Chapter%204:%20Glow%20in%20the%20Dark

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Chapter 4 Glow in the Dark
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Introductory Activity

• List as many things as you can think of that
  glow
• What do you have to do to make these glowing
  things glow?

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Glow in the dark

• This chapter will introduce the chemistry needed
  to understand how glowing things work
• Section 4.1 History of Atomic Theory
• Section 4.2 Atomic Structure
• Section 4.3 Electron Structure
• Section 4.4 Periodic table
• Section 4.5 Periodicity
• Section 4.6 Light
• Section 4.7 Light and matter

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Glow in the dark
Light
Electron structure
Periodic Table
Atomic structure
Periodicity
Atomic theory
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Section 4.1Development of Atomic Theory
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Daltons Atomic Theory

• John Daltons theory based on experiments in
  early 1800s
• All matter is made of tiny particles atoms
• Atoms cannot be created, divided, destroyed or
  changed into other types of atoms
• Atoms of the same element have identical
  properties
• Atoms of different elements have different
  properties
• Atoms of different elements combine in
  whole-number ratios to form compounds
• Chemical changes join, separate or rearrange
  atoms in compounds

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Cathode Ray Tubes
A cathode ray is a ray of light traveling in a
vacuum (no other particles inside)
The ray travels from one metal plate to another
as the plates are connected to electricity
Cathode ray
Metal plate (anode) to which stream travels
Metal plate (cathode) releases stream
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Cathode Ray Tubes Charge
In the late 1800s, JJ Thomson put charged plates
outside the tube
Negatively charged plate
-

Ray is deflected away from negative plate and
towards positive plate
Positively charged plate
It made no difference what type of metal he used
in the tubeall material produced this stream
that curved towards the positive charge
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Thomsons conclusions

• The evidence from Thomsons work showed that
  there was something negatively charged in atoms
• Since all types of metal produced the same
  result, the negative charge is in all types of
  atoms
• Since atoms were overall neutral, if there was a
  negative charge there had to also be a positive
  charge
• In 1897, Thomson announced that the rays were
  electrons and they had a negative charge

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Theories change

• Thomsons evidence showed Daltons idea of solid,
  uniform atoms was incorrect.
• Eugene Goldstein conducted experiments to label
  the positive part protons and determined it has
  the same charge as the electron (with opposite
  sign) but is 1837 times heavier!
• Thomson developed the plum pudding model.
• Since most of us arent familiar with plum
  pudding, you can think of it as a chocolate
  cookie dough theory

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Thomsons Theory
The chips are the negative electrons. The
dough is the positive portion The chips are
stationary and dont move within the
dough Remember, officially this theory is
called plum pudding but its the same idea!
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Gold Foil Experiment

• Hans Geiger performed experiments in the early
  1900s where he bombarded very thin gold foil
  with radioactive particles (alpha particles ?)
• They expected these relatively heavy particles to
  go through the atoms with a small deflection

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What happened in the experiment?
a
Gold foil
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What did he see?

• Most of the alpha particles passed straight
  through with no deflection
• These particles did not run into anything
• Some did deflect slightly
• These particles ran into something much smaller
  than themselves
• A few were reflected back the direction they came
  from
• These particles ran into something very dense

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What did that mean?

• Atoms are mostly empty space
• Electrons (the smaller particles) were the cause
  of the small deflections
• There must be a small area of the atom with most
  of its mass (the protons) that caused the
  reflections.
• He called this small, dense area the nucleus

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A third particle

• The protons and electrons could explain the
  charges of the various parts of the atom
• They could not explain the total mass of the
  atoms
• Neutrons were proposed in 1920s but not
  confirmed until 1932 by James Chadwick
• Neutrons had mass similar to protons and no
  charge. They were located in the nucleus

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More changes to the theory

• Niels Bohr performed experiments with hydrogen
  atoms light
• He determined that electrons are in levels
  according to how much energy they have and that
  only certain energy amounts were allowed.

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The Bohr Model

• It consists of the nucleus with protons
  neutrons and electrons in concentric orbits
  (circles) outside the nucleus
• The circle closest to the nucleus contains the
  lowest energy electrons
• The first level can hold 2 electron, then the
  next two levels can each hold 8 and then levels
  farther out can hold 18.

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Pictures of the Bohr Models
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Use of the Bohr Model now

• We no longer believe electrons are in concentric
  circles, but this is still a convenient way to
  show energy levels on 2-dimensional paper

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Modern Atomic Theory

• In the 1920s, Bohrs research lead the way for
  the study of quantum mechanics (the study of tiny
  particles)
• Modern atomic theory uses calculus equations to
  show how the subatomic particles act as both
  particles and waves
• These equations show the most probable location
  of electrons in the atom (known as atomic
  orbitals)