Class Opener: Friday, September 27
1) Create a drawing that illustrates Dalton’s view
of an atom.
2) Based on what you learned about atoms in
other science classes, create a diagram that
represents the structure of an atom.
3) Explain how your two drawings are different.
• Recognize that science is a progressive endeavor that
reevaluates and extends what is already known.
(SPI 3221. Inq.1)
• Identify the contributions of major atomic theorists:
Dalton, Thomson, Rutherford, and Bohr.
• Compare and contrast historical models of the atom.
Models of the Atom
• JJ Thomson – 1897
– discovered that within atoms there are
negatively charged particles
– called these particles electrons.
• Thomson performed experiments that involved passing
electric current through gases at low pressure.
• The result was a glowing beam called a cathode ray.
– A cathode ray is deflected by electrically charged
– Thomson concluded that a cathode ray is a
stream of electrons. Electrons are parts
of the atoms of all elements.
– from his findings Thomson developed the
“Plum pudding” model.
– the atom was filled with positively charged
material and the electrons were evenly
• Ernest Rutherford (1911)– Gold foil experiment
– directed a narrow beam of alpha particles at a very
thin sheet of gold foil.
– Alpha particles are positively charged
particles that are released by certain
radioactive elements as they decay.
Rutherford’s Gold-Foil Experiment
Alpha particles scatter from the gold foil.
• Alpha particles scatter from the gold foil.
The Rutherford Atomic Model
• Rutherford concluded that the atom is mostly empty
• All the positive charge and almost all of the mass are
concentrated in a small region in the center of the atom.
• He called this tiny core the nucleus.
• Niels Bohr (1913) – Planetary Model
– electron is found only in specific circular paths,
or orbits, around the nucleus.
• Sentence Starters
• After reflecting on
complete three of the
sentence starters in
your chemistry journal
entry for today.
– I’ve learned…
– I was surprised…
– I’m beginning to wonder…
– I would conclude…
– I now realize that…