Transcript File

Atomic Theory
Chapter 8
The Composition of the atom
…In the Beginning…
• Ancient Greece
• Circa 400 BC
• “Democritus”
Democritus concluded...
Matter is composed of
Indivisible, invisible,
tiny pieces.
“Atomon”=Indivisible
Early Theories of Matter
The Great Aristotle
He Criticized Democritus and
did not believe his ideas!
People believed Aristotle because he was a great
philosopher. As a result, his view was unchallenged
for 2000 years!
Democritus and Aristotle just had ideas, not scientific
proof. They could not test if it was valid. He was the
first to propose the term “atom” to matter
Let There Be….nothing significant for 1700 years
• Chemistry was
definitely present
– Chinese Empire
– Dark Ages
1803 John Dalton
• A British Schoolteacher
• First to apply Democritus’ idea of the atom to
chemistry
• He noted that in experiments where compounds were
formed, the elements always combined in the same
percentage by mass
– What was this idea called again?
• “Law of Definite Composition”
– Proved that this law could be explained if matter is
composed of atoms
1803 John Dalton
• Dalton’s Model of the
Atom:
– Uniform Shape
– Uniform Density
– Indivisible
• So what did Dalton’s
model look like?
• A simple design that
promoted future research
into atomic theory…
Dalton’s Atomic Theory
1. All matter is made up of extremely
small particles called atoms.
2. All atoms of a given element are
identical, having the same size,
mass and chemical properties.
Atoms of a specific element are
different from those of any other
element.
3. Atoms cannot be created, divided
into smaller particles, or destroyed.
4. Different atoms combine in simple
whole-number ratios to form
compounds.
5. In a chemical reaction, atoms are
separated, combined, or rearranged.
Was all of Dalton’s Theory accurate?
1. NO! His theory had to be
revised as additional info was
learned that could not be
explained by his theory.
2. He was wrong about
atoms being indivisible!
They are divisible into
several subatomic
particles!
3. He was mistaken about atoms of a given
element having identical properties.
Atoms of an element may have slightly
different masses.
The Cathode Ray Tube (CRT)
Developed by Sir William Crookes in
the late 1800s
Thank you Sir William Crookes-for your
Accidental Discovery!
• Discovery of
cathode rays led
to invention of the
TV!!!!
• TV and computer
monitor images are
formed as radiation from
the cathode strikes the
light producing
chemicals that coat the
backside of the screen.
Discoveries from the CRT
• Wilhelm Roentgen
(1895)
?
Henri Becquerel 1896
• Roentgen’s Discovery of X-rays provoked
further research by many others, including
Becquerel
• Since fluorescent materials could be
charged by X-rays, Henri wondered if
naturally fluorescent minerals could give
off X-rays….
• Like any good scientist, he set up a test…
Henri Becquerel 1896
Photographic film
sealed away
Henri Becquerel 1896
The photographic
paper was still
exposed!
The mineral contained
Uranium and was
RADIOACTIVE
JJ Thompson 1897
• Using the CRT…
Cathode
(--)
Anode (+)
Electromagnets
(turned off)
Beam of “light”
Fluorescent Screen
JJ Thompson 1897
• When the magnets were turned on, the beam
was pulled towards the + plate
• What 2 things could Thompson conclude about
the beam?
+ side
-- side
J. J. Thomson 1890’s
• J.J. proposed a model
of the atom known as
the “plum pudding
model.”
Thomson’s model
consisted of a
spherically shaped atom
made up of uniformly
distributed positive
charge within which the
negatively charged
particles resided.
JJ Thompson 1897
• What did his experiment discover?
• ELECTRONS! (negatively-charged particles)
• How did the model of the atom change
since Dalton?
AKA The Chocolate-Chip Cookie
Model
Robert Millikan 1909
• Determined the
charge of an
electron.
• Mass of an electron
= 9.1 x 10-28 g
• Mass of an electron
is extremely small.
Ernst Rutherford 1898
• After the discoveries of Becquerel and
Thompson, Rutherford was dissatisfied
with the atomic models and set out to
disprove it.
• Using the recent discovery of Radiation
(from Bacquerel), he and his lab assistants
(Geiger and Marsden) set out…
Ernst Rutherford Early 1900s
Ernest Rutherford 1911
• Studied how positively
charged alpha particles
interacted with solid
matter.
Conducted an
experiment to see if
alpha particles would be
deflected as they passed
through a thin foil of
gold. Also known as
Rutherford’s Gold Foil
Experiment.
Ernest Rutherford’s Gold Foil Experiment
1. A narrow beam of alpha
particles was aimed at a
thin sheet of gold foil.
2. A zinc sulfide coated
screen surrounding the
gold foil produced a flash of
light whenever it was struck
by an alpha particle.
Rutherford’s Gold Foil Experiment Results
1.
Most all of the alpha
particles passed straight
through the gold foil,
without deflection.
2. Some alpha particles
were scattered at small
angles.
3. A few times they were
deflected at very large
angles.
Rutherford’s Gold Foil Conclusions
1. Rutherford said plum
pudding model was
incorrect.
2. An atom consisted mostly
of empty space through
which the electrons move.
3. There was a tiny, dense,
positive region, called the
nucleus, centrally located
within the atom that
contained all of an atom’s
positive charge and almost
all of its mass.
Rutherford’s Gold Foil Conclusions
1. If a nucleus were the size
of the dot at the end of this
sentence, its mass would
be as much as that of 70
cars!
2. Rutherford’s atomic
model: most of an atom
consists of electrons
moving rapidly through
empty space. The
electrons move through
available space and are
held within the atom by
their attraction to the
positively charged
nucleus!
Rutherford’s Completing the Atom
• How did the model of
the atom change?
• What should it look
like based on
Rutherford’s
conclusions?
Summary
• Models of the atom:
Dalton
Rutherford
Thompson
Modern