Transcript Investigating Atoms and Atomic Theory

Atomos: Not to Be Cut
The History of Atomic Theory
Who are these men and what are
they doing in our classroom?
Democritus
Aristotle
Democritus
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began the search more than
2400 years ago.
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proposed that matter could
not be divided into smaller
and smaller pieces forever
named the smallest piece
of matter “atomos,”
meaning “not to be cut.”
•
400 BC
This theory was ignored and
forgotten for more than 2000
years!
Why?
Aristotle and Plato, had
a more respected, but
wrong, theory.
Aristotle and Plato favored the earth, fire,
air and water approach to the nature of
matter. Their ideas held sway because of
their eminence as philosophers. The
atomos idea was buried for
approximately 2000 years.
Ancient Philosophy
Who: Aristotle, Democritus
 When: More than 2000 years ago
 Where: Greece
 What: Aristotle believed in 4 elements: Earth,
Air, Fire, and Water. Democritus believed
that matter was made of small particles he
named “atoms”.
 Why: Aristotle and Democritus used
observation and inference to explain the
existence of everything.

Alchemists
Who: European Scientists
 When: 800 – 900 years ago
 Where: Europe
 What: Their work developed into what is now
modern chemistry.
 Why: Trying to change ordinary materials into
gold.
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Alchemic Symbols
John Dalton
Dalton’s Theory
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All elements are composed
of atoms.
Atoms are indivisible and
indestructible particles.
Atoms of the same element
are exactly alike.
Atoms of different elements
are different.
Compounds are formed by
the joining of atoms of two
or more elements.
Particle Theory
Who: John Dalton
 When: 1808
 Where: England
 What: Described atoms as tiny particles that
could not be divided. Thought each element
was made of its own kind of atom.
 Why: Building on the ideas of Democritus in
ancient Greece.
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.
 This
theory
became one
of the
foundations
of modern
chemistry.
J. J. Thompson
Thomson Model
“Plum Pudding”
model.
 Atoms were made
from a positively
charged substance
with negatively
charged electrons
scattered about, like
raisins in a pudding.

Thomson Model
 Thomson
studied
the passage of an
electric current
through a gas.
 As the current
passed through
the gas, it gave off
rays of negatively
charged particles.
Chapter 11
Section 1 Development of the
Atomic Theory
Thomson Model
 This
surprised
Thomson,
because the
atoms of the gas
were uncharged.
Where had the
negative charges
come from?
Where did
they come
from?
Discovery of Electrons
Who: J. J. Thompson
 When: 1897
 Where: England
 What: Thompson discovered that electrons
were smaller particles of an atom and were
negatively charged.
 Why: Thompson knew atoms were neutrally
charged, but couldn’t find the positive
particle.

Ernest Rutherford
 He
fired a stream of tiny positively
charged particles at a thin sheet of
gold foil (2000 atoms thick)
Chapter 11
Section 1 Development of the
Atomic Theory
What did this mean?
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all of an atom’s
positively charged
particles were
contained in the
nucleus
The negatively
charged particles
were scattered
outside the nucleus
around the atom’s
edge.
Atomic Structure I
Who: Ernest Rutherford
 When: 1911
 Where: England
 What: Conducted an experiment to isolate
the positive particles in an atom. Decided
that the atoms were mostly empty space, but
had a dense central core.
 Why: He knew that atoms had positive and
negative particles, but could not decide how
they were arranged.
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Niels Bohr
Bohr Model
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placed each electron
in a specific energy
level.

electrons move in definite
orbits around the nucleus
These paths are located
at certain distances from
the nucleus.
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Atomic Structure II
Who: Niels Bohr
 When: 1913
 Where: England
 What: Proposed that electrons traveled in
fixed paths around the nucleus. Scientists
still use the Bohr model to show the number
of electrons in each orbit around the nucleus.
 Why: Bohr was trying to show why the
negative electrons were not drawn into the
nucleus of the atom.
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S.MORRIS 2006
The Wave Model
based on the
principles of wave
mechanics.
 electrons do not
move about an
atom in a definite
path, like the
planets around the
sun.
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The Wave Model
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it is impossible to determine the exact location of
an electron. The probable location of an electron
is based on how much energy the electron has.
An atom has a small positively charged nucleus
surrounded by a large region in which there are
enough electrons to make an atom neutral.
Electron Cloud:
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A space in which
electrons are likely to be
found.
Electrons whirl about the
nucleus billions of times
in one second
They are not moving
around in random
patterns.
Location of electrons
depends upon how much
energy the electron has.
Electron Cloud:
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Depending on their energy they are locked into a
certain area in the cloud.
Electrons with the lowest energy are found in
the energy level closest to the nucleus
Electrons with the highest energy are found
in the outermost energy levels, farther from
the nucleus.
Electron Cloud Model
Electrons travel around the nucleus in
random orbits.
 Scientists cannot predict where they will be
at any given moment.
 Electrons travel so fast, they appear to form
a “cloud” around the nucleus.
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Indivisible Electron
Greek
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Dalton
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Nucleus
Thomson
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Rutherford
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Bohr
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Wave
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Orbit
Electron
Cloud
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