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Section 3: Particles of Matter
• Objectives:
 identify atoms as the
smallest particles of an
 describe Dalton’s
theory of atoms
 identify chemical
bonds as the force
holding atoms together
in molecules
I. Early Ideas About Atoms
FYI: This coin was in use during Democritus’s time. (440 B.C.)
A. Democritus thought the smallest particle in an object like this
silver coin was called an atomos.
B. Dalton’s Ideas About Atoms
C. In 1897, British scientist J.J. Thomson discovered small partic
inside the atom.
Thomson’s cathode-Ray tube experiment
Discovered negatively charged particles called elelctrons
FYI: Thomson revised atomic theory to include discovery of electrons.
He know overall charge of atom was neutral, but didn’t know
Location positive charged particles. Proposed possible model
called Plum Pudding model.
D. 1909: Rutherford’s “Shooting Gallery” Experiment
Ernest Rutherford further tested Thomson’s Plum Pudding Model.
Send stream of radium at gold foil. Some particles were deflected.
Rutherford proposed positively charged region called nucleus.
Most of atom’s mass is located here. Proposed positively charged
that passed close by the nucleus were pushed away by positive
D. Bohr’s Atomic Model
1913 Niels Bohr, a Danish scientist working for Rutherford suggested
electrons travel around nucleus in definite paths. Paths were located
certain distances from nucleus. Electrons could jump from level to
E. Modern Atomic Theory: Electron Cloud Model
Austrian scientists Schrodinger and Heisenberg revised Bohr’s model.
Stated electrons do not travel in definite path as Bohr suggested.
exact path of moving electrons can not be predicted.
Where electrons are likely to be found is region called electron
Ideas About Atoms today
The pattern that forms when a beam of electrons is directed at a sample
of aluminum. Analysis shows atoms have a diameter of about
0.00000003 cm. Three hundred-millionths of a centimeter. It would take
50,000 aluminum atoms to equal thickness of sheet of foil.
II. Atoms and Molecules
A. force that holds atoms together is a chemical bond.
a. two or more atoms bonded together is a molecule.