Announcements

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Announcements
1 Assignment 2 is on the web. 2 In-class
problem today. 3 Diffusion lab this week.
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Models of the Atom
1 The Greek Model the atom is a small
indivisible particle like a solid sphere.
NEUTRAL PARTICLE
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2 The Jellium Model The atom is composed of a
jelly-like substance in which electrons embedded.
ELECTRONS (negative charge)
JELLIUM (positive charge)
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3 The Planetary Model
Gold atom
NUCLEUS (positive charge)
ELECTRONS (negative charge)
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A LITTLE ABOUT CHARGE
1 It comes it two types positive and
negative. 2 Charge is conserved. 3 Like
charges repel opposites attract. 4 It only
comes in discrete amounts (the amount that comes
with an electron).
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We know that atoms are neutral (atoms that carry
a net charge have some electrons missing and are
called ions.
Since the nucleus contains positive charges
called protons, there must be the same number of
protons in the nucleus as there are electrons in
the orbits.
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The Nucleus
The nucleus is composed of two types of objects
protons an neutrons.
The protons and neutrons are collectively called
nucleons.
The atomic number is just the number of protons
in the nucleus.
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Example Hydrogen (the most simple atom)
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The Helium Nucleus how many electrons?
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The Hydrogen nucleus requires one electron. But
what happens when something is added
This is an isotope of hydrogen called deuterium.
What element is this??
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Isotope atoms with the same number of protons
(but possibly a different number of neutrons) are
called isotopes. Isotopes have similar chemical
properties. For instance, you can make a form of
water called heavy water with deuterium (D2O).
(10 heavier)
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The nucleus gets unstable if there are too many
extra neutrons. The nuclei sometimes break down
because of this instability this is where
radioactivity comes from.
TRITIUM
Well study the nucleus later in the course!
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What atom does this represent?
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The Quantum Model of the Atom (Bohrs model)
The electrons can only occupy SPECIFIC orbits!
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They can CHANGE orbits (make a quantum leap) if
they are given the right push
A BIG push makes it jump to the next allowed
orbit.
Too small of a push does nothing!
Note we skipped the second orbit! Why?!
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The reason is that each orbit has a maximum
number of electrons that it can hold. This is
due to the Pauli exclusion principle.
1st orbit 2 electrons 2nd orbit 8 electrons
The electrons fill the orbits from the inside
out. See the lithium atom here.
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It is the outermost electrons that determine the
chemical properties. All elements with one
outer, or valence, electron have similar
properties.
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All atoms with one valence electron appear in the
same column on the periodic table.
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The atoms with complete outer shells (no valence
electrons) do not interact well with any other
elements. (the noble gasses).
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The Modern Quantum Description of the Atom
What does the atom really look like? Do the
electrons really orbit like the planets?
No. The negative charge the electrons carry
looks smeared outor like a cloud.
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Another possible formation (note this is ONE
electron)
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Other possible configurations (or symmetries)
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These symmetries are responsible for the symmetry
in the bonding of solids!
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Summary Atomic models
Greek Jellium Planetary Bohr (Quantum) Modern
Quantum
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Scientific Notation
How do we write 1,000,000,000,000,000,000 in a
more compact form?
1,000,000,000,000,000,000
There are 18 zeros
We write this as 1 x 1018
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How do we write 1,200,000,000,000,000,000?
Again there are 18 spots after the first number,
but we have to account for the 2.
Answer 1.2 x 1018
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What is (1.82 x 1012) x (3.87 x 109)?
Step 1 group the numbers as follows (1.82 x
3.87) x (1012 x 109)
Step 2 multiply these 7.04
Step 3 add the exponents of the 10s 21
Step 4 Write the result 7.04 x 1021
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What is (2.4 x 1020) / (1.6 x 1011)?
Step 1 (Group) (2.4/1.6) x (1020 / 1011)

Step 2 (Divide numbers) 1.5
Step 3 SUBTRACT exponents 9
Step 4 Write in scientific notation
1.5 x 109
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What is 0.0000000000005 in scientific notation?
There are 13 digits. Thus we can write this as
5.0 x 10-13
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What is (3.0 x 1012) x (2.0 x 10-4)?
6.0 x 108
What is (6.0 x 104) / (3.0 x 10-3)?
2.0 x 107
And now to use scientific notation
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Order of magnitude calculations (estimations)
An order of magnitude is a power of ten.
Lets estimate the height of a trillion dollar
bills stacked on top of one another.
1 We simply multiply the thickness of a single
dollar bill by a trillion. One
trillion1,000,000,000,000
1.0 x 1012
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How thick is a dollar bill?
A ream of paper is about 2 inches thick and it
has 500 pages. Assuming a dollar is about the
same thickness
thickness of a dollar (2.0 x 100) / ( 5.0 x 102)
0.4 x 10-2
4.0 x 10-3 (inches)
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Now multiply this by a trillion to get the answer
(1.0 x 1012) x (4.0 x 10-3)
4.0 x 109 inches
How tall is the stack in feet?
12 inches in a foot so divide by 1.2 x 101
(4.0 x 109) / (1.2 x 101)
3.3 x 108 feet
How many miles is this?
5,280 feet in a mile, so divide by 5.3 x 103
(3.3 x 108) / (5.3 x 103)
0.62 x 105 6.2 x 104 miles!!!
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IN-CLASS EXERCISE
Estimate the number of cigarettes that will be
smoked in the united states in a single week.
Use Scientific Notation Show all math
work Clearly state what each number is (e.g.,
number of smokers in U. S., etc.) This will be
collected at the end of class!
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Announcements
1 Assignment 2 is on the web. 2 In-class
problem today. 3 Diffusion lab this week.