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Chemistry
Atomic Theory
Democritus
• Proposed that if you keep on
cutting an object in half
eventually you will have a
uncuttable particle. He called
this particle the “atom”
• Aristotle disagreed with this
theory, he felt you could never
end up with an indivisible
particle, however later he was
proved incorrect.
• The word atom comes from
the Greek word atomos,
meaning indivisible
• He also claimed that atoms
are always moving and they
form different materials by
joining together
John Dalton
• British chemist and school
teacher
• Results of experiments
demonstrated that
elements combine in
specific proportions
because they are made of
individual atoms.
• His theory stated…
• All substances are made of
atoms. Atoms are small
particles that cannot be
created, divided, or destroyed
• Atoms of the same element
are exactly alike
• Atoms join with other atoms
to make new substances
• The above theory was not
quite correct
Earnest Rutherford
• Student of Thompson
• Aimed a beam of positively charged particles
at a thin sheet of gold foil. He surrounded the
foil with with Zinc Sulfide a substance that
glowed when struck by particles.
• Rutherford thought that since the atoms were
soft blobs (plum putting model) it would
shoot straight through. But his data was a
little surprising
Rutherford’s Results
• Some particles shot straight through, while
others were deflected.
• He presented a new atomic theory (revised
Thompson)
• He proposed that in the center of the atom is a
small dense area which is positively charged
called the nucleus.
• From his results he calculated that the diameter
of the nucleus was 100,000 times smaller that an
atom of gold.
Niels Bohr
• Danish Scientist
• Suggested that electrons travel around the
nucleus in particular paths.
• These path’s are located in certain levels as
well as distances.
• Electrons can jump levels
• Good theory, however still needed to be
improved
The Modern Theory
• Many of the scientists that we discussed
contributed to the modern theory in one way,
shape or form. Allthough not one scientist
perfectly figured it out all helped to reach the
modern day theory .
• The modern day theory states that we have a
nucleus which holds P/N. The electrons are
held in shells that (electron cloud) that circle
the nucleus of an atom
Assignment
• Please write a 2 paragraph essay stating the
two scientists that you feel are the most
important in their contributions to the
modern day atomic theory (20 pts)