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4.1 Studying Atoms
Studying the structure of
atoms is a little like studying
wind. Because you cannot see
air, you must use indirect
evidence to tell the direction of
the wind. Atoms pose a similar
problem because they are
extremely small. Even with a
microscope, scientists cannot
see the structure of an atom.
4.1 Studying Atoms
Ancient Greek Models of Atoms
If you cut a piece of aluminum foil in half,
you have two smaller pieces of the same
shiny, flexible substance. You could cut
the pieces again and again. Can you keep
dividing the aluminum into smaller
pieces? Greek philosophers debated a
similar question about 2500 years ago.
4.1 Studying Atoms
Ancient Greek Models of Atoms
Democritus
– all matter consisted of extremely
small particles that could not be
divided
– called these particles atoms
– Greek word atomos, which
means “uncut” or “indivisible”
4.1 Studying Atoms
Ancient Greek Models of Atoms
Aristotle
– substances made of four elements
•
•
•
•
Earth
Air
Fire
Water
– Thought matter could be divided
indefinitely
By the 1800s, scientists had
enough experimental data to
support an atomic model
4.1 Studying Atoms
Which philosopher came up with
the term “atom?”
1. Aristotle
2. Democritus
4.1 Studying Atoms
Dalton’s Atomic Theory
– all matter is made up of individual
particles called atoms, that cannot be
divided
– Atoms of the same element are identical
– Atoms of different elements can join to
form molecules
– In a particular compound, atoms of
different elements always combine in the
same way
4.1 Studying Atoms
Dalton’s Atomic Theory
Evidence for Atoms
John Dalton
 Determined that because gases exert
pressure, they consists of individual
particles
 Determined that masses of elements that
make up a compound was equal to the
mass of the compound
 compounds have a fixed composition
4.1 Studying Atoms
Dalton’s Atomic Theory
When magnesium burns, it
combines with oxygen. In
magnesium oxide, the ratio of
the mass of magnesium to
the mass of oxygen is always
about 3 : 2. Magnesium
dioxide has a fixed
composition.
4.1 Studying Atoms
True or False? A compound has a
greater mass than the total mass of
all the atoms that make up that
compound.
1. True
2. False
4.1 Studying Atoms
Dalton’s Atomic Theory
Dalton’s Theory
• All elements are composed of atoms.
• All atoms of the same element have the
same mass, and atoms of different
elements have different masses.
• Compounds contain atoms of more than
one element.
• In a particular compound, atoms of different
elements always combine in the same way.
4.1 Studying Atoms
Dalton’s Atomic Theory
Dalton made these wooden spheres as a model
to represent the atoms of different elements. A
tiny, solid sphere with a different mass
represents each type of atom.
4.1 Studying Atoms
Which of the following is not part of
Dalton’s atomic theory?
1. Elements are made of atoms
2. Atoms of the same element have the
same mass
3. Compounds contain more than one
element
4. Atoms in a particular compound differ
from one sample to the next
4.1 Studying Atoms
Dalton’s Atomic Theory
A theory must explain the data from many
experiments
– Dalton’s theory was supported by experiments
and became widely accepted.
In time, some of Dalton’s ideas were found to
be incorrect
– theory was revised due to new discoveries.
4.1 Studying Atoms
Thomson’s Model of the Atom
Thomson’s experiments provided the first
evidence that atoms are made of even
smaller particles.
4.1 Studying Atoms
Cathode Ray Tube
• http://www.youtube.com/watch?v=O9Goys
cbazk
4.1 Studying Atoms
Thomson’s Model of the Atom
Some materials are said to have either a
positive or a negative electric charge.
• Objects with like charges repel, or push apart.
• Objects with opposite charges attract, or pull
together.
4.1 Studying Atoms
Thomson’s Model of the Atom
Amber is the hardened form
of a sticky, viscous liquid that
protects trees from insects
and disease. If amber is
rubbed with wool, it becomes
charged and can attract a
feather.
4.1 Studying Atoms
Thomson’s Model of the Atom
Thomson’s Experiments
In his experiments, Joseph John Thomson
used a sealed tube containing a very small
amount of gas.
Sealed tube
Glowing beam
filled with gas at
low pressure
Metal disk
Metal disk
Source of
electric current
Metal disk
Source of
electric current
4.1 Studying Atoms
Thomson’s Model of the Atom
Thomson’s Experiments
In his experiments, Joseph John Thomson
used a sealed tube containing a very small
amount of gas.
Sealed tube
Glowing beam
filled with gas at
low pressure
Positive plate
Metal disk
Metal disk
Source of
electric current
Negative plate
Metal disk
Source of
electric current
4.1 Studying Atoms
Thomson’s Model of the Atom
When the current was turned on, the disks
became charged, and a glowing beam
appeared in the tube.
• hypothesized that the beam was a stream of
charged particles that interacted with the air in
the tube and caused the air to glow
• beam was repelled by the negatively charged
plate
• beam was attracted by the positively charged
plate
4.1 Studying Atoms
Thomson’s Model of the Atom
Evidence for Subatomic Particles
He concluded that the particles in the beam
had a negative charge
– they were attracted to the positive plate
– hypothesized that the particles came from
inside atoms because
• no matter what metal Thomson used for the disk,
the particles produced were identical
• the particles had about 1/2000 the mass of a
hydrogen atom, the lightest atom
4.1 Studying Atoms
Thomson’s Model of the Atom
Thomson’s Model
He revised Dalton’s model to account for
these subatomic particles
• atom has neither a positive nor a negative
charge, but there must always be some positive
charge in the atom
• atom is filled with a positively charged mass of
matter that has negative charges evenly
scattered throughout it.
4.1 Studying Atoms
In Thompson’s experiment, the beam
was attracted to the _____ plate.
1. Positive
2. Negative
4.1 Studying Atoms
Thomson’s Model of the Atom
Thomson’s model is called the
“plum pudding” model. Today, it
might be called the “chocolate
chip ice cream” model.
The chips represent negatively
charged particles, which are
spread evenly through a mass of
positively charged matter—the
vanilla ice cream.
4.1 Studying Atoms
Rutherford’s Atomic Theory
According to Rutherford’s model, all of an
atom’s positive charge is concentrated in its
nucleus.
4.1 Studying Atoms
Gold Foil Experiment
• http://www.youtube.com/watch?v=XBqHkr
af8iE
4.1 Studying Atoms
Rutherford’s Atomic Theory
Rutherford’s Hypothesis
designed an experiment to find out what happens to
alpha particles when they pass through a thin sheet
of gold.
•Alpha particles are fast-moving, positively charged
particles.
• He hypothesized that the mass and charge at any
location in the gold would be too small to change the
path of an alpha particle.
• He predicted that most particles would travel in a straight
path from their source to a screen that lit up when struck.
4.1 Studying Atoms
Rutherford’s Atomic Theory
The Gold Foil Experiment
Deflected
particle
Undeflected
particle
Alpha
particles
Gold atoms
Slit
Beam of alpha
particles
Alpha
particles
Screen
Source of
alpha particles
Nucleus
4.1 Studying Atoms
Rutherford’s Atomic Theory
Discovery of the Nucleus
alpha particles whose paths were
deflected must have come close to
another charged object. The closer
they came, the greater the deflection.
many alpha particles passed through
the gold without being deflected.
These particles did not pass close to a
charged object.
4.1 Studying Atoms
Rutherford’s Atomic Theory
Thomson’s model did not explain all of the
evidence from Rutherford's experiment.
Rutherford proposed a new model.
• The positive charge of an atom is not evenly
spread throughout the atom.
• Positive charge is concentrated in a very
small, central area.
• The nucleus of the atom is a dense,
positively charged mass located in the center
of the atom.
4.1 Studying Atoms
A fast-moving, positively charged
particle is called a(n) _____.
1.
2.
3.
4.
Nucleus
Atom
Alpha particle
Beam
4.1 Studying Atoms
Rutherford’s Atomic Theory
The Houston Astrodome
occupies more than nine
acres and seats 60,000
people. If the stadium
were a model for an atom,
a marble could represent
its nucleus.
The total volume of an
atom is about a trillion
(1012) times the volume of
its nucleus.
4.1 Studying Atoms
Assessment Questions
1. Dalton’s theory did not include which of the
following points?
a.
b.
c.
d.
All elements are composed of atoms.
Most of an atom’s mass is in its nucleus.
Compounds contain atoms of more than one element.
In a specific compound, atoms of different elements
always combine in the same way.
4.1 Studying Atoms
Assessment Questions
2. J. J. Thomson’s experiments provided the first
evidence of
a.
b.
c.
d.
atoms.
a nucleus.
subatomic particles.
elements.
4.1 Studying Atoms
Assessment Questions
1. The concept of an atom as a small particle of
matter that cannot be divided was proposed by
the ancient Greek philosopher, Democritus.
True
False