4.1 Studying Atoms
4-1 notes
• Make a chart like the one below
Scientist
Theory/discovery
Experiment/
evidence
Model
Democritus
x
x
Aristotle
x
x
Dalton
Thomson
Rutherford
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
I.
Studying Atoms
A. Ancient Greek Models of Atoms
1. Democritus
a. Created the term called atoms- all matter
consisted of extremely small particles that could
not be divided.
2. Aristotle
a. Stated that all substances were made of only
four elements—earth, air, fire, and water.
4.1 Studying Atoms
Aristotle did not think there was
a limit to the division of matter.
For many centuries, most people
accepted Aristotle’s views on
the structure of matter. By the
1800s, scientists had enough
experimental data to support an
atomic model.
4.1 Studying Atoms
B. Dalton’s Atomic Theory
1. Evidence for Atoms
a. Dalton proposed the theory that all matter is made up
of individual particles called atoms, which cannot be
divided.
b. Dalton measured masses of elements that combine
when compounds form and noticed the ratio of the
masses of the elements in each compound was always
the same. (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
2. Dalton’s Atomic Theory
a. All elements are composed of atoms.
b. All atoms of the same element have the same
mass, and atoms of different elements have
different masses.
c. Compounds contain atoms of more than one
element.
d. 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
Dalton’s Atomic Theory
A theory must explain the data from many
experiments. Because Dalton’s atomic theory
met that goal, the theory became widely
accepted.
Over time, scientists found that not all of
Dalton’s ideas about atoms were completely
correct. They revised the theory to take into
account new discoveries.
4.1 Studying Atoms
Thomson’s Model of the Atom
When some materials are rubbed, they gain
the ability to attract or repel other materials.
Such 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
C. Thomson’s Model of the Atom
1. Thomson’s Experiment- provided the first evidence that
atoms are made of even smaller particles.
a. He used a sealed tube containing a very small amount of
gas. When a current was turned on, the disks became
charged, and a glowing beam appeared in the tube.
1) He hypothesized that the beam was a stream of
charged particles that interacted with the air in the
tube and caused the air to glow.
2) He observed that the beam was repelled by the
negatively charged plate and attracted by the
positively charged plate.
4.1 Studying Atoms
2. Evidence for Subatomic Particles
a. Thomson concluded that the particles in the beam had a
negative charge because they were attracted to the
positive plate. He hypothesized that the particles came
from inside atoms.
1) no matter what metal Thomson used for the disk, the
particles produced were identical.
2) the particles had about 1/2000 the mass of a
hydrogen atom, the lightest atom.
4.1 Studying Atoms
3. Thomson’s Model
a. The atom has neither a positive nor a negative
charge, but there must always be some positive
charge in the atom.
b. The atom is filled with a positively charged mass of
matter that has negative charges evenly scattered
throughout it.
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
D. Rutherford’s Atomic Theory
1. Rutherford’s Hypothesis
a. Based on Thomson’s model, Rutherford hypothesized
that the mass and charge at any location in the gold
would be too small to change the path of an alpha
particle.
b. 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
2. Gold Foil Experimenta. Rutherford designed an experiment to find out what
happens to alpha particles (fast-moving, positively charged
particles) when they pass through a thin sheet of gold
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
Discovery of the Nucleus
The alpha particles whose paths were
deflected must have come close to
another charged object. The closer
they came, the greater the deflection.
However, many alpha particles passed
through the gold without being
deflected. These particles did not pass
close to a charged object.
4.1 Studying Atoms
3. Discovery of the Nucleus
a. Thomson’s model did not explain all of the evidence from
Rutherford's experiment. Rutherford proposed a new
model.
1) The positive charge of an atom is not evenly spread
throughout the atom.
2) Positive charge is concentrated in a very small, central
area.
3) The nucleus of the atom is a dense, positively charged
mass located in the center of the atom.
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
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.
ANS: B
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
2. J. J. Thomson’s experiments provided the first
evidence of
a.
b.
c.
d.
atoms.
a nucleus.
subatomic particles.
elements.
ANS: C
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
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
ANS:
T