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Atomic Theory and Structure
what everything is made of
What is an atom?
• The smallest particle into which an element
can be divided and still be the same
– The smallest unit of of a substance
• And when I say small, I mean small
– A penny has 20,000,000,000,000,000,000,000
atoms (20 thousand billion billion)
If it’s so small, how was it
• Not long after Greece
finally defeated Persia,
a Greek philosopher
named Democritus did
some thinking
– If you cut something in
half, and then cut that
in half, and keep going,
you’ll end up with
something you can’t
Over 2000 years later
• John Dalton, a teacher,
wondered why
hydrogen and oxygen
always combine in the
same way to make
water (a compound)
– It must have something
to do with how single
atoms interact
So, what’s in an atom?
• Atoms have a nucleus
– Made of protons and
– The nucleus is very
small, dense, and
– Contains most of the
atom’s mass
• Outside the nucleus,
electrons zip around in
electron clouds
• Particles in the atomic
nucleus with a positive
(+) charge
• We use the number of
protons in an atom’s
nucleus to determine
what element it is
– Atomic number
• A particle in the
nucleus with NO
• Isotopes are atoms that
have the same number
of protons, but
different numbers of
• Mass Number is the
sum of protons and
neutrons in an atom
• Atomic Mass is the
average mass of an
element’s isotopes
• A particle outside the
nucleus with a negative (-)
– Found within electron clouds
• Very small
– 1,800 electrons are about the
size of one proton
• Their (-) charge cancels out
the proton’s (+) charge
• An atom with unequal
protons and electrons is
called an ion
– Either (+) or (-)
So what?