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The key features of this ppt are:
1. Dalton's Model
2. Thomson's Model
3. Rutherford Experiment
4. The Wave Model
1.
Investigating Atoms and Atomic
Theory
Students should be able to:
Describe the particle theory of matter. PS.2a
Use the Bohr model to differentiate among the
three basic particles in the atom (proton, neutron,
and electron) and their charges, relative masses,
and locations. PS.3
Compare the Bohr atomic model to the electron
cloud model with respect to their ability to
represent accurately the structure of the
atom.PS.3
2.
Atomos: Not to Be Cut
The History of Atomic Theory
3.
Atomic Models
This model of the
atom may look
familiar to you. This is
the Bohr model. In
this model, the
nucleus is orbited by
electrons, which are
in different energy
levels.
A model uses familiar ideas to
explain unfamiliar facts
observed in nature.
A model can be changed as
new information is collected.
4.
The atomic model
has changed
throughout the
centuries, starting
in 400 BC, when it
looked like a
billiard ball →
5.
Who are these men?
In this lesson, we’ll learn
about the men whose quests
for knowledge about the
fundamental nature of the
universe helped define our
6.
Democritus 400 BC
This is the Greek
philosopher Democritus
who began the search for
a description of matter
more than 2400 years ago.
He asked: Could matter
be divided into smaller
and smaller pieces
forever, or was there a
limit to the number of
times a piece of matter
could be divided?
7.
His theory: Matter could not
be divided into smaller and
smaller pieces forever,
eventually the smallest
possible piece would be
obtained.
This piece would be
indivisible.
He named the smallest
piece of matter “atomos,”
meaning “not to be cut.”
8.
To Democritus, atoms
were small, hard
particles that were all
made of the same
material but were
different shapes and
sizes.
Atoms were infinite in
number, always moving
and capable of joining
together.
9.
This theory was
ignored and forgotten
for more than 2000
10.
Why?
The eminent
philosophers of
the time,
Aristotle and
Plato, had a
more
respected,
(and ultimately Aristotle and Plato favored the earth, fire, air
and water approach to the nature of matter.
wrong) theory. Their ideas held sway because of their
eminence as philosophers. The atomos idea
was buried for approximately 2000 years.
12.
Dalton’s Model
In
the early 1800s,
the English
Chemist John
Dalton performed a
number of
experiments that
eventually led to
the acceptance of
the idea of atoms.
13.
Dalton’s Theory
He deduced that 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.
14.
.
This theory
became one
of the
foundations
of modern
chemistry.
15.
Thomson’s Plum Pudding
Model
In 1897, the
English scientist
J.J. Thomson
provided the first
hint that an atom
is made of even
smaller particles.
16.
Thomson Model
He proposed a
model of the atom
that is sometimes
called the “Plum
Pudding” model.
Atoms were made
from a positively
charged substance
with negatively
charged electrons
scattered about, like
raisins in a pudding.
17.
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.
18.
Thomson Model
Where did
they come
Thissurprised from?
Thomson,
because the
atoms of the gas
were uncharged.
Where had the
negative charges
come from?
19.
Thomson concluded that the
negative charges came from within
the atom.
A particle smaller than an atom had
to exist.
The atom was divisible!
Thomson called the negatively
charged “corpuscles,” today known
as electrons.
Since the gas was known to be
neutral, having no charge, he
reasoned that there must be
positively charged particles in the
But he could never find them.
20.
Rutherford’s Gold Foil
Experiment
In1908, the English
physicist Ernest
Rutherford was hard
at work on an
experiment that
seemed to have little
to do with unraveling
the mysteries of the
atomic structure.
21.
Rutherford’s experiment Involved firing
a stream of tiny positively charged
particles at a thin sheet of gold foil
(2000 atoms thick)
22.
Most of the positively
charged “bullets” passed
right through the gold
atoms in the sheet of gold
foil without changing
course at all.
Some of the positively
charged “bullets,” however,
did bounce away from the
gold sheet as if they had
hit something solid. He
knew that positive charges
repel positive charges.
24.
http://chemmovies.unl.edu/ChemAnime/R
UTHERFD/RUTHERFD.html
25.
This could only mean that the gold atoms in the
sheet were mostly open space. Atoms were not
a pudding filled with a positively charged
material.
Rutherford concluded that an atom had a small,
dense, positively charged center that repelled
his positively charged “bullets.”
He called the center of the atom the “nucleus”
The nucleus is tiny compared to the atom as a
whole.
26.
Rutherford reasoned
that 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.
27.
Bohr Model
In 1913, the Danish
scientist Niels Bohr
proposed an
improvement. In his
model, he placed
each electron in a
specific energy
level.
28.
Bohr Model
According to Bohr’s
atomic model,
electrons move in
definite orbits around
the nucleus, much
like planets circle the
sun. These orbits, or
energy levels, are
located at certain
distances from the
nucleus.
30.
The Wave Model
Today’s atomic
model is based on
the principles of
wave mechanics.
According to the
theory of wave
mechanics, electrons
do not move about
an atom in a definite
path, like the planets
around the sun.
31.
The Wave Model
In fact, 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.
According to the modern atomic model, at atom
has a small positively charged nucleus surrounded
by a large region in which there are enough
electrons to make an atom neutral.
32.
Electron Cloud:
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.
33.
Electron Cloud:
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.
34.
Indivisible Electron Nucleus Orbit Electron
Cloud
Greek X
Dalton X
Thomson X
Rutherford X X
Bohr X X X
Wave X X X
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