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Quiz
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Compare Aristotle’s and Democritus’s ideas
about Matter
Why was Aristotle held to be correct?
List 3 ideas from Dalton’s Atomic Theory
Who discovered electrons?
Draw a model of Thomson’s atom.
What allowed John Dalton to verify his atomic
theory?
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Why did the existence of electrons
account for the existence of protons?
Describe Rutherford’s experiment
thoroughly
Explain Rutherford’s model of the atom
What did Millikan discover?
What do opposite charges do to each
other?
What do like charges do to each other?
Chemistry February 26, 2007
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Review for Quiz on Factor Label
Quiz on Factor Label
Notes on History of Atomic Theory
HW: WS 4.1
Turn in Vocabulary in box now, please
Chemistry Unit 4:
Atomic Structure
Spring 2007
1st, 2nd, & 4th Periods
Answer the following questions in
your notes
 1)
Write your definition of an atom
 2) If you could see atoms, sketch (or
describe) a model of the atom
Chapter Objectives
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1. Discuss early theories about matter
2. Discuss the discovery of subatomic particles
3. Use terms such as mass number, atomic
number, atomic mass to describe elements
Early Theories About Matter
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Democritus (460-370 BC)
Greek Philosopher
Around 420 BC, he proposed that matter was
discontinuous
There was some point at which matter could
NOT be divided further
Early Theories About Matter
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Aristotle (384-322 BC)
Around 360 BC, he proposed matter was
continuous
Matter could be subdivided indefinitely without
ever reaching a limit
He believed there was no ultimate underlying
structure to matter
Early Theories About Matter
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Aristotle’s hypothesis was held to be
correct
In ancient Greece, hypotheses were not tested
Hypotheses were accepted or rejected based
on the reputation of the philosopher
Early Theories About Matter
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In the 1500s and 1600s, a basic change in the
way science was done occurred.
Hypotheses were now TESTED!
Development of the Modern Atomic Theory
• In 1782, a French chemist, Antoine
Lavoisier (1743-1794), made measurements
of chemical change in a sealed container.
• He observed that the mass of reactants in
the container before a chemical reaction
was equal to the mass of the products after
the reaction.
Development of the Modern Atomic Theory
• Lavoisier concluded that when a chemical
reaction occurs, mass is neither created nor
destroyed but only changed.
• Lavoisier’s conclusion became known as
the law of conservation of mass.
Development of the Modern Atomic Theory
Click box to view movie clip.
Development of the Modern Atomic Theory
• In 1799, another French chemist, Joseph Proust,
observed that the composition of water is always 11
percent hydrogen and 89 percent oxygen by mass.
• Regardless of the source of
the water, it always contains
these same percentages of
hydrogen and oxygen.
H
H
O
Development of the Modern Atomic Theory
• Proust studied many other compounds and
observed that the elements that composed the
compounds were always in a certain proportion
by mass. This principle is now referred to as the
law of definite proportions.
H
H
O
Dalton’s Atomic Theory
• John Dalton (17661844), an English
schoolteacher and
chemist, studied the
results of experiments
by Lavoisier, Proust,
and many other
scientists.
Dalton’s Atomic Theory
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John Dalton conducted various
experiments and formulated his atomic
theory. He found that:
1) All matter is composed of
extremely small particles
called atoms
Dalton’s Atomic Theory
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2) Atoms of a given element are
identical in size, mass, and other
properties; atoms of different elements
differ in size, mass, and other properties
3) Atoms cannot be subdivided, created, or
destroyed
Dalton’s Atomic Theory
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4) Atoms of different elements can
combine in simple, whole number
ratios to form chemical compounds
5) And finally, Dalton found that in chemical reactions,
atoms are combined, separated, or rearranged
Dalton’s Atomic Theory
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Dalton was able to develop his theory based on
the invention of the chemical balance
Although his theory has been modified
slightly to accommodate new discoveries,
Dalton’s theory was so insightful that it has
remained essentially intact up to the present
time.
Copy & Answer in Notes
Answer the following questions on a
separate sheet of paper
 1) Compare and contrast Democritus’s and
Aristotle’s early theories about the atom
 2) Which philosopher (Democritus or
Aristotle) was correct {Hint: Look at
Dalton’s Atomic Theory}
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Questions Continued
3) On what basis did Aristotle and
Democritus propose hypotheses about
the nature of matter?
 4) How did Dalton’s Atomic Theory
differ from Democritus’s or Aristotle’s
Theories?
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Homework
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Construct & complete a chart with the following
1) Individuals Who Proposed Various Atomic
Theories
2) Approximate Date Theory Proposed
3) Theory
4) Description of Experiment Used
5) Any discoveries made
6) Other important information
Discovery of the Electron
Because of Dalton’s atomic theory, most
scientists in the 1800s believed that the
atom was like a tiny solid ball that could
not be broken up into parts.
• In 1897, a British physicist, J.J.
Thomson, discovered that this solid-ball
model was not accurate.
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JJ Thomson’s Experiment
JJ Thomson (1856-1940)-British Physicist
 Credited with discovery of electrons in
1897
 Reasoned that negative particles resided in
the atom
 He called these particles electrons
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The Electron
• Because of Dalton’s atomic theory, most
scientists in the 1800s believed that the atom
was like a tiny solid ball that could not be
broken up into parts.
• In 1897, a British physicist, J.J. Thomson,
discovered that this solid-ball model was not
accurate.
• Thomson’s experiments used a vacuum tube.
The Electron
• A vacuum tube has had all
gases pumped out of it.
• At each end of the tube is a metal piece called
an electrode, which is connected through the
glass to a metal terminal outside the tube.
• These electrodes become electrically charged
when they are connected to a high-voltage
electrical source.
Cathode-Ray Tube
• When the electrodes
are charged, rays
travel in the tube
from the negative
electrode, which is • Because these rays
originate at the
the cathode, to the
cathode, they are
positive electrode,
called cathode rays.
the anode.
Video of Thomson’s Experiment
Click box to view movie clip.
Cathode-Ray Tube
• Thomson found that the rays bent toward a
positively charged plate and away from a
negatively charged plate.
• He knew that objects with like charges repel
each other, and objects with unlike charges
attract each other.
Cathode-Ray Tube
• Thomson concluded that cathode rays are
made up of invisible, negatively charged
particles referred to as electrons.
• These electrons had to come from the matter
(atoms) of the negative electrode.
Animation of Thomson’s
Experiment
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http://www.dlt.ncssm.edu/core/Chapter3Atomic_Str_%20Part1/Chapter3Animations/Canal_Ray-CRT.html
He reasoned that since atoms are
NEUTRAL, there must be some positive
charge to balance out this negative charge
 He proposed a dense cloud of positive
charge with the negative charge randomly
embedded within the cloud
 His model is called the “plum pudding
model”
 Similar to a pepperoni pizza model
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Thomson’s Plum Pudding
Model
The dense
cloud of
positive
charge
Electrons
Thomson’s Experiment
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1) What did he discover?
2) In what year did he make his discovery?
3) Adequately describe the experiment that he
used.
4) How did his discovery support or disprove
Dalton’s atomic theory?
Thomson’s Experiment
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5) Why were the electrons in the cathode tube
attracted to the positive plates?
6) Draw a model of what Thomson believed
the atom looked like.
Millikan’s Oil Drop Experiment
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Robert Millikan (1868-1953)
American scientist, University of Chicago
Discovered the charge on an electron using
the oil drop experiment
Animation of Millikan’s
Experiment
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http://www.dlt.ncssm.edu/core/Chapter3Atomic_Str_%20Part1/Chapter3Animations/OilDrop.html
Millikan’s Experiment
Liquid goes through an atomizer to
produce droplets
 These droplets enter a chamber
 The bottom of the chamber has a hole
in the center
 The droplets fall through the hole due
to gravity
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An electrical field is produced using
electricity
 This field offsets the effect of gravity
causing the droplets to be suspended in
mid air
 Through a series of known relationships
and mathematical equations, Millikan
calculates the charge of the electron in
Coulombs.
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Thomson’s and Millikan’s work allowed
for the mass of the electron to be
determined using known relationships
and equations
Discovery of Neutron
 James
Chadwick (1891-1974) is
credited with the discovery of the
neutron in 1932
Rutherford’s Model of the Atom
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Ernest Rutherford (1871-1937) from New
Zealand
Thomson’s atomic model had not yet been
tested due to the tiny size of the atom
Rutherford’s Model of the Atom
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Rutherford had been studying radioactivity
In 1911, he bombarded a thin sheet of metal
with alpha particles
 Alpha particles are positively charged
The Gold Foil Experiment
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http://micro.magnet.fsu.edu/electromag/java/r
utherford/
Video of Rutherford’s
Experiment
Click box to view movie clip.
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If Thomson’s “plum pudding model” was correct,
he expected the alpha particles to crash through
the gold foil (with only minor deflections)
He placed a screen around the foil. After the
alpha particles passed through the gold foil, they
would light up the screen. Most of the particles
should pass through unaffected
Rutherford’s Results
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Some of the alpha particles passed through
and hit the screen as predicted
BUT a high number of alpha particles were
deflected. This indicated that there must be
something in the center of the atom that
caused deflection
Rutherford’s Conclusion
 He
reasoned that a positive charge
center rested in the atom
 He called this center the nucleus
Thomson’s Movie
Video of Rutherford’s
Experiment
Click box to view movie clip.
Rutherford’s Experiment
Click box to view movie clip.
Neils Bohr’s Model
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His model is called the “solar system”model
The center of the atom contains protons and
neutrons
This area is collectively called the nucleus
The electrons orbit around the nucleus like the
planets do the sun
Question (copy and answer in
your notes)
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The planets orbit around the sun due to
gravity. Why do electrons orbit the nucleus?
The Three Subatomic Particles
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After various molecular models of the atoms had
been tested, it was determined that three
subatomic particles made up the atom
Protons
Neutrons
Electrons
Protons
Protons are found in the nucleus
 Protons have an actual charge of
+1.6 x 10-19 C and a relative charge of +1
 The actual mass of a proton is 1.67 x 10-24
g
 The relative mass of a proton is 1 atomic
mass unit (amu)
 The symbol is p+
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Neutrons
Neutrons are found in the nucleus
 Neutrons have an actual charge of 0 C and a
relative charge of 0
 The actual mass of a neutron is 1.67 x 10-24 g
 The relative mass of a neutron is 1 atomic
mass unit (amu)
 The symbol is n0
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Electrons
Electrons are found outside the nucleus
 Electrons have an actual charge of -1.6 x
10-19 C and a relative charge of -1
 The actual mass of an electron is 9.11 x
10-28 g
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Electrons
The relative mass of a electron is 1/1840
atomic mass unit (amu)
 The electron has a very small mass
 The symbol is e
Nuclear Forces
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Neutrons exert an attraction toward protons and
other neutrons
Overall the forces produce enough attraction to
keep protons relative close together in the nucleus
Construct and complete a chart with the
following information
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List the particles of the atoms
List the particles’ location in the atom
List the relative and actual charge of the particle
List the relative and actual mass of the particles
List the symbol of the particle
Atomic Number
Each element has a certain number of
protons in its nucleus
 The number of protons in the nucleus is
called the atomic number
 Each element has its own atomic number
because each element has its own, unique
number of protons
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An element has certain chemical properties
based on the number of protons in its
nucleus
 Elements are numbered in the periodic
table based on the atomic number
 Example: If an element has two protons,
what is its atomic number? What is the
identity of the element?
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On Your Own
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Which element has
a) 12 protons
b) 35 protons
c) 50 protons
d) 92 protons
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Since atoms are neutral, the number of protons
must equal the number of electrons
Example: If an element (atom) has 7 electrons,
how many protons does it have? What is the
elements identity?
On Your Own:
 Determine the number of protons in
the following atoms as well as each
atom’s identity
 a) 6 electrons
b) 14 electrons
 c) 72 electrons
d) 55 electrons
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Mass Number
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Mass Number: the number of protons and
neutrons in an atom added together
Mathematically
Mass Number = Protons + Neutrons
Example
 What
is the mass number of an
atom with 16 protons and 16
neutrons?
On Your Own
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Determine the Mass Number for the following
atoms
a) 17 protons and 18 neutrons
b) 11 protons and 12 neutrons
c) 1 proton and NO neutrons
d) 3 protons and 4 neutrons
Nuclide Symbols
Mass Numbers are written in the
upper left preceding the chemical
symbol
 Atomic Number is written directly
under the mass number
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Example: Write the correct nuclide
symbol for an element with 79 p+
and 118 n0
Mass
197
Element
Number
Symbol
79
Au
Atomic Number
Copy and Answer in Notes
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1. List the three subatomic particles, their relative
charge, relative mass, and location
2. What does atomic number mean?
3. What atom has an atomic number of 32?
4. Determine the number of protons and
electrons in
a. Ne
b. Poc. Pt
Review-Complete the chart
Element Atomic Mass p+
#
#
O
n0
e-
17
53
74
235
92
Isotopes
Atoms of the same elements have the
same number of protons
 HOWEVER there may be different
numbers of neutrons
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Isotopes
When an element’s atom has different
numbers of neutrons, it is said to have
isotopes
 Isotopes are common in nature
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Hydrogen’s Three Isotopes
Hydrogen has the following isotopes:
 Protium-a hydrogen atom with one
proton and NO neutrons
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Hydrogen’s Three Isotopes
Deuterium-a hydrogen atom with one
proton and only one neutron
 Tritium-a hydrogen atom with one
proton and two neutrons
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On Your Own
 Determine
the mass and atomic
numbers for each of hydrogen’s
isotopes. Put this information into
nuclide symbol form
Average Atomic Mass
Atomic mass is the mass of an atom
expressed in atomic mass units or amu
 The atomic mass unit is based in
relation to the standard of carbon-12
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Average Atomic Mass Continued
Carbon-12 has a mass of 12.000 00 amu
 If an atoms weighs half as much as
carbon-12, its atomic mass will be 6.000
00 amu
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If an atom weighs four times as much as
carbon-12, it will have a mass of 48.000
00 amu
 The atomic mass that is reported in the
periodic table is a weighted average based
on the relative abundance of each
element
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Review - Answer in Notes
1. What mass would an atom have that has
a mass equal to 1/12 that of carbon-12?
What element is this?
 2. Determine the mass each atom would
have if:
 a. It weighed 12 times as much as C-12
 b. It weighed 3/17 as much as C-12
 3. How do all isotopes differ from each
other?
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Relative abundance refers to how
commonly the isotope occurs in nature
 Certain isotopes will occur more
commonly than other isotopes
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To Determine Avg. Atomic Mass
1) First convert relative abundance (%) to
decimal equivalent
 2) Multiply mass (in amu) by decimal
equivalent
 3) Add the numbers together
 4) The sum (in amu) is the average atomic
mass
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Copy & Answer in Notes
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1. What is an isotope?
2. How is mass number different from atomic
number?
3. How is relative (%) abundance determined?
 For
example, an element has two
naturally occurring isotopes. One
isotope has a relative abundance of
19.91% and a mass of 10.012 amu.
A second isotope has a relative
abundance of 80.08% and a mass of
11.009 amu. Calculate the atomic
mass
Additional Example
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Calculate the average atomic masses for the
following:
Isotope:
Rel. Abund.
Rel. Mass
hydrogen-1 99.985%
1.007 825
hydrogen-2 0.015%
2.0140
Ans) 1.008 amu
Copy & Answer in Notes
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3. Determine Avg. Atomic Mass for oxygen:
Isotope
Rel. Abund.
Actual Mass
O-16
99.762
15.994 915
O-17
0.038
16.999 13
O-18
0.200
17.999 160
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Isotope
rel.
abund. (%)
C-12 98.90
1.10
U-235
0.720
U-238
99.280
O-16
99.762
O-17
0.038
O-18
0.200
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b.
13
c.
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d.
rel. mass
(in amu)
12.0
C13.003 355
235.043
238.050 78
15.994 915
16.999 131
17.999 160