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Atomic Theory
The History of the Atom
What do I need to know?

How did the following people impact the
development of the atomic model?
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Democritus
John Dalton
JJ Thomson
Ernest Rutherford
James Chadwick
Sir William Crookes
Niels Bohr
Democritus
A contemporary of
Aristotle around
400 BC.
The world was made
up of two things:
1. empty space
2. tiny particles
called “atomos”.
Isaac Newton and Robert Boyle
Lived during the late1600s.
Spoke out in support of Democritus.
No experimental evidence of this
belief.
John Dalton
 English
schoolmaster and
chemist
 Lived in the 1800s
 Called “The Father of the
Modern Atomic Theory”
because he offered the first
rational atomic theory
based on the scientific
evidence of the time.
John Dalton’s Atomic Theory
 Supported
Democritus’ idea of the atom.
 Was the first atomic theory based on
scientific evidence.
 Combined ideas of elements with that of
atoms
 Published in 1808
Scientific Evidence available to
Dalton 1
Antoine Lavoisier
 French Chemist
 Total mass before chemical reaction was
the same as the total mass after chemical
reaction.
 Mass
of reactants = Mass of products
Law of Conservation of Mass
 Antoine
 For
Lavoisier
a closed system, the total mass before
chemical reaction is equal to the total
mass after chemical reaction.
 Total mass of the universe is a constant
 Matter can be changed in many ways, but
it cannot be created or destroyed (for non
nuclear processes)
Scientific Evidence Available to
Dalton 2
Joseph Proust
French Chemist

Observed that specific substances
always contain elements in the same
ratio by mass.
Law of Definite Proportions (#3)
 Each
compound has a specific ratio of
elements
 It is a ratio by mass
 Water is always 8 grams of oxygen for
each gram of hydrogen
Law of Definite Proportions

Joseph Proust

Substances always contain elements in the
same ratio by mass.

Example: For NaCl, for a 100.0 g sample, it will
always have 39.3 g Na and 60.7 g Cl
 i.e 39.3 % Na and 60.7 % Cl
John Dalton
 Armed
with this experimental work, John
Dalton proposed his atomic theory
 He only had 2 pieces of experimental work
when he made his proposal although other
evidence shortly followed (within 3-4
years)
 He kept Democritus’ idea of particulate
matter
Dalton’s Atomic Theory (1808)
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An element is composed of extremely small, indivisible
particles called Atoms
All of the atoms of a given element have identical
properties, which differ from the properties of the other
elements.
Atoms cannot be created, destroyed, or transformed
into atoms of another element. Experiment 1 Law
of Conservation of Mass
Compounds are formed when atoms of different
elements combine with each other in small wholenumber ratios.
 Law of Multiple Proportions (Dalton proposal not
based on experiment but proven later to be true by
Amadeo Avagadro and J.L. Gay-Lussac)
The relative numbers and kinds of atoms are constant
in a given compound. Experiment 2  Law of
Definite Proportions
Law of Multiple Proportions
 CO
or CO2, but not CO1.8
Law of Multiple Proportions
 if
two elements form more than one
compound, the ratio of the second element
that combines with 1 gram of the first
element in each is a simple whole number.
What?
 Water
is 8 grams of oxygen per gram of
hydrogen.
 Hydrogen Peroxide is 16 grams of oxygen
per gram of hydrogen.
 16 to 8 is a 2 to 1 ratio
 True because you have to add a whole
atom, you can’t add a piece of an atom.
Subatomic Theory in the
1800s
Sir William Crookes
JJ Thomson
Millikan
Rutherford
Summary of Dalton’s Atomic
Theory
1.
2.
3.
4.
It was based on experiment
It explained the concepts of atoms,
elements, and compounds
It explained why there were differences
b/w the properties of the elements.
Included the three foundational laws of
chemistry (LCM, LDP, LMP)
Discovery of the Electron
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1897
It required to two separate experiments
by two different individuals:
JJ Thomson and Robert Millikan
JJ Thomson and the Cathode Ray
Tube (CRT)—1897
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JJ Thomson used the CRT to measure the
charge-to-mass ratio (charge : mass) of a
kg of electrons.
He won the 1906 Nobel Prize in Physics
for this work.
Dalton’s Atomic Theory
 All matter is made of tiny indivisible particles
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called atoms.
Atoms of the same element are identical,
those of different elements are different.
Atoms of different elements combine in
whole number ratios to form compounds
In Chemical reactions, atoms are rearranged,
combined or separated. No new atoms are
created or destroyed.
J. J. Thomson English physicist
1897
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Used the cathode ray tube
It is a vacuum tube - all the air has been
pumped out.
Couldn’t measure the charge directly so he
determined the ratio of charge to mass.
Thomson’s Experiment
CATHODE
Voltage source
-
ANODE
+
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
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+
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
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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
CRT Expt
Next, he placed an object in the path of the
CR.
Part of the beam
was blocked and…
Cathode (-)
This proved conclusively
that the CR was made of
particles.
Anode (+)
A shadow was created
on the anode.
CRT Expt
Finally he placed a paddle wheel in the path
of the CR—the paddle wheel spun.
Cathode (-)
Anode (+)
This proved that the particles
that made up the CR had mass.
CRT Expt Summary
Thomson showed:
1. The CR was charged
2. The CR had a negative charge
3. The CR was made of particles
4. The CR particles had mass.
** The deflection of the cathode rays gave evidence for the negatively
charged nature of electrons. **
What does it really look like?
CRT Expt Summary
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Thomson won the Nobel prize because he
measured the charge : mass ratio of a kg
of electrons.
He measured this to be 1.759 X 1011 C/kg
and the charge was negative (electrons
are negatively charged).
Contributions…
WHO?
Democritus
DID WHAT?
Proposed idea
of Atoms
Sir William
Crookes
Discovered
cathode ray
(charged particles)
J.J. Thomson Discovered Electron
charge to mass
ratio
PROBLEM?
What holds
atoms
together?
Unable to
determine
exact
charge.
Millikan / Thomson Result
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With the results of these 2 experiments, the
mass of the electron could then be calculated.
Thomson:
Millikan:
1.759 X 1011 C / kg
1.6 X 10-19 C / e-
Today, the electron is the standard unit of
negative charge
Thomson’s Model /
Plum Pudding
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Found the electron
Couldn’t find positive
Said the atom was like
plum pudding
Positive & negative
charges evenly spread
through out, with the
electrons able to be
removed
Rutherford’s experiment
(Aware of Thomson’s plum
pudding
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Used radioactivity
Alpha particles - positively charged pieces
given off by uranium
Shot them at gold foil which can be made
a few atoms thick
The modern view of the atom was
developed by Ernest Rutherford
(1871-1937).
He Expected
The alpha particles to pass through
without changing direction very much
Because…
 The positive charges were spread out
evenly. Alone they were not enough to
stop the alpha particles

Results of
foil
experiment
if Plum
Pudding
model had
been
correct.
What he expected
He thought the mass was evenly
distributed in the atom
Because, he thought
the mass was evenly
distributed in the atom
What he got
How he explained it
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Atom is mostly
empty
Small dense,
positive center
Alpha particles
are deflected by
it if they get
close enough
+
+
What Actually Happened
Rutherford’s experiment.
Plum Pudding Model must be
Incorrect
YOU MUST KNOW THIS…WRITE
THE FOLLOWING DOWN…
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Concluded:
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Atom is mostly empty space
Center (nucleus) Positive particles and most
of the atom’s mass will be found here.
Nucleus is DENSE
Structure of the Atom
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There are two regions
The nucleus
With protons and neutrons
Positive charge
Almost all the mass
Electron cloud- Most of the volume of an
atom
The region where the electron can be
found
Modern View
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The atom is mostly
empty space
Two regions
Nucleus- protons
and neutrons
Electron cloudregion where you
might find an
electron
Subatomic particles
Relative Actual
mass (g)
Name Symbol Charge mass
Electron
e-
-1
1/1840 9.11 x 10-28
Proton
p+
+1
1
1.67 x 10-24
Neutron
n0
0
1
1.67 x 10-24
Contributions…
WHO?
Millikan
Rutherford
DID WHAT?
Accurately determined
• Charge (-1)
• Mass of electron
(1/1840 the mass of
Hydrogen atom) Problem –
couldn’t account
for all of the mass.
Concluded
• Discovered Proton
• Nucleus is positive
• Nucleus is dense
• Atom mostly empty
space
Contributions…
WHO?
Neils Bohr
DID WHAT?
Electrons orbit nucleus
like planets “planetary
model”
Electrons travel in fixed
orbits
James Chadwick
(Rutherford’s lab assistant)
Discovered Neutron
click here for video on discovery
Atomic Number
 Moseley found that different elements
have different numbers of protons.
 In fact, he discovered that elements had
incrementally different numbers of
protons
 Called this number the Atomic Number
Discovery of the Proton

Atomic Number tells us:
1. Number of protons in an atom
2. Identity of the element (Au, Ag, etc)
Implications for Dalton’s
Atomic Theory
 The discovery of the proton, electron,
and the neutron was the first major
contradiction to Dalton’s Atomic Theory
 Dalton’s Atomic Theory postulated that
the atom was the smallest piece of
matter—we now know this not to be true.
Subatomic Particle
Summary (pg 111)
Particle
Actual Charge Relative Charge
Electron -1.9 X 10-19 C
Proton
Neutron
+1.9 X 10-19 C
0
-1
+1
0
Subatomic Particle Summary
* KNOW Relative Mass *
Particle
Actual Mass
Relative Mass
Electron 9.10939 X 10-31 kg
0
1.67262 X 10-27 kg
1
Neutron 1.67493 X 10-27 kg
1
Proton
Subatomic Particle Summary
SO WHAT DO I NEED TO KNOW?
Test on Tue. (10/7)
 You need to remember:
 --Who discovered what and how
(Thomson & Millikan, Crookes, Rutherford,
Bohr & Chadwick)
--The following in this table:
Particle
Electron
Proton
Neutron
Relative Mass
0
1
1
Relative Charge
-1
+1
0
Symbols
 Contain the symbol of the element, the
mass number and the atomic number
Mass
number
Atomic
number
X
Mass Number
 Number of protons +
number of Neutrons
Atomic Number
 The number of protons in the
nucleus of an atom.
Find the number of
electrons and protons
contain in an atom of:
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A.
B.
C.
D.
E.
F.
33 p+
Arsenic
Gold
79 p+
Fluorine
9 p+
Molybdenum 42 p+
Polonium 84 p+
Barium
56 p+
33 e-
79 e9 e42 e84 e56 e-
Identify the atom containing
the following number:
 A.
 B.
 C.
 D.
 E.
34
5
31
61
94
Selenium
Boron
Gallium
Promethium
Plutonium
Identify the atom containing
the following number of
protons:
 A. 74 Tungsten
 B. 20 Calcium
 C. 49 Indium
 D. 70 Ytterbium
 E. 93 Neptunium
Nuclear Symbol and PEN
diagrams
P protons
E electrons
N neutrons
 PEN diagram is nothing more than a
diagram that tells how many protons,
electron, and neutrons are in an atom.
Nuclear Symbols and PEN
diagrams
 Mass Number (A): # protons + # neutrons
 Atomic Number (Z): # protons
Nuclear Symbol: symbol that designates
the specific atomic number and mass
number of a nuclide.
Nuclear Symbol and
names

37Cl
is the same as chlorine-37

15N
is the same as nitrogen-15
PEN Example
 Copper-65
 65Cu
P
E
N
29
29
36
A – Z = # neutrons
from PT
if neutral, same as Z
A-Z
Your turn – class work –
complete for homework
 Practice Atomic Structure Worksheet#1
and #2
Isotopes and Atomic Mass
While scientists were discovering the
subatomic particles, other scientists were
involved with isolating and characterizing
the naturally occurring elements.
 These scientists were having difficulty
understanding their mass measurements
especially considering the now widely
accepted fact of subatomic particles

Isotopes and Atomic Mass
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

Scientists determined that the atomic mass was
actually a weighted average of the naturally
occurring isotopes of an element.
Isotopes are atoms of the same element that
have different numbers of neutrons
This gives atoms of the same element different
masses and the measured atomic mass of an
element is then a weighted average of the
naturally occurring isotopes.
Isotopes and Atomic Mass

Mass Number: The number of protons
and neutrons in an isotope of an atom

Atomic Mass: Weighted average of all of
the naturally occurring isotopes of an
element.
What’s the standard for the mass of
an atom?
STANDARD IS THE CARBON – 12 ATOM
The Carbon-12 atom was assigned
a mass of exactly 12 atomic mass
units (amu)
ATOMIC MASS UNIT

amu = 1/12 of a carbon 12 atom
Although 1 amu is
approximately equal to
1 proton or 1 neutron there are differences
Subatomic Particles Masses
(p.119)
KNOW
Electron = 0.000549 = 0 amu
 Proton = 1.007276 = 1.00 amu
 Neutron = 1.008665 = 1.00 amu

Isotopes and Atomic Mass
What is a weighted average?
 Consider your grade:

40 % Daily
60 % Tests
Avg. = 80?
70
90
Isotopes and Atomic Mass

Not if it’s a weighted average. The test grade is
weighted more heavily so the average should
reflect this.
40 % Daily
60 % Tests
70 X .40 =
90 X .60 =
28
54
28 + 54 = 82
Avg. =
82
Atomic Mass
 Atomic
mass is a weighted
average.

To calculate the atomic mass - you need
the following to be given to you:
 relative
abundance of the isotopes
 mass of the isotope.
Steps to determine atomic mass:
Step 1: Convert the relative abundance (%)
into decimal format.
Step 2: Multiply the decimal from step 1 by the
mass of that isotope.
Step 3: Do this for each isotope.
Step 4: Add the results of all isotope calculations
This will provide you with the atomic mass.
Let’s take a look…
Atomic Mass Calculation
Isotope
Rel. Abundance
Mass (amu)
16O
18O
99.759 %
0.037 %
0.204 %
15.995 amu
16.995 amu
17.999 amu
At. Mass
=
17O
0.99759(15.995) +
0.00037(16.995) +
0.00204(17.999)
= 15.9995 = 16.00 amu
Bellringer (10/6/08)
Take out Atomic Mass Worksheet
On a separate sheet of paper answer the following:

1.
2.
An ion always contains:
A. equal number of p+ and eB. equal number of p+ and n0
C. Unequal number of p+ and eD. Unequal number of n0 and eWhat is the approximate mass of a neutron in amu’s?
Let’s Review from Friday…
Measuring Atomic Mass
Unit is the Atomic Mass Unit (amu)
 One twelfth the mass of a carbon-12 atom.
 Each isotope has its own atomic mass we
need the average from percent
abundance.

Naming Isotopes
Put the mass number after the name of
the element
 carbon- 12
 carbon -14
 uranium-235

How do isotopes Hydrogen – 1 and
Hydrogen -2 Differ ?
H – 2 has one neutron and H – 1 has no neutrons
Atomic Mass

are the decimal numbers on the periodic
table.

Is not a whole number because it is an
average.
Atomic Mass

Calculate the atomic mass of copper if
copper has two isotopes. 69.1% has a mass
of 62.93 amu and the rest has a mass of
64.93 amu. (.691) (62.93 amu) = 43.48
100% - 69.1% = 30.90 %
(.3090) (64.93 amu) = 20.06
43.48 + 20.06 = 63.54 amu
Check your work – what is the
atomic mass for Copper?
Atomic Mass

Magnesium has three isotopes. 78.99%
magnesium 24 with a mass of 23.9850 amu,
10.00% magnesium 25 with a mass of 24.9858
amu, and the rest magnesium 25 with a mass of
25.9826 amu. What is the atomic mass of
magnesium?

If not told otherwise, the mass of the isotope is the
mass number in amu
Need more Practice / information
see p. 119-121
See top of my web page for all
downloads for this unit.