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Atomic Theory
History
The Parade of Old Guys
Who Discovered the
Atom
Who first theorized
atoms?
The History of the Atom
• Fourth century BC, Greek philosopher
named Democritus was the first to theorize
the existence of atoms.
• Democritus did not have the technology to
test his Atmos Theory and it was not widely
accepted.
“Atmos” is Greek
for indivisible
What was the first
atomic theory of
atoms supported by
data?
The History of the Atom
• 1805 - John Dalton
I took Lavoisier's work
and made an atomic
• the first atomic theory
theory out of it.
supported by scientific
evidence
• Used evidence derived from
the Law of Conservation of
Mass, the Law of Definite
Proportions, and the Law of
Multiple Proportions
• called the Billiard Ball atomic model
Dalton’s Atomic Theory States:
1)Matter is made of small particles called atoms.
2)All atoms of an element are identical in size,
mass, and other properties; atoms of different
elements are different in size, mass, and other
properties.
3) Atoms can't be subdivided, created, or
destroyed.
4) Atoms combine in whole-number ratios to
form chemical compounds.
5) In chemical reactions, atoms are combined,
separated, or rearranged.
Atomic Structure
• Not all aspects of Dalton’s atomic theory
have proven to be correct. We now know
that:
• Atoms are divisible into even smaller
particles: protons, neutrons &
electrons.
• One element can have atoms with
different masses- isotopes.
What was the first
subatomic particle
discovered?
Atomic Structure
Discovery of the Electron
• J. J. Thomson - English physicist. 1897
• Thomson discovered the electron
• Used a cathode ray tube and a magnet
called the plum pudding atomic model
Plum pudding is like
oatmeal with raisins.
The raisins are the negative
particles, and the oatmeal is
positive.
Thomson’s Plum Pudding Model
• Said the atom was
like plum pudding:
• A blob of positive
stuff with the
negative electrons
scattered
throughout
How did Thompson
discover the
electron?
Cathode Ray Tube Experiment
Copy
This!
_
cathode
voltage source
+
vacuum
tube
anode
Cathode Ray Tube Experiment
Copy
This!
_
voltage source
+
When turned on, the tube shoots a
beam of light from the cathode to the
anode.
Cathode Ray Tube Experiment
Copy
This!
_
voltage source
+
+
When Thomson added a magnetic
field, the beam bent toward the
positive pole.
Watch this video of the Cathode Ray
experiment
Click the button
Watch Video
Or copy & paste this address into your web browser:
http://www.youtube.com/watch?v=O9Goyscbazk
How did Einstein
prove the existence of
atoms?
Einstein’s Atomic Proof
Brownian Motions- irregular,
random motions of pollen
particles floating in water.
In 1905 Einstein proved
mathematically that Brownian
Motions were caused by the
atoms in the liquid impacting
on the larger suspended
particles.
“Brownian motions
are caused by
moving atoms.”
“Therefore atoms
must exist”
Who discovered the
nucleus?
Atomic Structure
Discovery of the Atomic Nucleus
• In 1911Ernest Rutherford
discovered the nucleus.
• The gold foil experiment
led to the Nuclear Model
of the atom
Rutherford’s Nuclear Model
• An atom has a central, small, dense
positive nucleus
• Electrons revolve around the nucleus
•Most of the atom’s volume is empty
space
• Most of the atom’s mass is in the
nucleus
How was the nucleus
discovered?
Atomic Structure
The Gold Foil Experiment:
• Rutherford used Alpha particles positively charged pieces given off by
uranium
• Shot them at gold foil which can be
made a few atoms thick
• When the alpha particles hit a
fluorescent screen, it glows.
Lead
block
Polonium
Fluorescent
Screen
Gold Foil
Copy
This!
What he expected
What he got
Copy
This!
Watch this video of the Gold Foil
experiment
Click the button
Watch Video
Or copy & paste this address into your web browser:
http://www.youtube.com/watch?v=Q8RuO2ekNGw
How he explained it
• Atom is mostly
empty space
• Small dense, positive
particle at center
• Alpha particles are
deflected by it if they
get close enough
+
How were electron
energy levels
discovered?
Bohr’s Planetary Model
• In1913 Niels Bohr replaced
the Nuclear model with the
Planetary Model.
The Planetary Model is
often called the Bohr
Model.
•Bohr studied the light waves given off by
hydrogen plasma to calculate the energy
of hydrogen’s electrons as it moved from
one orbit to the next.
Bohr’s Planetary Model
• According to Bohr’s model, electrons can
be only certain distances (orbits) from
the nucleus, like planets orbiting a star.
• Each orbit corresponds to a certain
quantity of energy that an electron can
have.
Bohr’s Planetary Model
An electron will gain
energy as it moves
from a low to a high
orbit and lose
energy as it moves
from a high to low
orbit.
This quantum of
energy is seen as
light.
Watch this video of the Bohr Model
Click the button
Watch Video
Or copy & paste this address into your web browser:
http://www.youtube.com/watch?v=mRNNccii-No
What is the current
atomic theory?
Electron-Cloud Model
Electron-Cloud Model/ Standard
Model- the current theory of the atom
•Nucleus: contains protons &
neutrons held together by the strong
nuclear force
•The exact location of
individual electrons
cannot be determined
Orbitals
• In this model, electrons are located in
orbitals, regions within specific energy
levels in the electron cloud.
• Orbitals are regions where electrons are
likely to be found.