Atomic Theory
Mr. Tollefson
Chemistry Class
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.
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.
Dalton and Atomic Theory
• John Dalton’s (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.
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
Electron’s: 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.
Cathode Rays
• Thompson thus created CRT’s 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.
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.
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.
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.
Alpha
Radiation
Gold Foil
Deflected
radiation
Phosphorescent
Screen
Radioactive Source
Diagram for gold foil experiment
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.
Nucleus
• The results of Rutherford’s
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.
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.
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.
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 it’s
energy state.
Planetary atom
Electrons in orbit, nucleus at center.
Electrons = 2n2
The Modern Atom
• Today’s 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.
Quantum Mechanics
• Todays Atom is only different from
Bohr’s 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 it’s energy.
Schrodinger
• The main mathematics for the
quantum atom were performed by
Erwin Schrodinger in the 20’s and
30’s. The equation that describes the
position of an electron around an
atom is called Schrodinger’s
Equation and deals in differential
calculus.
Neutron
• The Final sub-atomic particle was
found in the 1930’s 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 it’s neutral charge.
Today
• Today we think there are sub-subatomic 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.