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Chemistry
10/14 The History of the
Atom
ChemSaver p 17
• (400 B.C.) Democritus of Adbera
(northern Greece) asserted that all material things
are composed of extremely small irreducible
particles called atoms.
• The atomic theory was roundly rejected by
Aristotle, and, thus, by almost everybody else for
the next two millennia.
Democritus’ Atomic Model
Dalton’s Atomic Theory
(1808)
John Dalton
1. All matter is composed of
extremely small particles called atoms
2. Atoms of a given element are
identical in size, mass, and other
properties; atoms of different
elements differ in size, mass, and
other properties
Dalton’s Atomic Theory
Continued (1808)
John Dalton
3. Atoms cannot be subdivided,
created, or destroyed
4. Atoms of different elements
combine in simple whole-number ratios
to form chemical compounds
5. In chemical reactions, atoms are
combined, separated, or rearranged
Dalton’s Atomic Model
Cathode Ray Experiment
1. What will the particles in the tube do
when the current is turned on?
2. What evidence did this experiment
provide that electrons are negatively
charged?
3. How do you think this experiment would
be different if more powerful magnets
were used?
Click here for demonstration
Chem Saver
page 17
• JJ Thomson
• The Cathode ray tube experiment
showed:
• That atoms are divisible.
• That negatively charged particles can be
easily ejected from an atom.
• Thomson developed the “plum pudding
model” of the atom (chocolate chip cookie
dough from my point of view!)
JJ Thomson’s Atomic Model
Rutherford’s Gold Foil Experiment
1. Why did most of the alpha particles pass
through the gold foil undeflected?
2. Why were a few of the alpha particles
deflected at wide angles when they passed
through the gold foil?
3. Why did some alpha particles appear to
bounce off the gold foil at very large
angles, some even back toward their
source?
Click here for demonstration
Chem Saver
page 17
• Ernest Rutherford
• Since some alpha particles “bounced” back
Rutherford concluded:
• Atoms are mostly empty space (99+%)
• Atoms have a small, dense, and very
positive nuclei
• Most of the mass of an atom is in its
nucleus
Rutherford’s Atomic Model
The size of a nucleus to the electron cloud of an
atom is similar to the size of one blade of turf in
reliant stadium