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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
• Atoms join with other atoms to form new
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
Niels Bohr (1913)
• Proposed that electron moved around the
nucleus in energy levels (shells), but no
electrons between the energy level (think
• Electrons can jump from one level to
• 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