Atomic Theory

1
Atomic Theory

• Mr. Tollefson
• Chemistry Class

2
Introduction

• Students will be introduced to the atom and the
  development of the atomic theory from ancient
  times to the present day. It is your job to take
  notes on the pertinent information in this
  presentation.

3
The Atom

• Democritus, an Ancient Greek philosopher, first
  purposed the word Atom around 375 BC. The atom
  was defined as a small round indivisible
  particle. Democritus described the atom by
  talking about the constant division of an object
  until the object could no longer be divided. The
  idea of the Atom had to wait for 2200 years to be
  proven.

4
Dalton and Atomic Theory

• John Daltons (d1844) Atomic Theory consists of 4
  parts. It was the first departure from the
  ancient idea of the atom. Most of the elements
  of this theory are still true today.

5
Dalton and Atomic Theory

• All elements are composed of indivisible
  particles called atoms.
• Atoms of the same element are identical. Atoms
  of element are different from others.
• Atoms can mix together in whole number ratios
• Chemical reactions occur when atoms combine,
  separate or rearrange

6
Electrons Discovery 1897

• The English physicist Sir JJ Thompson discovered
  the electron in 1897. He experimented with
  sealed tubes with gases in them. He passed high
  voltage electric current through the tubes. The
  glowing beam created is called a cathode ray.

7
Cathode Rays

• Thompson thus created CRTs or Cathode Ray Tubes,
  the basis for televisions and computer monitors.
• Thompson found that the rays were attracted to
  metal plates that carry positive charges.
  Thompson proposed that a cathode ray is composed
  of tiny negatively charged particles about 1/2000
  the mass of the hydrogen atom. He called them
  electrons.

8
Plum Pudding

• Thompson also worked with modified cathode ray
  tubes that gave rays traveling in the opposite
  direction. These particles were found to have
  the same amount of charge as electrons, just the
  opposite sign. These were later called protons.

9
PLUM PUDDING

• From what Thompson learned about protons and
  electrons he proposed his Plum Pudding model of
  the atom. Protons and electrons are mixed
  together in a big blob with no particular order.

10
Nucleus

• Ernst Rutherford made a major advance in atomic
  theory with a simple experiment with complex
  results (1912). Sometimes called the gold foil
  experiment it is the foundation of the modern
  atom.

11
Alpha Radiation
Deflected radiation
Gold Foil
Phosphorescent Screen
Radioactive Source
Diagram for gold foil experiment
12
GOLD FOIL EXPERIMENT

• Using a radioactive source Rutherford fired alpha
  particles at a very thin piece of gold foil. He
  expected the alpha particle to easily pass
  through the foil and strike the screen on the
  opposite side. He found that some of the
  particles were deflected at small angles and some
  were reflected straight back.

13
Nucleus

• The results of Rutherfords experiment was his
  proposal of the nuclear atom. He proposed that
  in the center of an atom was a very small and
  dense nucleus and that the nucleus was composed
  of protons.

14
Rutherford Atom

• The electrons in the Rutherford atom hold no
  specific place in the atom, they are free to move
  about. So, the Rutherford model of the atom has
  a small dense nucleus with free roaming electrons.

15
BOAR? BOOR? BOHR? BAOR

• Neils Bohr worked with Lord Rutherford for a
  short while and came up with his own hypothesis
  for the structure of an atom. It is called the
  planetary model of the atom.
  

16
Planetary Atom

• The only difference between the planetary atom
  and the Rutherford atom is the placement of the
  electrons in orbitals. Electrons can only occupy
  specific orbits at specific energy levels around
  an atom. The farther away an electron is from the
  nucleus the higher its energy state.

17
Planetary atom

• Electrons in orbit, nucleus at center.
• Electrons 2n2

18
The Modern Atom

• Todays atom is very similar to the Bohr atom.
  The modern atom is sometimes called the quantum
  mechanical atom. That is because the theory was
  based on a type of physics called quantum
  mechanics.

19
Quantum Mechanics

• Todays Atom is only different from Bohrs Atom in
  that it does not actually occupy orbits. It has
  a probability of occupying certain places around
  the atom and 90 of the time it spends in the
  Bohr Orbits. The position of the electron is
  determined by its energy.

20
Schrodinger

• The main mathematics for the quantum atom were
  performed by Erwin Schrodinger in the 20s and
  30s. The equation that describes the position
  of an electron around an atom is called
  Schrodingers Equation and deals in differential
  calculus.

21
Neutron

• The Final sub-atomic particle was found in the
  1930s by James Chadwick. He proposed this
  particle had no charge but was as massive as the
  proton and also resided in the nucleus. He
  called it the neutron, after its neutral charge.
  

22
Today

• Today we think there are sub-sub-atomic particles
  called quarks. Each Proton, neutron and
  electron is made up of 3 quarks. Quarks come in
  many different types and have partial charges of
  2/3 and -1/3. Quarks also have other properties
  named, strangeness, charm, flavor, and color.