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Chapter 5: Atomic Theory
Mr. Albaugh
• Democritus (440 BCE)- realized that if you
continued to cut something, eventually you
would end up with something that couldn’t
be cut anymore, atomos – meaning not
able to divide
• Atoms are the smallest particle that an
element can be divided and still be the
same substance
• All matter is made of atoms
John Dalton (1803)
• realized that atoms combine in very specific
proportions (ratios) based on mass
• all substances are made of atoms and they can
not be created, divided or destroyed because
they were made of a single substance
• All atoms of the same element are exactly alike
and different from other elements, they are
unique
• Atoms join with other atoms to form new
substance
Dalton’s model of the atom (1805)
J. J. Thomson (1897)
• discovered that there were small particles
inside the atom, meaning that atoms can
be divided into smaller substances
• Electrons – negatively charged particles
attracted to positively charged particles
• Plum pudding model – electrons are mixed
throughout the atom, soft blobs of matter
J.J. Thomson model (1897)
Ernest Rutherford (1909)
• Discovered that an atom contains a
nucleus with positively charged particles
and that the electrons must be “floating”
around the nucleus
Rutherford’s Gold Foil Experiment
What happened in the Au atoms during the
experiment?
Niels Bohr (1913)
• Proposed that electron moved around the
nucleus in energy levels (shells), but no
electrons between the energy level (think
ladder)
• Electrons can jump from one level to
another
• Travel in a definite path
Bohr’s Planetary Model of the atom
Modern Atomic Theory
• Erwin Shroedinger & Werner Heisenberg
• Electrons have no predictable pattern and
move in a region where electrons are likely
to be found called the electron cloud
“Modern” atomic theory