Section 1.3
The Atomic Theory
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People in general are curious…
scientists in specific !
Theories in science are proposed to explain the evidence available at the
time.
As new evidence is discovered, theories are adapted to explain new data.
This is the nature of Science.
In the future this will continue.
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Plato
Democritus
460-370 BC
First Greek philosopher to
suggest that tiny different
pieces of particles exist
“atoms”
Atoms make up the world
His idea was refuted by
Aristotle
Plato
Teacher of Aristotle
Aristotle
384-322 BC
Four Elements
Air, fire, earth
water
The Four Element Theory lasted for about 2 thousand years because no
one tested the theory with scientific experiments.
It was not a scientific theory- which is tested by experiment.
Science back then was more of a philosophical moralistic field that
philosophers rather than experimentation decided.
To sum up the very ancient
times on science understanding
1) Greek philosophers believed that matter was made of
atomos that were the smallest pieces of matter.
2) Aristotle believed matter was made of different
combinations of earth, air, fire, and water.
3) Alchemists experimented with matter and tried to turn
common metals into gold.
4) Their activities marked the beginning of
our understanding of matter.
Development of Atomic Theory (I)
John Dalton (1766 - 1844)
1) Credited with developing a theory that was a new way of explaining
matter.
2) He studied gases that make up Earth’s atmosphere.
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John Dalton (1766 - 1844)
Based on his studies, he suggested that:
A) matter is made of small, hard spheres that are
different for different elements
B) the smallest particle of an element is called an atom
• This is the basis for Dalton’s Atomic Theory.
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Dalton’s Atomic Theory
See page 30
1. All matter is made of small particles called atoms.
2. Atoms cannot be created, destroyed, or divided into
smaller particles.
3. All atoms of the same element are identical in mass
and size, but they are different in mass and size from
the atoms of other elements.
4. Compounds are created when atoms of different
elements link together in definite proportions.
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Atomic Theory (II)
See page 30
J. J. Thomson (1856 - 1940)
He discovered the “electron”
Raisin bun/ blueberry muffin/
Plum pudding model
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Atomic Theory II
See page 30
J. J. Thomson (1856 - 1940)
Thomson studied electric currents in gas tubes (like
today’s fluorescent lights).
From his studies, he determined that the currents were
streams of negatively charged particles. These were later
called electrons.
He hypothesized that atoms are made of smaller
particles. He proposed the “raisin bun” model of the
atom.
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Atomic Theory II
See page 30
J. J. Thomson (1856 - 1940)
Raisin bun model
This model is best visualized as a positively charged bun
with negatively charged particles spread out in it like
raisins.
This was a very startling proposal because atoms were
thought to be the smallest part of matter!!!
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Raisin bun model is wrong!!!
(Tough luck my friend)
Raisin bun model says: The atom is made up of positive
material with negative particles throughout- like raisins in
a raisin bun.
We now know this is not true…electrons are not found
inside the nucleus. They are outside the nucleus!
Ernest Rutherford 1871-1937
He was J. J Thomson’s student
Atomic Theory (III)
See page 31
Ernest Rutherford (1871 - 1937)
He wanted to explore inside the atom.
He took a very thin gold piece and shot at it with
alpha particles (positively-charged). He noticed
the following:
Almost 99% of the alpha particles went straight
through the gold foil.
About 1% bounced right back!
What does this mean?!
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Atomic Theory III
See page 31
Ernest Rutherford (1871 - 1937)
From his gold experiment, he concluded the
following:
Most of the atom must be empty space to explain
why the alpha particles went straight through it.
There must be a small, dense part inside the
atom that is positively-charged which made the
alpha particles bounce right back!
This small dense part is what we now know as the
nucleus 
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Atomic Theory IV
See page 31 - 32
Niels Bohr (1885 - 1962)
1) He wanted to see what happens to the
electrons that surround the nucleus when
electric current is passed through hydrogen
gas
2) He noticed that when we give the gas extra
energy from the electricity, light is released!
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Atomic Theory IV
See page 31 - 32
Niels Bohr (1885 - 1962)
Based on his observations, Bohr proposed that
electrons surround the nucleus in specific “energy
levels” or “shells.”
When we supply electricity, we give electrons extra
energy so they jump from a low energy level to a
high energy level.
When an electron jumps from a high energy level to
a low energy level, it releases or gives away
energy in the form of light!
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Energy levels of electrons
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Inside the Atom
See pages 32 - 33
1) An atom is the smallest particle of an element that
retains the properties of the element.
2) All atoms are made up of three kinds of particles called subatomic
particles. These particles are:
• electrons
• protons
• neutrons
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A closer look at the subatomic
particles
Protons
Neutrons
Electrons
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Discovery of the Nucleus
Size of the Atom
Homework
Page 33
#3,5,6,7
Page 37
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