A Brief History of Atomic
Theory
This Section will focus on Scientists
who have had an impact on the
study of the atom.
Ready To Meet Them?
Democritus
470-380 B.C.
Democritus proposed that
matter cannot be broken
down indefinitely. At some
point you end up with a
piece that can’t be
divided. That smallest
piece he called an atom,
from the Greek word
atomos, which means
“indivisible”.
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Democritus’ Model
“Atomos”
ATOMOS was the word Democritus used the
point, or stage where matter cannot be broken
down any further. ATOMOS literally means
“indivisible”
John Dalton
John Dalton was a
British chemist.He was
the first modern
scientist to propose the
existence of atoms.He
described an atom as an
invisible indestructible,
solid sphere, like a
billiard ball.
1766 - 1844
Dalton’s Model
The “Indivisible Sphere”
Sir Joseph “J.J.”
Thomson
1856 - 1940
J.J. Thomson was a
British physicist who was
the first scientist to
propose the plum pudding
model. He was also the
first to propose the theory
of the negatively charged
electron. He was credited
for the discovery. He also
proposed the isotope.
Thomson “plum pudding” model
+
-
Ernest Rutherford
1871- 1937
Ernest Rutherford
experiments proved
that atoms are mostly
empty space
Discovered the nuclear,
which contains
positively charged
particles .Was the first
to suggest that
electrons circle the
dense nucleus.
Rutherford’s Model
Nucleus
It has a
+ charge
Electron Negative charge
Niels Bohr
Niels Bohr stated that
electrons move in
different orbits, or
energy levels, around the
nucleus like planets orbit
the sun. Each energy
level is located a specific
1885 - 1962
distance from the
nucleus and contains a
certain number of
electrons.
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Bohr Model
Nucleus
Energy
levels
Electronnegative charge
Current Model
This model is
based upon Bohr’s
model, except that
electrons orbit the
nucleus in random
patterns. The
region where these
particles are found
is referred to as
the electron cloud.
Electron Clouds
Nucleus
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