Chapter 4
The Atom – Part 1
In Search of Giants Part 1: Atoms and the Periodic Table
What is the atom?
• Sulfur
What is the atom?
• Lead
• An atom is the smallest particle of an
element that retains that element’s
properties.
• From the Greek “a-tomos” which means
“not divisible”.
• The concept of the atom was first
proposed in Greece over 5000 years ago.
Democritus
• First proposed the
concept that matter
was not continuous
but was instead made
up of tiny particles he
termed the “atoms”.
Aristotle
• Believed in the four
elements: Earth, Air, Fire
and Water.
• Proposed a fifth element
“ether”.
• Because of Aristotle’s
vast influence on later
generations his ideas
would persist until the
Renaissance.
(Philosophy vs. Science)
Sir Francis Bacon (1561-1626)
• Father of “Inductive
Reasoning”.
• In deductive reasoning,
a conclusion is reached
from general
statements, but in
inductive reasoning the
conclusion is reached
from specific examples.
• “Knowledge is Power”
Robert Boyle (1627 – 1691)
•Robert Boyle is generally regarded as the first modern chemist
Antoine Lavoisier (1743 – 1794)
• Father of Modern
Chemistry.
• Law of Conservation of
Mass.
• Abolished the Phlogiston
Theory.
• Recognized and named
oxygen and hydrogen.
• Introduced the metric
system.
• Wrote the first extensive
list of elements.
Antoine Lavoisier (1743 – 1794)
• Declared a traitor during
“The Reign of Terror”.
• “It took them only an
instant to cut off his head,
but France may not
produce another like it in
a century” (LaGrange).
John Dalton (1766 – 1844)
• Proposed the first
scientifically supported
atomic theory.
Dalton’s Model of the Atom
• Dalton's model was that
the atoms were tiny,
indivisible, indestructible
particles and that each
one had a certain mass,
size, and chemical
behavior that was
determined by what kind
of element they were.
• What made us change
Dalton’s Atomic Model?
Cathode Ray Tube
• Physicists in the
19th century found
out that if they
constructed a glass
tube with wires
inserted in both
ends, and pumped
out as much of the
air as they could, an
electric charge
passed across the
tube from the wires
would create a
fluorescent glow.
Cathode Ray Tubes
Scientific Debate
• William Crookes discovered that a tube coated in a
fluorescing material at the positive end, would produce a
focused ‘dot’ when cathode rays hit it.
• With more experimentation, researchers found that the
cathode rays moved with the properties of waves.
However, other researchers argued that the focused nature
of the cathode ray beam meant that they had to be made up
of particles.
Scientific Debate
• Physicists knew that the ray carried a negative charge but
were not sure whether the charge could be separated from
the ray. They debated whether the rays were waves or
particles, as they seemed to exhibit some of the properties
of both. In response, J. J. Thomson constructed some
unique experiments to find a definitive and comprehensive
answer about the nature of cathode rays.
• Wave – Particle Duality is a concept that will be discussed
at a later time.
THOMSON’S FIRST CATHODE
RAY EXPERIMENT
• Thomson built a cathode ray tube with an electrometer, a
device for catching and measuring electrical charge.
electrometer
THOMSON’S FIRST CATHODE
RAY EXPERIMENT
• Thomson wanted to see if, by bending the rays with a magnet,
he could separate the charge from the rays.
• He found that when the rays were bent toward the electrometer
that it registered a large amount of negative charge and that
when they were bent away the electrometer did not register
much electric charge.
• This showed that the negative charge and the cathode rays
must somehow be “stuck together”.
electrometer
THOMSON’S FIRST CATHODE
RAY EXPERIMENT
• Thomson’s first experiment
therefore allowed him to
“speculate” that the cathode rays
were made of negatively charged
particles.
THOMSON’S SECOND
CATHODE RAY EXPERIMENT
• Thomson developed the second stage of the
experiment, to prove that the rays carried a
negative charge.
• To prove this hypothesis, he attempted to deflect
the cathode rays with an electric field. Earlier
experiments had failed to back this up, but
Thomson thought that the vacuum in the tube was
not good enough, and found ways to improve the
quality of the vacuum.
THOMSON’S SECOND
CATHODE RAY EXPERIMENT
anode
cathode
• Thomson's second experiment proving cathode
rays had electric charge.
• Cathode rays have charge: they are attracted
toward the positive plate D and repelled by the
negative plate E.
THOMSON’S SECOND
CATHODE RAY EXPERIMENT
• Thomson concluded from his first two
experiments: "I can see no escape from the
conclusion that [cathode rays] are charges of
negative electricity carried by particles of matter."
• By deflecting the cathode rays with the electric
charge, Thomson’s second experiment proved,
that the cathode rays were made up of negatively
charged particles.
THOMSON’S SECOND
CATHODE RAY EXPERIMENT
• This result was a major discovery in itself, but
Thomson resolved to understand more about the
nature of these particles.
• Thomson asked: "What are these particles? Are
they atoms, or molecules, or matter in a still finer
state of subdivision?“
• He then moved on to a third experiment.
THOMSON’S THIRD CATHODE
RAY EXPERIMENT
• Thomson's third experiment sought to determine
the basic properties of the particles.
• He could measure how much the rays were bent
by a magnetic field, and how much energy they
carried.
• From this data he could calculate the ratio of the
mass of a particle to its electric charge (m/e).
THOMSON’S THIRD CATHODE
RAY EXPERIMENT
• Thomson found out that the mass to charge ratio
was so small that the particles had to be nearly 2000
times smaller than a hydrogen atom.
Do you understand the significance of this?
THOMSON’S THIRD CATHODE
RAY EXPERIMENT
• Thomson speculated that the cathode rays
were made of negatively charged particles that
must come from within the atoms. Since they
were much smaller than the atom itself.
• Therefore the atom was not the smallest
particle of matter and could be broken apart.
• He had discovered the first subatomic particle.
The Electron
THOMSON’S CATHODE RAY
EXPERIMENTS
• Thomson’s work with cathode rays show how
a series of experiments can gradually
uncover truths.
• Many great scientific discoveries involve
performing a series of interconnected
experiments, gradually accumulating data
and proving a hypothesis.
THOMSON’S CATHODE RAY
EXPERIMENTS
• Thomson received the Nobel prize for physics in 1906
for this work.
J. J. Thomson (1856 – 1940)
• Credited for the
discovery of the
electron.
• Invented the mass
spectrometer which
led to his discovery
of isotopes.
Thomson’s Model of the Atom
Thomson’s model of the atom stated that the atom is a
tiny, spherical and neutral body, which is made up of negatively
charged particles (electrons) in a positive mass which
neutralized the charge of the negative electrons.
This model is commonly referred to as the plum-pudding
model of the atom.
In Search of Giants Part 2: The Discovery of the Electron (2:50)
James Chadwick (1891 – 1974)
• Discovered the
neutron.
• Paved the way for
nuclear power and
the nuclear bomb.
Protons
• Somewhere between Thomson and
Chadwick, physicists realized that there are
positively charged particles in the atom
which we call 'protons'.
• The way this happened was a gradual
process, and that is why it is hard to say
exactly who discovered the proton, although
Ernest Rutherford, is often given credit.
Lord Ernest Rutherford (1871 – 1937)
• Discovered the
nucleus of the atom.
• Pioneered the orbital
theory (planetary
model) of the atom.
Rutherford’s Gold Foil Experiment
Rutherford’s Gold Foil Experiment
Rutherford’s Gold Foil Experiment
Rutherford’s Atomic Model
The “Planetary Model” of the Atom
• The nucleus is very small, dense, and positively
charged.
• Electrons surround the nucleus.
• Most of the atom is empty space
In Search of Giants Part 2: The Discovery of the Nucleus (3:20)