TRANSMUTATION AND
DECAY SERIES
HOW URANIUM CHANGES TO LEAD
or
WHAT HAPPENS IN BETWEEN
Kevin Lavarnway
Schroon Lake Central School
START WITH A RADIOISOTOPE
• An isotope is an atom of the same element
with the same number of protons and a
different number of neutrons
• Some isotopes are naturally unstable and
spontaneously change to another isotope
of a different element
• This change from one element to another
is called TRANSMUTATION.
THIS IS NOT A CHEMICAL
CHANGE!
• Chemical reactions involve atoms
rearranging by breaking and forming
bonds involving electrons
• TRANSMUTATION involves changes in
the nucleus that change the actual identity
of the element
• These reactions are called NUCLEAR
because they involve the atom’s nucleus
NOTATION
Uranium – 238
238
mass number ( p + n )
U
92
atomic number ( # p )
TRANSMUTATION INVOLVES
THE RELEASE OF RADIATION
• A Radioisotope emits, or gives off,
radiation from its nucleus
• Each isotope emits a certain type of
radiation; it has a specific decay mode
• The radiation can be in the form of
particles or rays
ELECTROMAGNETIC SPECTRUM
LOWER ENERGY
HIGHER ENERGY
TYPES OF RADIATION
• Pure energy given off as rays is called
gamma radiation
• Radiation particles could be alpha or beta
particles
ALPHA PARTICLE
• The same as a helium nucleus
• 2 protons and 2 neutrons
4
He
2
mass of 4 amu
Has a 2+ charge
because of 2 protons
LOWEST PENETRATING POWER
BETA PARTICLE
• The same characteristics as an electron
(not from an energy level—emitted from the
nucleus)
0
essentially no mass
e
-1
charge of -1
HIGHER PENETRATING POWER
GAMMA RADIATION
• Pure electromagnetic energy with high
frequency and short wavelength
• No mass or charge associated with it
MOST PENETRATING POWER
PENETRATING POWER OF NUCLEAR
RADIATION
NUCLEAR TRANSMUTATION
(natural)
Original
Radioisotope
238
U
92
New
Isotope
234
Th
90
+
Radiation
4
+
He
2
A CLOSER LOOK…
U-238 has alpha decay
Total mass before and after must be “equal”
238
U
92
234
Th
90
4
+
He
2
Total of atomic numbers must also balance
How can you use decay mode
information?
Can you determine the new isotope if you
know the decay mode?
For example, Th-232 is an alpha emitter.
232
Th
90
?
+
4
He
2
STRATEGY…
*mass numbers must balance
232
=
228 + 4
Th
?
+
He
90
=
88
+ 2
*atomic numbers must balance
How do we know what the new isotope is?
CHECK THE PERIODIC TABLE!
The new isotope has a mass number of 228
and an atomic number of 88.
228
?
88 each element has a unique
atomic number…
The new isotope is Radium-228 which is a
beta emitter!
What happens to Ra-228?
228
Ra
88
0
?
+
e
-1
(beta particle)
Balance the mass numbers and the atomic
numbers…careful of that -1)
And the result of the decay is…
mass number stays same
atomic number increases by 1
228
Ac
89
Actinium a new element with a greater atomic #
This is a beta emitter as well! When will it stop???
…not until a stable isotope is formed!
The change from one radioisotope to
another in a specific sequence is called a
decay series
We will explore some transmutations that
are part of the uranium-238 decay series...
You will be given two radioisotopes and their
decay modes. Determine what the
product of each transmutation will be…
Start off examples are:
U-238 is an alpha emitter:
238
234
U
Th
92
90
Th-234 is a beta emitter:
234
234
Th
Pa
90
91
+
4
He
2
0
+
e
-1
Take your time and think it out…
1.
Write the symbol of the radioisotope in the proper
notation on the reactant side
2.
Write the notation for the radiation type emitted on the
product side
3.
Total up the mass numbers on top so the total mass is
balanced on each side
4.
Total up the atomic numbers on the bottom so they are
balanced as well
5.
Use Periodic Table to identify the element by its atomic
number
Some practice for you…
• You and a partner will receive a sheet of
paper with two sample transmutations--tear in half so you each have two reactions
• Complete each transmutation individually
and then compare the results with your
partner…
• Decide on the correct answers and we will
share them with the entire class by
completing the decay series…
U-238 decay series
U-238 Th-234 Pa-234 U-234
Th-230 Ra-226 Rn-222
Pb-214 Bi-214
Bi-210
Po-218
Po-214 Pb-210
Po-210 Pb-206 (stable)
One very special isotope…
U-238 Th-234 Pa-234 U-234
Th-230 Ra-226 Rn-222 Po-218
Pb-214 Bi-214 Po-214 Pb-210
Bi-210 Po-210 Pb-206 (stable)
Rn-222 is a gas!
Are all of the steps just alike?
It looks like all transmutations are identical
to every other when it is written out in a
straight line or sequence…
but some are alpha and some are beta
decays…
they are not all the same.
Is there a better way to show what is really
happening?
Let’s try an x-y axis to show it…
Follow the directions on the
hand-out to change the linear
picture into a 2-dimensional
display.
Once you figure it out, place
your Post-It note for your
radioisotope on the wall chart in
the right location.
Mass number
Use the results to get the range
of atomic numbers you need
and the range of mass numbers
you need to fit on your graph.
Atomic number
Now for some practice on your own
You have just worked with the decay series
for U-238 and discovered a sequence of
radioisotopes (including radon gas!)
U-235 is a different isotope of uranium. It
has its own decay series as it too
eventually changes to lead.
Give the U-235 decay series a try and
practice your skills…
good luck!