Download Atomos: Not to Be Cut

Brief History of
Atomic Theory
1st atomic models
In 400 BC, the
model looked like a
solid indivisible ball
Atomos

Democritus’ theory:
Matter could not be
divided into smaller and
smaller pieces forever

He named the
smallest piece of
matter “atomos,” an
indivisible particle
This theory was ignored and
forgotten for more than 2000
years!
Why?
Aristotle & Plato thought earth, fire, air
and water were the elements.
Alchemy used this model for about
2000 years.
Dalton’s Model
In the early 1800s, the
English chemist
John Dalton
performed experiments
that confirmed the idea
of atoms.
Dalton’s Atomic Theory
All elements are made of
indivisible atoms
 Atoms of the same element
are identical, but differ from
atoms of other elements
 Different elements’ atoms can
combine to form compounds.
 Reactions occur when atoms
are separated, joined, or
rearranged.
WHICH OF THESE ARE NO
LONGER CONSIDERED TO
BE TRUE ?

Thomson’s Plum Pudding Model
In 1897, J.J.
Thomson
discovered that
atoms are made of
even smaller
particles.
Thomson Model
Thomson used
cathode ray tube
 The ray consisted of
negatively charged
“electrons”.

Thomson Model
He proposed a model of the
atom called the “Plum
Pudding” model.*
 Atoms were made from a
positively charged
substance with negatively
charged electrons scattered
about, like raisins in a
pudding.
 Aka: “raisin pudding model”

Millikan (1909)


OIL DROP
experiment
+
Measures charge on
electron
-
 Rutherford’s
gold foil experiment
(1911) Involved firing a stream of tiny
positively charged particles at a thin
sheet of gold foil (2000 atoms thick)
 Result: most of space occupied by
atom is empty space! Except for
small, dense, positively charged
NUCLEUS.
 Positive particles named PROTONS
all protons are in the nucleus.
Rutherford Nuclear Model

all of an atom’s
positively charged
particles are
contained in the
nucleus. The
negatively charged
particles were
scattered outside the
nucleus.
Bohr Model
 In
1913, the
Danish scientist
Niels Bohr
proposed that
each electron is in
a specific energy
level.
Bohr Model

According to
Bohr’s atomic
model, electrons
move in definite
orbits around the
nucleus, much like
planets circle the
sun.
Wave Model
Louis
DeBroglie
(1924)
e’s have
wavelike
properties
Wave Model
Erwin Schrodinger
(1927)
Quantum mechanics
model
e’s location
probability
Chadwick (1932)

Discovers particle with same mass as
proton but no charge …the
NEUTRON
Indivisible Electron
Greek
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Dalton
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Nucleus
Thomson
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Rutherford
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Bohr
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Wave
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Orbit
Electron
Cloud
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