Atomic Theory

1
Atomic Theory

• Chapter 3

2
Dalton (1803)

• 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.

3
Thompson (1896)

• 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
  chargedparticles which also must carry the mass
  of theatom.  The main finding is that negatively
  charged electrons carried the cathode ray towards
  the positively charged anode.

4
Cathode Ray Tube Experiment

• 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.

5
Cathode Ray Tube
6
Millikan (1909)

• Millikan measured the charge on an electron with
  his oil-drop 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 upwardor 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

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Millikans Apparatus
8
Rutherford (1909)

• 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.

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Alpha Scattering Experiment
10
Niels Bohr (1911)

• 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.

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Quantum Mechanical Model

• Max Planck (1900) suggests that radiation is
  quantized (it comes in discrete amounts.)
• 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
  particle-wave duality of photons, the equivalence
  principle, and special relativity.

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Atomic Models

• Thomson

• Rutherford

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Atomic Models

• Bohr Model

• Quantum Mechanical Model

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Electrons Particles or Waves?

• 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.

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Remember

• Scientists
• Dalton
• Thomson Cathode Ray Tube
• Millikan Oil dropper Apparatus
• Rutherford Gold Foil Alpha Scattering
• Bohr
• Planck
• Einstein

• Models
• Thomson Model
• Rutherford Model
• Bohr Model
• Quantum Mechanical

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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

• Isotopes same elements but different number of
  neutrons.
• The mass number will INDIRECTLY give you the
  number of neutrons.
• Ions charged particles.
• 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.