Atomic Structure

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Atomic Structure

• History of Atomic Theory

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Democritus (460 - 370 BC)

• Was the first person to come up with the idea of
  atom
• Believed that all matter was composed of
  indivisible particles he called ATOMS
• Which is derived from the Greek word Atomos
  meaning indivisible
• He also believed that different atoms
• Are different sizes
• Have different properties
• Other philosophers of that time did not agree
  with his theories.

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John Dalton (1766-1844)

• Dalton is the Father of Atomic Theory
• Daltons ideas were so brilliant that they have
  remained essentially intact up to the present
  time and has only been slightly corrected.

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Daltons Atomic Theory (1803)aka 5 Postulates

1. All matter is composed of extremely small
   particles called atoms. (I agree with Democritus!)

 
2. All atoms of a given element are identical,
having the same - size - mass - chemical
properties.
3. All atoms of different elements are different.
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Daltons Atomic Theory (1803)aka 5 Postulates

1. Atoms cannot be created, divided into smaller
   particles, or destroyed.

In a chemical reaction, atoms of different
elements are separated, joined or rearranged.
They are never changed into the atoms of another
element. We will learn more later
5. Atoms combine in definite whole number ratios
to make compounds (you cant have a ½ of a Carbon
bonding with Oxygen its a whole atom or no atom)
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Daltons Atomic Model

• Based on Daltons Atomic Theory (5 postulates),
  most scientists in the 1800s believed that the
  atom was like a tiny solid ball that could not be
  broken up into parts.
• Dalton was credited for the three Atomic Laws
  that were proven after his time.

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JJ Thomson (1856-1940)

• Used cathode rays to prove that Daltons
  Solid-ball model could be broken into smaller
  particles
• Thomson is credited with discovering electrons

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Cathode Ray Tubes

• Cathode rays had been used for some time before
  Thompsons experiments.
• A cathode ray is a tube that has a piece of
  metal, called an electrode, at each end. Each
  electrode is connected to a power source
  (battery).

• When the power is turned on, the electrodes
  become charged and produce a stream of charged
  particles. They travel from cathode, across the
  tube to the anode.

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Cathode Ray Tubes

• Thomson put the tube in a magnetic field. He
  predicted that the stream would travel in a
  straight path.
• Instead, he found that the path curved away from
  a negatively charged plate and toward a
  positively charged plate
• Why?

• Like charges repel each other, and objects with
  unlike charges attract each other, Thomson
  concluded that the stream of charged particles
  had electrons in them.

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Cathode Ray Tube Experiment

• Thompson Concluded
• Cathode rays are made up of invisible, negatively
  charged particles called Electrons.

• These electrons had to come from the matter
  (atoms) of the negative electrode.
• Since the electrodes could be made from a variety
  of metals, then all atoms must contain electrons!

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Thomsons Plum Pudding Model

• Thomsons Plum Pudding model is a charge sphere
  that has (- )charged electrons scattered inside,
  like raisins in plum pudding.
• Overall, the atom is neutral atom because the
  atom had the same number of positive and negative
  charges.

• From Thomsons experiments, scientists concluded
  that atoms were not just neutral spheres, but
  somehow were composed of electrically charged
  particles.
• The balance of positive and negative charge
  supports the neutral atom.

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Rutherford (1871-1937)

• Took Thomsons Plum Pudding Model and added to it
• Used the Gold Foil Experiment to discover the
  existence of
• Protons
• Nucleus
• You must be able to explain the Gold Foil
  Experimentit will be on the CST

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Gold Foil Experiment

• Rutherford directed a narrow beam of alpha
  particles ( charges) at a thin piece of gold
  foil
• Based on the Plum Pudding Model, he predicted
  that the charges would go through the foil
• Because the atom overall was neutral, he thought
  the electrons would travel straight through

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The Gold Foil Experiment
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Conclusions from Gold Foil

• Rutherford found that every once and a while, a
  particle was deflected bounced back
• Why?
• Because the charge hit a central mass of
  positive charge and was repelled.

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Conclusions from Rutherfords Gold Foil
Experiment (memorize this!)

• The atom contains a positively charged nucleus

• This nucleus contains almost all of the mass of
  the atom, but occupies a very small volume of the
  atom.

• The negatively charged electrons occupied most of
  the volume of the atom.

•  The atom is mostly empty space.

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Rutherfords Planetary Model

• To explain his observations, Rutherford
  developed a new model
• The electrons orbit the nucleus like the planets
  revolve around the sun.

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Bohr (1885-1962)

• Worked in Rutherfords lab
• Wondered why electrons are not attracted to the
  nucleus and cluster around it
• Disproved Rutherfords Planetary Model
• Designed and proved the way electrons are
  arranged in the atom

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Bohrs Energy Level Model

• Energy Level Model Electrons are arranged in
  circles around the nucleus. Each circle has a
  different energy.
• Electrons are in constant motion, traveling
  around the circle
• Electrons can jump from one circle to the next
• But they cant go to the nucleus they traveling
  too fast to be fully attracted.

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Bohrs Energy Level Model
  He proposed the following
1. Protons and neutrons are in the nucleus
2. Electrons can only be certain distances from
the nucleus.
3. The electrons orbit the nucleus at fixed
energy levels.
4. The electrons must absorb or emit a fixed
amount of energy to travel between these energy
levels
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Electron Cloud Model

• As a result of continuing research throughout the
  20th century, scientists today realize that
  energy levels are not neat, planet-like orbits
  around the nucleus of an atom.

• Instead, they are spherical regions of space
  around the nucleus in which electrons are most
  likely to be found.
• Electrons travel in lanes like a runner on a
  track. Electrons can be anywhere in their lane.
  The lanes are called electron clouds.

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Electron Cloud Model

• Electrons themselves take up little space but
  travel rapidly through the space surrounding the
  nucleus.

• These spherical regions where electrons travel
  may be depicted as clouds around the nucleus.

• The space around the nucleus of an atom where the
  atoms electrons are found is called the electron
  cloud.

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Electron Cloud Model
No electrons here.
Nucleus
Electron clouds
Electrons will be found here 90 of the time.

• Each cloud is determined by the probability of an
  electrons location. An electron will be found in
  its cloud about 90 of the time.