Atomic Theory
Catalyst (5 min)
1. What does the atomic number of each atom
represent?
2. What does the mass number of each atom
represent?
3. How many protons and neutrons are in the
nuclei of the following elements?
a. phosphorus (P)
b. cadmium (Cd)
c. lead (Pb)
Let’s Take a Trip Through
Time!
``Nothing exists except
atoms and empty space;
everything else is opinion.''
Democritus
Democritus
460 – 370 B.C.
• the first to propose that
matter was not infinitely
divisible
• Believed that matter was
made up of tiny indivisible
particles called atomos
• Everything is composed of
small atoms moving in a
void
• Ideas rejected by leading
philosophers because they
did not believe in empty
space
First Concept of an Atom
John Dalton
1766-1844
• English School Teacher
• Introduced his ideas in
1803
• Used experiments that
studied the ratio that
elements combine in
chemical reactions to
transform Democritus’s
ideas on atoms into a
scientific theory
Dalton’s Atomic Theory
1. All elements are composed of tiny indivisible particles called
atoms
2. Atoms of the same element are identical. The atoms of any one
element are different from those of any other element.
3. Atoms of different elements can physically mix together or
chemically combine to form compounds
4. In a chemical reaction, atoms are separated, combined or
rerranged. Atoms of one element are never changed into
atoms of another element.
5. Atoms cannot be created, divided into smaller particles or
destroyed.
Dalton’s Model
But if the scientists couldn’t
SEE the atom, how did they
even know it existed?!
``To the electron:
may it never be of
any use!''
J.J. Thomson
J.J. Thomson
1856-1940
• Hypothesized that the
structure of an atom was like
‘plum pudding’
• Discovered the electron in
the Cathode Ray Experiment
• Discovered that this
subatomic particle has a
negative charge
• Discovered isotopes 1913
Cathode Ray Tube Experiment
• Identified the first subatomic particle, the electron
• Determined the charge to mass ratio of the electron
• Mass of the electron is MUCH less then the mass of a hydrogen
atom
• Meant there are particles smaller than atoms!
Video!
Cathode Ray
Experiment
Altering the gas in the tube and
the material used for the
cathode have no effect
Cathode Ray is deflected toward
the positively charged plate by an
electric field
the ray’s
negative particles
(electrons) are
found in all forms
of matter
Proves: the particles
Proves:
in the ray must
have a negative
charge
Plum Pudding Model
“Fullness of knowledge always means
some understanding of the depths of
our ignorance; and that is always
conducive to humility and reverence. “
Robert Millikan
Robert A. Millikan
1868-1953
• Conduced oil
drop experiment
to determine
the mass and
quantity of an
electrons charge
“You should never bet
against anything in
science at odds of more
than about 1012 to 1.”
Ernest Rutherford
Ernest Rutherford
1871-1937
• Student of J.J. Thomson
• Conducted the gold foil
experiment
• Discovered that an atom
consists of mostly empty space
through which the electrons
move
• Concluded there is a tiny dense
region called the nucleus which
is located in the center of the
atoms and contain all its mass
• Electrons are held in the atom
by attraction with positively
charged protons.
Gold Foil Experiment
• Shot alpha particles at a thin piece of gold foil
• Most of particles passed through foil which
indicated that atoms are composed of mostly
empty space
• Alpha particle sometimes bounced back which
indicated they were deflected from the nucleus
Video!
“If an atom were expanded to the size of
a cathedral, the nucleus would be about
the size of a fly - but a fly many
thousands of times heavier than the
cathedral.”
“This is almost as incredible as if
you fired a 15-inch shell at a
piece of tissue paper and it came
back and hit you”
Ernest Rutherford
Rutherford Model
But what keeps the
electrons from spiraling
into the nucleus?
``An expert is a person who
has made all the mistakes
that can be made in a very
narrow field.''
Niels Bohr
Niels Bohr
1885-1962
• Developed theory that
electrons orbited the
nucleus in neat, planet-like
orbits
• Planetary Model 1913
• Nucleus surrounded by
orbiting electrons at different
energy levels
• Electrons have definite orbits
• Utilized Planck’s Quantum
Energy theory
• Worked on the Manhattan
Project (US atomic bomb)
Bohr Model for Nitrogen
How can the protons bundle
inside the nucleus without
blowing the atom apart?
James Chadwick
1932
showed that
the nucleus
contained a
neutral particle
called the
neutron.
Quantum Model (learn next class)
Developed from wave equation by Erwin Schrodinger (1926)
Fill in the chart using your notes (5 min)
Name
Time
Frame
Key
Points
Model
Democritus
Dalton
Thomson
Rutherford
Bohr
Who Did It? (5 min)
Six scientists were sitting around a
table discussing their contributions to
Atomic Theory. Use the clues to
determine where they were sitting and
what contribution they made.
Subatomic Particles
Table Summary
In your notes fill in the table comparing the relative charge,
mass and location of each of the subatomic particles (3 min)
In your notes, make a timeline graph
showing the development of modern
atomic theory (5 min)
Atomic Theory
JJ
Thomson
Democratus
400 BC
1803
John
Dalton
1904
Niels
Bohr
1910
Ernest
Rutherford
1913
1926
Schroedinger
/ Heisenberg
You Do:
Complete the following book
problems on a separate sheet of
paper:
p.104 #1, 4
p.105 #13, 15
p.122 #39, 62
Closing
How did the concept of the
atom change over time and
what key experiments and
scientists led to our current
understanding of the atom?
Homework
1. Describe the location of the
electrons in Thompson’s ‘plum
pudding’ model of the atom
2. In the Rutherford atomic model,
which subatomic particles are
located in the nucleus?
3. Describe Thomson’s and Millikan’s
contributions to atomic theory.