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Atomic Theory
A Brief History
The Greek Model
Democritus concluded that matter couldn’t
be divided into smaller and smaller
pieces forever. Eventually the smallest
piece would be obtained. This piece
would be indivisible.
Democritus – 400 B.C.
The word atom
comes from
the Greek word
atomos meaning
indivisible or
uncuttable
Dalton Model – Early 1800’s
There are 4 basic ideas of
Dalton’s Atomic Theory
Dalton’s Theory
All elements are
composed of atoms that
cannot be divided.
Dalton’s Theory
All atoms of the same
element are exactly alike
and have the same mass.
Atoms of different elements
are different and have
different masses.
Dalton’s theory
An atom of one element
cannot be changed into an
atom of a different element.
Atoms cannot be created or
destroyed in any chemical
change, only rearranged.
Dalton’s Theory
Every compound is
composed of atoms of
different elements,
combined in a specific
ratio.
J. J. Thomson - 1897


Discovered that the atom is divisible and
negatively charged particles he called
rays.
Today they are known as electrons. He
also reasoned that positive charges
existed to make the atom neutral, but he
couldn’t find them.
Plum Pudding Model
The pudding represents the
sphere of positive electricity and
the bits of plum scattered in the
pudding are the electrons.
I like to call it the "blueberry muffin"
model. All those round little
blueberries surrounded by the
bread of the muffin. Ummmm, good.
Some butter on top of a muffin
hot from the oven and some nice,
COLD milk. Oh my.
Ernst Rutherford

Rutherford concluded that atoms were
mostly made of empty space and
proposed that the atom has a small,
dense, positively charged center called
the nucleus. He thought the electrons
were moving around the nucleus in the
empty space.
Gold Foil Experiment
Gold Foil Experiment

Rutherford’s research team aimed a
beam of positively charged particles at a
thin sheet of gold foil. They predicted
that, if Thomson’s model were correct,
the charged particles would pass right
through the foil in a straight line. The
gold atoms would not have enough
positive charge in any one region to
strongly repel the charged particles.
Gold Foil Experiment

Rutherford’s team observed that most of the
particles passed through the foil undisturbed,
as expected. But, to their surprise, a few
particles were deflected strongly. Since like
charges repel each other, Rutherford inferred
that an atom’s positive charge must be
clustered in a tiny region in its center, called
the nucleus. Any particle that was deflected
strongly had been repelled by a gold atom’s
nucleus.
Bohr Model
Niels Bohr – 1913
Showed that electrons
could only have specific
amounts of energy leading
them to move in certain orbits
Like layers of an onion.
Modern Day Model
Electrons do not have a specific path
around the nucleus and it’s impossible to
determine the exact location of an
electron. The probable location of an
electron is based on how much energy
the electron has.
Also called the Cloud Model
Modern Atomic Model
“According to the modern
atomic model, an atom has a
positively charged nucleus
surrounded by a large region in
which there are enough
electrons to make the atom
neutral.”