and size. These
properties are different from the properties
of the atoms of any other element.

• 4. Atoms of different elements combine in
  specific proportions to form compounds. (Law of
  Definite Proportions)

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Electrons
cathode ray tube
J. J. Thomson
paddlewheel

-
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Protons
Eugen Goldstein
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(1904)
J. J. Thomson
pudding
(electrons)
http//www.youtube.com/watch?vWmmglVNl9OQ
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Rutherfords Gold Foil Experiment
tiny, dense positive core (called the nucleus)
alpha particles
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Rutherfords Nuclear Model
(1911)
nucleus (contains protons)
electron
empty space
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Problems With Rutherfords Model
1. Why doesnt the electron spiral into the
nucleus?
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2. How do you explain line spectra?
http//www.youtube.com/watch?vFfY4R5mkMY8feature
related
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Quantum Theory of Light
Light consists of a stream of energy packets (or
quanta) called photons.
Fire photon torpedo, Mr. Sulu!
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Evidence for the Quantum Theory
Photoelectric Effect the release of electrons
from a substance due
to light striking the surface of a
metal
According to the classical theory of light, the
brightness (intensity) of the light shone on the
metal would determine the kinetic energy of the
liberated electrons. This prediction was shown to
be false. The frequency (colour/energy ) of the
light was the most important characteristic of
the light producing the effect.
http//phet.colorado.edu/en/simulation/photoelectr
ic
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Einsteins Explanation for the Photoelectric
Effect (1905)

1. Light is transmitted in the form of energy
   packets called photons.

2. The energy of a photon depends on its
frequency.
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Neils Bohr

• An electron is so small that it DOESNT obey the
  classical
• (Newtonian) laws of motion.

Therefore, an electron does NOT necessarily lose
(radiate) energy as it orbits the nucleus.

2. An electron can only have certain energies,
just as the gearbox in a car can only have
certain gears. (Its energy is quantized.)
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Bohrs Model of the Hydrogen Atom (1914)
electron dropping to a lower orbit
orbits
electron in excited state
photon of light emitted when electron drops
?
electron in ground state
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Line Spectrum of Hydrogen
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The line spectrum of each element is unique, like
a fingerprint.
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Helium was first discovered when scientists
looked at the pattern of spectral lines during
a solar eclipse.
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The colours in fireworks are a result of
electrons becoming excited and then dropping
back down to orbits closer to the nucleus.
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Problems with Bohrs Model
Although Bohrs model successfully predicted the
line spectrum for hydrogen, the extension of
his model to atoms with two (or more) electrons
did not agree with experimental evidence.
Gimme a break!!!
http//www.youtube.com/watch?vR7OKPaKr5QMfeature
related http//www.youtube.com/watch?v5z2ZfYVzef
sfeaturerelated
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The Wave Mechanics Model of the Atom
(Schrodinger)
A model of the atom based on the fact that the
electron exhibits properties of a particle AND a
wave (but never both simultaneously).
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The idea that an electron had a dual nature was
first proposed by Louis de Broglie in 1923.
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Particle-Like Properties of Electrons

• Electrons are deflected
• by magnetic or electric fields.

-
2. Electrons can make a paddlewheel turn.
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Wave-Like Properties of Electrons
Diffraction of electrons through a crystal.
diffraction the spreading out of a wave when
passing through a small opening
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Moving Waves
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Standing Waves
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Since electrons are tied down by the nucleus,
they create standing wave patterns like a guitar
string called orbitals.
Unlike the standing wave on a guitar string,
these standing electron waves/orbitals occur in
3-D.
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Heisenberg Uncertainty Principle
If the velocity of an electron is known, its
position cannot be determined exactly.
(i.e.- While measuring the velocity of tiny
particles like electrons, you will affect their
position.)

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Electron Cloud 3-D representation of the
probability of finding an
electron in an orbital.
Where there are large s of dots, the probability
of finding the electron is high.
http//www.youtube.com/watch?v5z2ZfYVzefsfeature
related
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Neutrons
James Chadwick