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History of Atoms
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History of Atoms
Democritus
Who first thought of the atom?
The Greek philosopher Democritus
(about 400 BC) – he thought if you
cut an object over and over and over
again, there would eventually be a
piece that you couldn’t cut anymore.
He called that piece an atom. In
Greek, atom means “not to be cut”
or “indivisible”.
Why is this a big deal?
Because …
Democritus by Agostino Carracci (15571602)
History of Atoms
Aristotle
Aristotle was another philosopher and he believed that
everything was made of four elements: wind, water, earth and
fire. His was the accepted explanation, because no one could see
atoms!
Water. Earth. Fire. Air. Long ago, the four nations lived together
in harmony. Then, everything changed when the Fire Nation
attacked …
History of Atoms
John Dalton
The Atomic Theory
John Dalton was an English Scientist.
In 1803 he proposed the atomic
theory that states:
-Atoms cannot be divided
-Atoms are invisible
-All elements are made of atoms
-Atoms of the same element are the
same
-Atoms of different elements are
different
-Molecules are formed by two or
more atoms joined together
Ideas about Atoms
Democritus’s Deductions
• Atoms are small solid
objects that cannot be
divided, created, or
destroyed
• Atoms are constantly
moving in empty space
• Different types of matter
are made of different types
of atoms.
• The properties of atoms
determine the properties of
matter.
Dalton’s Atomic Theory
• All matter is made of atoms
that cannot be divided,
created or destroyed.
• During a chemical reaction,
atoms of one element cannot
be converted into atoms of
another element.
• Atoms of one element are
identical to each other but
different from atoms of
another element.
• Atoms combine in specific
ratios.
History of Atoms
Joseph J. Thomson
What are atoms made of?
In 1897, a scientist named Joseph J.
Thomson was experimenting with
gases and electricity and discovered
that atoms have negatively and
positively charged particles. The
negatively charged particles are
called electrons and the positively
charged particles are called protons.
See book, page 84-85
History of Atoms
Ernest Rutherford
What else are atoms made of?
Ernest Rutherford discovered in
1908 that atoms are mostly made of
empty space! In the center of the
atom is a dense nucleus of protons
surrounded by electrons. In fact, if
the nucleus of an atom was the size
of a pea, the atom would be as big as
a football stadium! The electrons
would be flying around the outside
of the stadium, and the rest of the
atom would be empty space!
See page 86-88
Atomic Models
Thomson
“Plum pudding” model
Rutherford
History of Atoms
Niels Bohr
What do atoms look like?
In 1913, Niels Bohr proposed that
the atoms look like a solar system,
with the protons in the nucleus
acting like the sun, and the electrons
flying around the nucleus like planets
orbiting the sun. See page 89
Proton
Neutron
Electron
History of Atoms
Werner Heisenberg
What do atoms look like?
In 1926, Werner Heisenberg
proposed that electrons don’t really
fly all around the atom. He said that
electrons had certain spots they
floated around in called orbitals
(kind of like balloons).
History of Atoms
James Chadwick
The Last Piece
In 1932, James Chadwick discovered
that atoms also contain neutrons.
Neutrons do not have a charge, they
are “neutral”. The neutrons and the
protons make up the nucleus, or the
center of the atom.