Transcript September 20th, 2012

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September 20 , 2012
Do Now:

Clear Desk except for calculator/writing utensil
for Quiz
Do Now (Part 2)
Complete this problem in your notebook
:
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a)The radius of an atom of krypton is
about 1.9 Angstroms. Express this
distance in nanometers and in
picometers.
b) If the atom is assumed to be a
sphere, what is the volume in cubic cm
of a single Kr atom?
Which Is Which!?
Discuss with your neighbor, what question could we
try and answer today?
How does an atom of silver differ from
an atom of platinum?
Subatomic Composition!

Compare Platinum and Silver on the
periodic table. If you were missing the
element symbol, how might you determine
which is which?
Let's break it down...
Each element has a specific letter symbol...
Elements are symbolized by one or two letters.
Why are some elements symbols not the first letter of their name?
Atomic Number (z)
Why do all atoms of the same element have the same number of
protons and electrons?
According to Dalton's law of constant composition, do all atoms of
the same element have the same atomic number? Why?
Atomic mass number (A): the total number of
protons and neutrons in the nucleus of an atom.
(Total # of nucleons)
A=Z+n
Practice!
Fill in the following information for each of the
following elements.
1) Ca __#p
2) S __#p
3) Ar __#p
__#n
__#e
__#n
__#e
__#n
__#e
5) X
__#p
__#n
__#e
4) X
__#p
__#n
__#e
Predict why those two x's have
different mass numbers?
Atoms with identical atomic numbers but
different mass numbers are called
isotopes.
How does carbon-12 differ from carbon-14?
Isotopic atoms have the same number of
protons, but different number of
neutrons.
Let's take a look...
Practice!

Magnesium has three isotopes, with
mass numbers 24, 25, and 26.
a) write the complete chemical symbol
(subscript and superscript) of each.
 b) how many neutrons are in an atom of
each isotope?
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How many protons, neutrons, and
electrons are in an atom of
phosphorus-31?
So...
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So if isotopes exist, how did we get the
atomic mass for the periodic table?
Discuss with your neighbor.
A sample of an element in nature consists of a
mixture of its naturally occurring isotopes.
Atomic Mass (with isotopes)
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Atomic Mass of an element is a weighted
average of all isotopes, resulting in
fractional values.
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Mass spectrometry is used to find the
mass and percentage of each isotope.
Mass Specto...what?
Here is another example...
So how does it look?
Calculating Atomic Mass
Weighted Average
Atomic mass = (%A) (mass of isotope A)+ (%B) (mass of isotope B)
100
100
Example!

Naturally occurring chlorine is 75.78%
chlorine-35 (which has an atomic mass
of 34.969 amu), and 24.22% chlorine-37
(which has an atomic mass of 36.966
amu).
Calculate the average atomic mass
(atomic weight) of chlorine.
Practice!
1) Naturally occurring carbon is a mixture of two
isotopes, 12C (98.89%) and 13C (1.11 %).
Individual carbon atoms therefore have a mass of
either 12.000 or 13.003354 amu. What is the
atomic mass of a carbon sample.
2) Calculate the atomic mass for Ne with these isotope:
Mass #
Mass (amu)
%
20
19.992
90.51
21
20.994
0.27
22
21.991
9.22