Components and Structure of the Atom

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Components and Structure of the Atom
Lecture 5

• Chemistry 142 B
• James B. Callis, Instructor
• Autumn Quarter, 2004

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Neon SignsLow pressure neon gas in evacuated
tube, between electrodes.High voltage separates
, - charges in Ne atoms particle goes to -
electrode.- particle goes to
electrode.Measure current between electrodes
Shows atoms made of , - charges.
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Movies to Illustrate the Nature of Atoms

• Primitive Cathode Ray Tube http//micro.magnet.fs
  u.edu/electromag/java/crookestube/
• Millikan Oil Droplet Experiment
  http//chemistry.umeche.maine.edu/fgamar/Millikan
  .html
• Rutherford Scattering Experiment
  http//www.howstuffworks.com/framed.htm?parentato
  m.htmurlhttp//micro.magnet.fsu.edu/electromag/j
  ava/rutherford/

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Deflection -gt mass/charge
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Millikans Experiment1. Measured rate of
droplets fall without voltage gave its
mass.2. Voltage across plates influenced speed,
due to charge of droplet.3. Quantitative effect
of voltage w/ laws of physics -gt amt. of charge
on droplet.4. RESULT Different droplets had
different charge, but always a multiple of same
number -gt elementary charge on
electron e 1.602x10-19 coulombs
(negative).5. (Mass/charge) x e- mass of
e-
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Rutherford Experiment

• Alpha (i.e., subatomic) particles bombarding the
  atom.
• Rationale - to study the internal structure of
  the atom, and to know more about the mass
  distribution in the atom.
• Bombarded a thin Gold foil with alpha particles
  from radium.

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Ernest Rutherford (1871-1937)

• Won the Nobel Prize in Chemistry in 1908
• It was quite the most incredible event..... It
  was almost as if a gunner were to fire a shell at
  a piece of tissue and the shell bounced right
  back!

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The Neutron

• Protons cannot be the only particle in the
  nucleus because the mass of the protons in is
  less than the mass of the nucleus.
• Therefore, a third, neutrally charged particle
  must exist!
• James Chadwick discovered the missing subatomic
  particle, the neutron. He bombarded beryllium
  foil with alpha particles and noticed a neutral
  stream of particles coming out. The particles
  have about the same mass as a proton.

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Notes mass of e- tiny relative to p, n.
p, n have same mass (almost). e-, p have
same charge, opposite sign.
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lt- D 2 x 10-8 cm -gt
Moving electron cloud surrounding nucleus. Almost
all the mass in the nucleus. Radius of Atom
10-8 cm Diameter of Nucleus 10-13 cm
__________________
Diameter 10-13 cm
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Atomic Definitions I Symbols, Isotopes,Numbers
A
X
The Nuclear Symbol of the Atom, or Isotope
Z
X Atomic symbol of the element, or element
symbol
A The Mass number A Z N Z The Atomic
Number, the Number of Protons in the
Nucleus (All atoms of the same element have the
same no. of protons.) N The Number of Neutrons
in the Nucleus
Isotopes atoms of an element with the same
number of protons, but
different numbers of Neutrons in the Nucleus
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Neutral ATOMS
If neutral, then number of electrons number of
protons. Numbers of each particle

• 51 Cr p ( ), e- ( ),
  n ( )
• 239 Pu p( ), e-( ),
  n ( )
• 15 N p( ), e-( ), n( )
• 56 Fe p( ), e-( ), n ( )
• 235 U p( ), e-( ), n ( )

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