Announcements
More and more people contacting me for help. Yay!
Principle of fairness: we can quiz you on stuff that’s on
the schedule, but not if we are way far behind it. Also,
questions will usually be simple when we haven’t
covered it in lecture or recitation.
I will post some new MC stuff very soon.
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Friday
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Today’s lecture is brought to you
by the numbers …
3&4
Russo & Silver Chapters #3 and #4.
Virtual Book Chapters #2 and #3
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Chapter 3 title: Evolution of Atomic
Theory
Chapter 4: Modern Atomic Theory
It is really about
Discreteness, subatomic particles & beginning
concepts of periodicity.
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The CEF guides also lay out the
facts, to wit:
Atoms exist.
Atom means un-cuttable or indestructible (which is a lie).
Atoms are incredibly small!
There are enormous numbers of them.
Composed of electrons, protons and neutrons.
Protons weigh 1836 times more than electrons.
Neutrons weigh about the same as protons.
Amazingly, all the mass is at the middle of the atom.
The atom’s behavior is dictated by the number of protons.
All atoms with the same number of protons behave the same,
even though they might have different numbers of neutrons.
If you lay out the elements according to mass (or number of
protons) their chemical properties repeat.
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This should all be supremely
unsatisfying to you:
1. How do we know?
2. Why should you care?
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Dr. Ivar Giaever…..1973 winner of the Nobel Prize in physics, announced his
resignation Tuesday, Sept. 13, 2011, from the premier physics society in disgust over its
officially stated policy that “global warming is occurring.”
The official position of the American Physical Society (APS) supports the theory that
man’s actions have inexorably led to the warming of the planet, through increased
emissions of carbon dioxide.
Giaever does not agree — and put it bluntly and succinctly in the subject line of his
email (to APS) , “I resign from APS,” Giaever wrote.
Giaever was cooled to the statement on warming theory by a line claiming that “the
evidence is incontrovertible.”
“In the APS it is ok to discuss whether the mass of the proton changes over time and
how a multi-universe behaves, but the evidence of global warming is
incontrovertible?” he wrote in an email to Kate Kirby, executive officer of the physics
society.
http://dailycaller.com/dc-trawler/#ixzz1Y2oB2T1L
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Funny he should mention protons. Just
recently there is news on protons: we
have a new radius estimate!
Old: rp = 0.8768(69) fm
New: rp = 0.8775(51) fm
rp
So…how many sig digs is that?
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Let’s calculate the density of a
proton. Assume m = 1.66 x 10-24
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Now we are going to dissect
some of the historical
observations that led us to
believe atoms are as we
depicted them in previous
lecture, which was advertised as
a “preview”. For example, how
do we know mass of proton?
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Chemistry is "discrete"; it
obeys integer arithmetic.
There is precisely ONE sodium atom per
ONE chlorine atom in salt.
There are TWO hydrogens per ONE oxygen
in water. Exactly.
WHY?
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But if there are so many atoms,
how can we count?
What beats counting?
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Chemists count by weighing. This
example shows polyethylene pellets.
0.0144 gram/pellet
5.94 grams of pellets
How many pellets?
Now, do you suppose all
polyethylene pellets have the
mass of exactly 0.0144 grams?
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http://www.bp.com/genericarticle.do?categoryId=9013612&contentId=7021458
Formation of HCl from H2 and Cl2
2 grams of H2 plus 71 grams of Cl2 = 73 grams of HCl
4 grams of H2 plus 142 grams of Cl2= 146 grams of HCl
etc.
It is always 35.5 g of Cl2 to 1 g of H2
Any other ratio results in "leftovers"
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Formation of HI from H2 and I2
2 grams of H2 plus 106 grams of I2 = 108 grams of HI
4 grams of H2 plus 212 grams of I2= 216 grams of HI
etc.
Any ratio mass other than 1:53 results in "leftovers"
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Some elements produced more than
one substance, like Carbon & Oxygen.
Substance 1:
12 g C + 16 g O
Ratio: 12/16 = 3/4
Substance 2:
12 g C + 32 g O
Ratio: 12/32 = 3/8
Substance 3:
12 g C + 48 g O
Ratio: 12/48 = 3/12 = 1/4
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Deduction, Hope and Guess
No compound was ever found
between C and O where the C/O
ratio was more than 3/4. So this
is probably CO.
Probably? What?!!?
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If the 4/3 compound is CO,
then….
The 3/8 compound is CO2
The 3/12 compound is CO3
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Apparently, THERE
ARE RULES…
for how atoms join. Not only is the idea
that just one atom could be put
together in multiple ways to get all the
materials wrong, so also is any notion
about mixing atoms in random ratios.
Matter is atomically discrete!
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There were LOTS of "relative mass" experiments.
Ones like these 3 helped decide the periodic table.
Oxygen Hydrogen
Water* Leftovers
100 g
8.2 g
73.2 g
35.1 g Oxygen
1g
4.1 g
1.13 g
3.98 g Hydrogen
65 g
8.2 g
73.2 g
None!
No leftovers
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…..which leads to Oxygen
being 8 times heavier than
Hydrogen…unless some new
H-O compound is
discovered!!!!
…….
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Hydrogen peroxide
In the words of a Chem1001 S07
student, Dalton eventually had to
learn that “water ain’t no ho”.
One of many uses: turning hair blonde.
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Example: Building the Mass Table
The very maximum of oxygen that will react with 1 g
of hydrogen, producing zero leftovers, is 16 g.
The very minimum of sulfur that will react with 1 g
of hydrogen, producing zero leftovers, is 32 g.
What does this say about the relative masses of
oxygen and sulfur?
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The Lightest
In looking at all the elements that could
react with 1 g of hydrogen, none was ever found
that took less than 1 gram.
Not only that, working through all other
combinations of elements, none lighter than
hydrogen was ever found.
So, hydrogen becomes a convenient basis for all
our masses.
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Dmitri Mendeleev noticed that, if you
put all the elements in order of their
masses, relative to hydrogen, periodic
trends appear…in how they react, how
easy it is to electrify them, etc.
Periodic table of the days
Mendeleev at
63 years of age
in 1897
http://en.wikipedia.org/wiki/Dmitri_Mendeleev
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Periodic Arrangement of Notes
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Periodic Table of Elements
http://periodic.lanl.gov/default.htm
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Spectroscopy (Emission spectroscopy shown)
provided further evidence for discreteness…but not
originally understood.
Discrete Lines!
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Cathode Ray Tubes were (still are!) one way
to deconstruct “indestructible” atoms.
All sorts of things once thought to be indestructible
have turned out to be quite delicate.
RMS_Titanic
http://en.wikipedia.org/wiki/RMS_Titanic
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Schematic of CRT
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What CRT experiments say:
You can see where the electrons are going,
how affected by magnet, etc.
•The negative stuff of this tube is similar
to the negative stuff of the gas discharge tube.
•This negative stuff behaves, in this experiment,
like a particle.
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Canal tubes were used to zero in
on the positive charges.
Canal Ray Tube
Accentuate the Positive!
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The point is: beating up on atoms. Knock the
stuffing out of them to see what happens,
just like a kid with a toy!
And what happens is:
•Some stuff goes to right collector (positively
charged stuff). This stuff depends on the
element is being deconstructed.
•Some stuff goes to left collector (negatively
charged stuff). This stuff does not depend on the
element being beat up.
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We don’t need no stinking canal
ray tube. Try a pickle.
http://www.youtube.com/watch?v=nNtvyI5BVEg
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Radioactive decay studies provided a mixture of positive,
negative & something new.
View Slide 3.6
CRT-like device for determining charge of radioactive decay products
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Here’s one of those gen ed lessons!
So the main point is that we are beating up on
atoms because we do not know what else to
do with them. This is a perfectly legitimate
take-home message for the whole course: when
you don’t know what to do, start anyway, either
boldly or carefully as the situation requires.
On the other hand, if you don’t know what to
do about your boyfriend, that doesn’t mean you should
start beating him!
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Because some of the laws of electricity
& magnetism were known by the time
of these experiments, scientists could
determine that….
charge/mass = e/m =
1.76 x 1011 coulombs per kilogram
Not very satisfying….what’s the electron mass, dammit?
And what is its charge?
Knowing the quotient is not enough!
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Robert Millikan’s oil drop experiment
was America’s first big success in physics.
http://www.trinity.edu/fwalmsle/Millikan/oildrop.gif
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Millikan oil drop demos.
http://www.youtube.com/watch?v=XMfYHag7Liw
http://www.youtube.com/watch?v=6ggP5W2xAP0
http://www.youtube.com/watch?v=3fNjWWew0k4
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What goes into Millikan’s calculations
Size of oil drops  mass of droplet
(from density = mass/volume, D=M/V)
Gravitational constant of Earth.
Electric field between top & bottom
plates of his device.
What comes out of Millikan’s calculation
The charge on oil droplets, one at a time!
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Frequency Distribution of Charge
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Suppose we instead gathered information on how
much money is in your wallet or purse.
Alicia: $25.05
Kevin: $17.02
Javoris: $31.53
Rachel: $5.84
Clay: $128.98
Lakiya: $64.02
What is the smallest unit of money?
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Suppose we instead listed scores on an exam.
Alicia: 39
Kevin: 69
Javoris: 75
Rachel: 54
Clay: 18
Lakiya: 93
How much is each problem worth?
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Millikan was really looking for the least common
denominator, that’s all. His LCD was the charge
on a single electron.
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With the charge determined, he could go on to
determine that electrons don’t weigh much!
e
11 Coul/kg (from charge/mass study)
1.76
x
10

m
and
e
So…
1.602 x 10-19 Coul (from Millikan)
e
m
 9.1 x 10-31 kg
e
 
m
= 9.1 x 10-28 g
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Oh, my goodness!
That’s lighter than my offensive playbook!
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Now go after the protons…
Mass spectroscopy—used in drug design today.
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Modern MS
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Proton is massive compared to electron!
•1836 times heavier than the electron.
• 1.673 x 10-27 kg/proton or 1.673 x 10-24 g/proton.
•Suppose we add the proton and electron mass:
•1.673 x 10-24 g + 9.1 x 10-28 g = 1.674 x 10-24 g.
•This is APPROXIMATELY the mass of most hydrogen atoms.
•We define the "atomic mass unit" or a.m.u. as 1.66 x 10-24 g.
•Compute the inverse of this number: 6.022 x 1023 hydrogen atoms/g.
•Hmm......keep that number in mind. It is called Avogadro's number
http://en.wikipedia.org/wiki/Proton-to-electron_mass_ratio
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Big Question: Where are the protons, relative to the
electrons: Two early models, only one is right.
Dark chocolate = protons
Cookie dough = electrons
http://www.elementsvillage.com/gallery/files/2/0/3/0/hershey-kiss-cookies_jpg_600x400_crop_q85.jpg
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The answer is: Hershey’s Kiss!
Rutherford proved his old boss Thompson was wrong. Here
is an example of a theory going down in flames.
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What it means…nuclear atoms,
more like hershey kiss cookie.
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The problem with protons....
....is that there aren't enough of them to
account for the actual mass of the atoms.
Rutherford actually thought that maybe
some protons might have captured an
electron and, therefore, not "come loose" in
the discharge chambers. About 1926 he
rather loosely suggested (at a Bakerian
Lecture---essentially a social function) the
concept of a "neutron".
http://upload.wikimedia.org/wikipedia/en/a/a3/TyndallLecturingAtTheRoyalInstitution,1870,LondonIllustratedNews.jpg
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Discovery of neutron took 6 years, and it
had important consequences:
N+P
It fixed the “weight” problem.
Some atoms have an extra neutron—
one of these is “skinny”—the other “fat”
The neutron proved to be a fabulous
destructor of atoms! Uh-oh.
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C
6
15
8O
P
2
1H
“Choose the form of the destructor.”
---Ghostbusters
James Chadwick, Rutherford’s best attack dog, worked almost a solid
fortnight to discover the neutron in 1932. Got Nobel prize in 1935.
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In December 1940 Franz Simon, who had been
commissioned by MAUD, reported that it was
possible to separate the isotope uranium-235.
Simon's report included cost estimates and technical
specifications for a large uranium enrichment plant.
James Chadwick later wrote that was when he
"realised that a nuclear bomb was not only
possible, it was inevitable. I had to then take
sleeping pills. It was the only remedy.“
Ref: Chadwick Wikipedia
MAUD = Military Application of Uranium Detonation
= British atomic bomb project
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When atoms are destroyed, mass gets converted to
energy. Lots and lots of energy…in a fraction of a
second….so that’s lots and lots of power.
E=
2
mc
2
mc 2
time
mc
P
time
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Next: where specifically are
those electrons in my Hershey
Kiss cookie?
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Stop here?
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Here’s what people said they learned.
.
Dalton was wrong!
“water ain’t no ho”
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Here’s what people said they learned.
.
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Here’s what people said they wanted to know
better.
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Rarely, people forget to tear off their “action item for
self study”; here is one such example.
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