Transcript Dalton (1803)

Atomic Theory
Chapter 3
Dalton (1803)
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Proposed that atoms are the smallest particles of an
element.
All atoms in that element are identical but they differ from
those of other elements.
There is a difference between a model of atoms and a
theory of atoms. A model focuses on describing what the
atoms are like, whereas the theory not only talks about
what the atoms are like but how they interact with one
another and so forth.
Dalton's model was that the atoms were tiny, indivisible,
indestructible particles and that each one had a certain
mass, size, and chemical behavior that was determined
by what kind of element they were.
Thompson (1896)
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Found that cathode rays could be deflected by
an electric field
Showed that cathode "rays" were actually
particles
Electron - (originally called corpuscles by
Thomson) particles given off by the cathode;
fundamental unit of negative electricity
Raisin (Plum) Pudding Model  Matter is electrically neutral and electrons are
much lighter than atoms.
 Conclusion: There must be positively charged
particles which also must carry the mass of the
atom. The main finding is that negatively
charged electrons carried the cathode ray
towards the positively charged anode.
Cathode Ray Tube Experiment
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Sealed tube experiments of gases under a high
voltage showed a stream of particles called
cathode rays moving from cathode to anode.
These rays were deflected towards the positive
plate of an electrical field showing that they are
negatively charged.
Regardless of the type of the gas inside the tube
unique cathode rays were produced. Cathode
rays are a stream of electrons.
This experiment leads to the discovery of
electrons and Thomson's "Plum-pudding"
(blueberry muffin) model of the atom.
Cathode Ray Tube
Millikan (1909)
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Millikan measured the charge on an electron with his oildrop apparatus.
An "atomizer" from a perfume bottle sprayed oil or water
droplets into the sample chamber. Some of the droplets
fell through the pinhole into an area between two plates
(one positive and one negative). This middle chamber
was ionized by x-rays. Particles that did not capture any
electrons fell to the bottom plate due to gravity. Particles
that did capture one or more electrons were attracted to
the positive upper plate and either floated upward
or fell more slowly.
Conclusion: The charge on a drop was always a
multiple of 1.59 x 10-19 Coulombs. He proved
Thomson's hypothesis that the mass of an electron is at
least 1000 times smaller than the smallest atom
Millikan’s Apparatus
Rutherford (1909)
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Studied the deflection of alpha particles as they
were targeted at thin gold foil sheets.
 Most of the alpha particles penetrated
straight through.
 However few were deflected at slight angles.
Conclusion: The positive charge and mass of
an atom were mainly in the center and only
made up a small fraction of the atom. He named
this concentrated center the nucleus.
Rutherford was also able to estimate the charge
of an atom by studying the deflection of alpha
particles. He found that the positive charge on
the atom was approximately half of the atomic
weight.
Alpha Scattering Experiment
Niels Bohr (1911)
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The Bohr Model is known as the "planetary
model" of the atom.
In the Bohr Model the neutrons and protons
occupy a dense central region called the
nucleus, and the electrons orbit the nucleus
much like planets orbiting the Sun (but the orbits
are not confined to a plane as is approximately
true in the Solar System).
Led to the calculation of possible energy levels
for these orbits.
Quantum Mechanical Model
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Max Planck (1900) suggests that radiation is
quantized (it comes in discrete amounts.)
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QUANTUM NUMBERS
Albert Einstein (1905), one of the few scientists
to take Planck's ideas seriously, proposes a
quantum of light (the photon) which behaves like
a particle. Einstein's other theories explained the
equivalence of mass and energy, the particlewave duality of photons, the equivalence
principle, and special relativity.
Atomic Models
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Thomson:
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Rutherford:
Atomic Models
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Bohr Model:
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Quantum Mechanical
Model:
Electrons: Particles or Waves?
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Sometimes light displays particle-like behavior,
and sometimes it acts like a wave; it all depends
on what sort of experiment you're doing. This is
known as wave/particle duality.
If we begin to think of electrons as waves, we'll
have to change our whole concept of what an
"orbit" is. Instead of having a little particle
whizzing around the nucleus in a circular path,
we'd have a wave sort of strung out around the
whole circle.
Remember…
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Scientists
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Dalton
Thomson – Cathode
Ray Tube
Millikan – Oil dropper
Apparatus
Rutherford – Gold Foil
Alpha Scattering
Bohr
Planck
Einstein
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Models
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Thomson Model
Rutherford Model
Bohr Model
Quantum Mechanical
Sub-Atomic Particles
Protons – positive charge, found in the
nucleus.
 Neutrons – no charge, found in the
nucleus.
 Electrons – negatively charged, found
around the nucleus (electron cloud).
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Atomic Structure
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Isotopes – same elements but different number
of neutrons.
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The mass number will INDIRECTLY give you the
number of neutrons.
Ions – charged particles.
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The loss of electrons gives the atom a positive charge
(+).
The gaining of electrons gives the atom a negative
charge.
The charge goes in the top right hand corner in a
chemical symbol.