Download A. What Is an Atom?

Bell Activity #1
The following is a quote by Democritus.
Paraphrase this quote in your own
words.
“Color exists by convention, sweet by
convention, bitter by convention; in
reality nothing exists but atoms and the
void.”
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Objectives
• Describe some of the experiments that
led to the current atomic theory.
• Compare the different models of the
atom.
• Explain how the atomic theory has
changed as scientists have discovered
new information about the atom.
I. The Beginning of The Atomic Theory
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A. What Is an Atom? Around 440
BCE, a Greek philosopher named
Democritus thought that you would
eventually end up with a particle that
could not be cut. He called this
particle an atom. An atom is the
smallest unit of an element that
maintains the properties of that
element.
It would take 1.05 x 1017
gold atoms to cover the
entire surface of a dollar
bill. That’s 105 quadrillion
gold atoms!
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I. The Beginning of The Atomic Theory
B. From Aristotle to Modern
Science Aristotle, another
Greek philosopher, disagreed
with Democritus’s ideas. He
believed that you would never
end up with a particle that could
not be cut. Most people
believed Aristotle, even though
Democritus was right: Matter is
made of particles, which we call
atoms. An atom is the smallest
particle into which an element
can be divided and still be the
same substance.
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Aristotle held the theory
that all matter could be
reduced to a combination
of four elements: earth,
water, air, and fire.
II. Dalton’s Atomic Theory Based on
Experiments
A. Dalton’s Theory John Dalton
published his atomic theory in 1803.
His theory stated that all substances
are made of atoms. Atoms are small
particles that cannot be created,
divided, or destroyed. Atoms of the
same element are exactly alike, and
atoms of different elements are
different. Atoms join with other atoms
to make new substances.
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B. Not Quite Correct The atomic
theory was then changed to describe
the atom more correctly.
Along with contributing
to the atomic theory, John
Dalton was also the first to
describe colorblindness.
Dalton Himself was
colorblind.
Red-Green Color Blindness is most common.
Can you see the numbers?
A person with total color blindness is unable to
distinguish individual hues.
In this photograph, the scene is shown as it might
look to a person with partial color blindness.
III. Thomson’s Discovery of Electrons
A. Negatively Charged Particles
Thomson experimented with a
cathode-ray tube like the one shown
on the next slide. He discovered
negatively charged particles that are
now known as electrons.
B. Like Plums in Pudding After
learning that atoms contain
electrons, Thomson proposed a new
model of the atom. Thomson
thought that electrons were mixed
throughout an atom, like plums in a
pudding.
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Thomson’s Cathode-Ray Tube
Experiment
IV. Rutherford’s Atomic “Shooting
Gallery”
A. Negatively Charged Particles
In 1909, Ernest Rutherford aimed a
beam of small, positively charged
particles at a thin sheet of gold foil.
The next slide shows his
experiment.
B. Surprising Results Rutherford
expected the particles to pass right
through the gold in a straight line. To
Rutherford’s great surprise, some of
the particles were deflected.
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Rutherford’s Gold-Foil Experiment
V. Where are the Electrons?
A. Far from the Nucleus
Rutherford proposed that in the
center of the atom is a tiny, positively
charged part called the nucleus.
B. Bohr’s Electron Levels In 1913,
Niels Bohr proposed that electrons
move around the nucleus in certain
paths, or energy levels.
C. The Modern Atomic Theory
According to the current theory, there
are regions inside the atom where
electrons are likely to found.
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