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History of Atomic Theory
Original content from Dr. Stephen L. Cotton of Charles Page High School
Modified by John Flanagan
History of Atomic Theory
Original idea Ancient Greece
(400 B.C.)
 Democritus - Greek
philosopher
 Atomos – the smallest particle
of a substance

Another Greek
 Aristotle
- Famous philosopher
 All substances are made of 4
elements
 Fire, Air, Earth, and Water
 Blend these in different proportions to
get all substances
 Alchemist strive to convert lead to
gold
Who’s Next?
 Late
1700’s
John Dalton- England.
 Dalton’s Atomic Theory
 Combined ideas of
elements with that of
atoms.
Dalton’s Atomic Theory
 All matter is made of tiny indivisible
particles called atoms.
 Atoms of the same element are
identical, those of different atoms are
different.
 Atoms of different elements combine in
simple whole number ratios to form
compounds.
 Chemical reactions involve the
rearrangement of atoms. No new
atoms are created or destroyed.
History of Atomic Theory
J. J. Thomson English physicist.
1897
 Made a piece of
equipment called a
cathode ray tube.
 It is a vacuum tube
where all the air
has been pumped
out.

Thomson’s Experiment
Voltage source
-
+
Vacuum tube
Metal Disks
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 Passing
+
an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field
Thomson’s Experiment
Voltage source
+
 By
adding an electric field he found
that the moving pieces were negative
Plum Pudding Model
 A mass
of positive
matter
 Tiny negative
particles (electrons)
spread evenly
throughout
 Like a “plum
pudding”
Ernest Rutherford (1911)
Types of Radiation
Found three types
Alpha is positive.
Beta is negative.
Gamma is neutral.
Rutherford’s Gold Foil experiment
Rutherford - English physicist.
 Believed in the plum pudding model of
the atom
 Used an observational experimental
model to find out more about the atom
 Informal Hypothesis – What would
happen if particles were shot at a thin
sheet of atoms?

Rutherford’s experiment
Procedure
 Place a radioactive positive alpha
source in a lead box
 Alpha particles were shot at gold foil
which was only a few atoms thick.
 A fluorescent screen round everything
shows where the particles are deflected.

Lead
block
Alpha source
Fluorescent
Screen
Gold Foil
What he expected
Was for the alpha particles to pass through without
changing direction very much.
Because he thought
the mass was evenly
distributed in the atom
What he got
Data: 19 out of 20 went through – 1 in 20 bounced back
Conclusions:
Atom is mostly
empty space.
 Small dense,
positive piece
at center (Nucleus.)

+
+
Ernest Rutherford’s Model




Discovered dense
positive piece at
the center of the
atom- nucleus
Electrons would
surround it,
stationary
Mostly empty
space
“Nuclear model”
Niels Bohr’s Model
He had a question: Why don’t the
electrons fall into the nucleus?
 Move like planets around the sun.
 In circular orbits at different levels.
 Amounts of energy separate one
level from another.
 “Planetary model”

The Quantum Mechanical Model



The nucleus is deep
inside a blurry
“electron cloud.”
An area of highest
probability of finding
an electron.
Think of spokes of a
bicycle wheel
The Quantum Mechanical Model




Has energy levels for
electrons.
No circular orbits
Electrons move in “quantum leaps.”
It can only tell us the probability of finding
an electron a certain distance from the
nucleus.
Three primary subatomic particles
Name Symbol Charge
Relative
mass
Electron
e-
-1
Proton
p+
+1
1
Neutron
n0
0
1
~0 (1/1840)
Counting the Pieces
 Atomic
Number = number of
protons in the nucleus
 # of protons determines kind of
atom
 the same as the number of
electrons (IF it is a neutral atom.)
 Mass Number = the number of
protons + neutrons.
Isotopes
 Dalton
was wrong.
 Atoms of the same element can
have different numbers of neutrons.
 Therefore, they have different mass
numbers.
 These are called isotopes.
Naming Isotopes
 Since
atoms of the same element
can have different mass number,
we name an isotope with the
element and the mass number.
 Examples:
–carbon- 12
–carbon -14
–uranium-235
Isotopic Notation
 Isotopic
Notation is another way to
write an isotope combining the
symbol of the element, the mass
number and the atomic number.
Mass
number
Atomic
number
X
Isotopic Notation






19
What is the name of the isotope?
– Fluorine 19
What is the Atomic Number?
–9
What is the Mass Number?
– 19
What is the number of protons?
–9
What is the number of neutrons?
– 10
What is the number of electrons?
–9
9F
Isotopic Notation Carbon 14






What is the Atomic Number?
–6
What is the Mass Number?
– 14
What is the number of protons?
–6
What is the number of neutrons?
–8
What is the number of electrons?
–6
What is its isotopic notation?
– 146C
Isotopic Notation





A neutral atom has 33 protons and 43
neutrons.
What is the name of the isotope?
– Arsenic 76
What is its Atomic Number?
– 33
What is its Mass Number?
– 76
What is its isotopic notation?
– 7633As
Atomic Mass
 Atomic
Mass is the average mass
number of an element.
 Atoms of the same element can have
different mass numbers (isotopes.)
 Atomic mass is based on abundance of
each isotope in nature.
Atomic Mass
 Is
not a whole number because it is
an average.
 are the decimal numbers on the
periodic table.