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Standard 5:
Atomic Theory
How it has
changed
over time
Democritus

Greek philosopher

Decided that all matter was made up of tiny
indivisible particles
Greek word for indivisible = “atomos”
First person to use the term atom
Idea faded as Aristotle’s ideas were more
popular (matter is divine, spiritual)



John Dalton

Scientist – performed experiments

After the renaissance, Democritus’ ideas of
atoms gained popularity
Dalton studied many ideas about atoms and
developed the first atomic theory

Dalton’s Atomic Theory
1.
2.
3.
4.
All matter is made of atoms.
Atoms are indivisible.
Atoms of the same element are identical,
but atoms of different elements are different.
Atoms of different elements unite to form
compounds.
J. J. Thomson

Discovered the electron while
studying cathode ray tubes.
Why does the
beam of electrons
move upward?
Downward?

He realized that if there was an
electron in the atom, there must be
a proton to cancel out the negative
charge.
J. J. Thomson’s Plum Pudding
Model

The atom is not a solid sphere, but has
electrons floating around in a positive
material.
Plum Pudding
Protons and Electrons
Proton = positive particle (+)
 Electron = negative particle (-)

# of Protons = # of Electrons
NEUTRAL atom

Atomic Number = # of protons in an atom
Rutherford’s
Gold Foil Experiment
Video:
Through the Wormhole: Episode 7
Rutherford’s Planetary Model
 Rutherford discovers the NUCLEUS
-- tiny, positive center of an atom
1.
2.
3.
Proton = positively charged particle
Electron = negatively charged particle
Neutron = particle with no charge
Electrons are found
orbiting the nucleus.
NUCLEUS
protons + neutrons
located inside
Calculating PEN numbers
Atomic Number
=
# of protons in the nucleus of an atom
# of protons = # of electrons
Element
Sulfur
S
Iron
Fe
Atomic #
# Protons
# Electrons
Calculating PEN numbers
Mass Number
# of protons
# of neutrons
in the nucleus of an atom

MUST round off the atomic mass found on
the periodic table  red number
Magnesium
Boron
Neon
Writing Symbols
Mass Number
Atomic Number
197
79
Au
X
16
8
O
Calculating PEN numbers
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Atomic # = PROTONS
protons = # of ELECTRONS
# of NEUTRONS = mass # – atomic #
P= ___
K
E= ___
N= ___
Bohr Models

Bohr model of the atom
 electrons travel in rings around the nucleus
288
NUCLEUS
Niels Bohr
1885 - 1962
Parts of a Bohr Model
Isotopes

Isotopes – atoms of the same element


same number of protons (same atomic number)
different number of neutrons (diff. mass number)
There are three isotopes of carbon:
Carbon – 12
12
6
Carbon – 13
C
6
Carbon – 14
C
C
6
P
P
P
E
E
E
N
N
N