Download Atomic Structure 3: ISOTOPES

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Period: _______
Atomic Structure 3: ISOTOPES
1. Introduction
By now you know that all atoms of an
element have the same number of protons,
as given by the atomic number. You also
know that the number of electrons in an
uncharged atom equals the number of
protons. The number of neutrons in the
nucleus of an atom is not so predictable.
The periodic table helps us out, though, by
giving the atomic mass of an element,
which lets you calculate the number of
neutrons. “How?” you ask? The number of
neutrons is just the atomic mass minus
the atomic number. Ta da!
Atomic
Number
Carbon-12 (12C)
Atomic Number
Symbol
Atomic Mass
Protons
CARBON
Neutrons
Electrons
Atomic
Mass
Find carbon on Side 1 of your periodic table, and complete the table and the element block above.
2. Numerous Neutrons
Now find carbon on Side 2 of your periodic table. Same atomic number, right? Same atomic mass, right? WRONG!
Side 2 gives the atomic mass of carbon as 12.011! What’s up with that?
(Technical detail for fussy people: Side 1 uses the old term “atomic weight”
first introduced by Dalton himself. It means the same thing as atomic
mass.)
Remember how I said all atoms of an element are the same? Well
… I lied. It IS true that all atoms of an element have exactly the
same number of protons. But, that AIN’T true about the number of
neutrons. Different atoms of the same element with different
numbers of neutrons are called isotopes — they have the same
atomic number and the same number of protons, but they have
different atomic masses and different numbers of neutrons.
EXAMPLE:
FOR ALL YOU WORD LOVERS:
“Isotope” comes from the Greek
words “iso” meaning “the
same” and “topos” meaning
“place”. All isotopes of an
element share the same place in
the periodic table.
A bonus point if you can name this
building …
•
100% of all carbon atoms have 6 protons no matter how many
neutrons they have. That’s it. Six. One more than five.
•
98.93% of naturally occurring carbon atoms have 6 neutrons to go with those 6 protons. This isotope of
carbon, with an atomic mass of 12, is called ... wait for it ... carbon-12! We also write it as “12C ”.
•
Since carbon-12 is by far the most
common isotope of carbon, it’s the only
one indicated on Side 1 of your periodic
table.
•
But wait – there’s more! 1.07% of all
carbon atoms have 6 protons and
7 neutrons and an atomic mass of 13.
This isotope is called “carbon-13”.
Complete the table and element block for
carbon-13.
Atomic
Number
Carbon-13 (13C)
Atomic Number
Symbol
Atomic Mass
Protons
Neutrons
Electrons
CARBON
Atomic
Mass
Side 2 of your periodic table gives the atomic mass of natural carbon. If you go out in your backyard and dig up a
shovelful of carbon, 98.93% of it will have an atomic mass of 12 and 1.07% will have an atomic mass of 13. On
average, the shovelful will have an atomic mass of 12.011.
Dr. V / Physical Science
AtomicStructure3.Isotopes.doc
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3 BONUS POINTS to anyone who can figure out precisely where the 12.011 (actually 12.0107) comes from! Show
your work here:
3. Radioactivity
At least one of you is thinking “What about carbon-14? I know I’ve heard
something about carbon-14.” Good boy, Sherman! About 1 trillionth of
naturally occurring carbon has 6 protons and 8 neutrons for an atomic
mass of ____. This isotope is special because it’s radioactive. It’s
radioactive because it’s unstable. It’s unstable because it has too many
neutrons for its own good. Since it’s unstable, it undergoes “radioactive
decay”.
So, Too many (or too few) neutrons leads to radioactive decay. Here’s how:
Once in a great while, a radioactive carbon-14 atom changes into an atom of
nitrogen-14, which is not radioactive (so it lasts forever). You put a lump of
14
C under your pillow, and you wake up with 14N instead; compliments of
the radioactive decay fairy. Complete the tables to figure out what happens
in that reaction:
Carbon-14 (14C)
Draw a circle around Sherman.
Nitrogen-14 (14N)
Atomic Number
Atomic Number
Atomic Mass
Atomic Mass
Protons
Protons
Neutrons
Neutrons
Electrons
Electrons
This is an example of a nuclear reaction not a chemical reaction. Something in the nucleus changed, and we
converted one element into another. (REMEMBER: The number of protons determines the identity of the atom!)
Which of these equations describes what happened in the carbon nucleus? Notation: p = proton, n = neutron,
e = electron, and “→” means “turns into”…



4.
p+n→e
p+e→n
n→p+e
HINT: Look at the tables and see what you lost and what you
gained, and then pick the equation that shows that …
Radiometric Dating
Some nuclear reactions occur quickly (like in nuclear bombs) and some occur slowly, like carbon-14 decay. The
“half-life” of carbon-14 is (roughly) 6000 years. That means it takes 6000 years for half of any given amount of
carbon-14 to be converted into nitrogen-14. In other words, if you put 10 grams of carbon-14 under your pillow
and wake up 6000 years later, you’ll have 5 grams left. The rest will have been turned into nitrogen-14 by the
radioactive decay fairy.
You’ve probably heard of carbon-14 because it’s used to determine the age of ancient plants and animals. If we
find a woolly mammoth carcass frozen in a glacier, we can tell how old the body is by measuring how much
carbon-14 is left in it. Fill out the table below to figure out how much carbon-14 would be left under your pillow
after 60,000 years:
AGE (YEARS)
14
C AMOUNT (grams)
30,000
0
10
36,000
6000
5
42,000
12,000
48,000
18,000
54,000
24,000
60,000
We can use other radioactive isotopes to date rocks that are billions of years old!
Dr. V / Physical Science
AtomicStructure3.Isotopes.doc
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