Investigating Atoms and Atomic Investigating Atoms and Atomic
Theory Theory
Students should be able to: Students should be able to:
Describe the particle theory of matter. PS.2aDescribe the particle theory of matter. PS.2a
Use the Bohr model to differentiate among the Use the Bohr model to differentiate among the
three basic particles in the atom (proton, neutron, three basic particles in the atom (proton, neutron, and electron) and their charges, relative masses, and electron) and their charges, relative masses,
and locations. PS.3 and locations. PS.3
Compare the Bohr atomic model to the electron Compare the Bohr atomic model to the electron cloud model with respect to their ability to
cloud model with respect to their ability to represent accurately the structure of the represent accurately the structure of the
atom.PS.3 atom.PS.3
Atomos: Not to Be Cut Atomos: Not to Be Cut
The History of Atomic Theory
The History of Atomic Theory
Atomic Models Atomic Models
This model of the This model of the atom may look atom may look
familiar to you. This is familiar to you. This is
the Bohr model. In the Bohr model. In
this model, the this model, the
nucleus is orbited by nucleus is orbited by electrons, which are electrons, which are
in different energy in different energy
levels.
levels.
A model uses familiar ideas to A model uses familiar ideas to explain unfamiliar facts
explain unfamiliar facts observed in nature.
observed in nature.
A model can be changed as A model can be changed as new information is collected.
new information is collected.
The atomic model The atomic model has changed
has changed throughout the throughout the
centuries, starting centuries, starting in 400 BC, when it in 400 BC, when it
looked like looked like a a
billiard ball
billiard ball → →
Who are these men?
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 views.
Democritus Democritus
This is the Greek This is the Greek
philosopher Democritus philosopher Democritus
who began the search for who began the search for
a description of matter a description of matter
more than
more than 2400 years ago.2400 years ago.
He asked: Could matter He asked: Could matter be divided into smaller be divided into smaller and smaller pieces
and smaller pieces
forever, or was there a forever, or was there a limit
limit to the number of to the number of times a piece of matter times a piece of matter could be
could be divideddivided? ?
400 BC
Atomos Atomos
His theory: Matter could not His theory: Matter could not be divided into smaller and be divided into smaller and
smaller pieces forever, smaller pieces forever, eventually the smallest eventually the smallest
possible piece would be possible piece would be
obtained.
obtained.
This piece would be This piece would be indivisible.
indivisible.
He named the smallest He named the smallest piece of matter “atomos,”
piece of matter “atomos,”
meaning “not to be cut.”
meaning “not to be cut.”
Atomos Atomos
To Democritus, atoms To Democritus, atoms were
were smallsmall, hard , hard
particles that were all particles that were all
made of the same made of the same
material but were material but were
different
different shapes and shapes and sizes.
sizes.
Atoms were Atoms were infiniteinfinite in in number, always moving number, always moving
and capable of joining and capable of joining
together.
together.
This theory was This theory was
ignored and forgotten ignored and forgotten
for more than
for more than 2000 2000 years!
years!
Why? Why?
The eminent The eminent philosophers of philosophers of
the time, the time,
Aristotle
Aristotle and and Plato, had a Plato, had a
more respected, more respected,
(and ultimately (and ultimately
wrong
wrong ) theory. ) 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.
Dalton’s Model Dalton’s Model
In the early 1800s, In the early 1800s, the English
the English
Chemist John Chemist John
Dalton
Dalton performed a performed a number of
number of
experiments that experiments that eventually led to eventually led to
the acceptance of the acceptance of the idea of atoms.
the idea of atoms.
Dalton’s Theory Dalton’s Theory
He deduced that all He deduced that all elements
elements are composed of are composed of atoms. Atoms are
atoms. Atoms are indivisible and
indivisible and
indestructible particles.
indestructible particles.
Atoms of the Atoms of the samesame element element are exactly alike.
are exactly alike.
Atoms of Atoms of differentdifferent elements elements are are differentdifferent..
CompoundsCompounds are formed by are formed by the joining of atoms of two the joining of atoms of two
or more elements.
or more elements.
. .
This theory This theory became one became one
of the of the
foundations foundations
of modern of modern
chemistry.
chemistry.
Thomson’s Plum Pudding Thomson’s Plum Pudding
Model Model
In In 1897 1897 , the , the
English scientist English scientist
J.J. Thomson J.J. Thomson
provided the first provided the first hint that an atom hint that an atom
is made of even is made of even
smaller
smaller particles. particles.
Thomson Model Thomson Model
He proposed a model He proposed a model of the atom that is
of the atom that is
sometimes called the sometimes called the
“ “ Plum Plum Pudding Pudding ” ” model.
model.
Atoms were made Atoms were made from a positively from a positively
charged
charged substance substance with negatively
with negatively charged electrons charged electrons
scattered
scattered about, like about, like raisins in a pudding.
raisins in a pudding.
Thomson Model Thomson Model
Thomson studied Thomson studied the the passage passage of an of an
electric current electric current
through a gas.
through a gas.
As the current As the current passed through passed through
the gas, it gave off the gas, it gave off
rays of
rays of negatively negatively charged particles.
charged particles.
Thomson Model Thomson Model
This surprised This surprised Thomson,
Thomson,
because the because the
atoms of the gas atoms of the gas were uncharged.
were uncharged.
Where had the Where had the
negative charges negative charges
come from?
come from?
Where did they come from?
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 atom.
Rutherford’s Gold Foil Rutherford’s Gold Foil
Experiment Experiment
In 1908, the English In 1908, the English physicist Ernest
physicist Ernest
Rutherford was hard Rutherford was hard
at work on an at work on an
experiment that experiment that
seemed to have little seemed to have little to do with unraveling to do with unraveling
the mysteries of the the mysteries of the
atomic structure.
atomic structure.
Rutherford’s experiment Involved firing Rutherford’s experiment Involved firing a stream of tiny
a stream of tiny positively charged positively charged particles at a thin sheet of
particles at a thin sheet of gold foil gold foil (2000 atoms thick)
(2000 atoms thick)
MostMost of the positively of the positively
charged “bullets” passed charged “bullets” passed
right through the gold right through the gold atoms in the sheet of
atoms in the sheet of gold gold foilfoil without changing without changing
course at all.
course at all.
SomeSome of the positively of the positively
charged “bullets,” however, charged “bullets,” however,
did bounce away from the did bounce away from the
gold sheet as if they had gold sheet as if they had
hit something
hit something solidsolid. He . He knew that positive charges knew that positive charges
repel
repel positive charges. positive charges.
http://chemmovies.unl.edu/ChemAnime http://chemmovies.unl.edu/ChemAnime /RUTHERFD/RUTHERFD.html
/RUTHERFD/RUTHERFD.html
http://chemmovies.unl.edu/ChemAnime http://chemmovies.unl.edu/ChemAnime /RUTHERFD/RUTHERFD.html
/RUTHERFD/RUTHERFD.html
This could only mean that the gold atoms in the This could only mean that the gold atoms in the sheet were mostly
sheet were mostly open space open space . Atoms were . Atoms were not not a pudding filled with a positively charged
a pudding filled with a positively charged material.
material.
Rutherford concluded that an atom had a Rutherford concluded that an atom had a small, small, dense, positively charged center
dense, positively charged center that that repelled repelled his positively charged “bullets.”
his positively charged “bullets.”
He called the center of the atom the “ He called the center of the atom the “ nucleus nucleus ” ”
The nucleus is The nucleus is tiny tiny compared to the atom as a compared to the atom as a whole.
whole.
Rutherford Rutherford
Rutherford reasoned Rutherford reasoned that all of an atom’s that all of an atom’s
positively charged positively charged
particles were particles were
contained
contained in the in the
nucleus. The negatively nucleus. The negatively
charged particles were charged particles were
scattered
scattered outside the outside the nucleus around the nucleus around the
atom’s
atom’s edgeedge..
Bohr Model Bohr Model
In 1913, the Danish In 1913, the Danish scientist Niels Bohr scientist Niels Bohr
proposed an proposed an
improvement. In his improvement. In his
model, he placed model, he placed
each electron in a each electron in a
specific
specific energy energy level.
level.
Bohr Model Bohr Model
According to Bohr’s According to Bohr’s atomic model,
atomic model,
electrons move in electrons move in
definite
definite orbits orbits around around the nucleus, much
the nucleus, much
like planets circle the like planets circle the sun. These orbits, or sun. These orbits, or
energy
energy levels levels , are , are located at certain located at certain
distances from the distances from the
nucleus.
nucleus.
Wave Model
The Wave Model The Wave Model
Today’s atomic Today’s atomic
model is based on model is based on
the principles of the principles of wave wave mechanics mechanics . .
According to the According to the theory of wave theory of wave
mechanics, electrons mechanics, electrons
do not move
do not move about about an atom in a
an atom in a definite definite path,
path, like the planets like the planets around the sun.
around the sun.
The Wave Model The Wave Model
In fact, it is In fact, it is impossibleimpossible to determine the exact to determine the exact location of an electron. The
location of an electron. The probableprobable location of an location of an electron is based on how much
electron is based on how much energyenergy the electron the electron has.has.
According to the modern atomic model, at atom has According to the modern atomic model, at atom has a a small positively charged nucleussmall positively charged nucleus surrounded by a surrounded by a large region in which there are enough electrons to large region in which there are enough electrons to make an atom neutral.
make an atom neutral.
Electron Cloud:
Electron Cloud:
A space in which A space in which
electrons are likely to be electrons are likely to be
found.
found.
Electrons Electrons whirlwhirl about the about the nucleus billions of times nucleus billions of times
in one second in one second
They are not moving They are not moving around in
around in randomrandom patterns.
patterns.
Location of electrons Location of electrons
depends upon how much depends upon how much
energy
energy the electron has. the electron has.
Electron Cloud:
Electron Cloud:
Depending on their energy they are locked into a Depending on their energy they are locked into a certain area in the cloud.
certain area in the cloud.
Electrons with the Electrons with the lowest lowest energy are found in energy are found in the energy level
the energy level closest closest to the nucleus to the nucleus
Electrons with the Electrons with the highest highest energy are found energy are found in the
in the outermost outermost energy levels, farther from energy levels, farther from the nucleus.
the nucleus.
Indivisible
Indivisible ElectronElectron NucleusNucleus OrbitOrbit Electron Electron Cloud Cloud
