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NUCLEAR CHEMISTRY
Discovery of Radioactivity
Radiation - energy emitted in the form of
electromagnetic waves (i.e. light, microwaves, x-rays,
etc..)
• 1895 – Roentgen – discoverer of x-rays - Nobel Prize
1901. video clip
• Becquerel – followed-up on Roentgen’s work story
• Marie and Pierre Curie
• 1899 – Rutherford – alpha and beta rays
Radioactivity – spontaneous emission of particles and
energy from atomic nuclei.
ELECTROMAGNETIC SPECTRUM
LOWER ENERGY
HIGHER ENERGY
Ionizing and Non-ionizing radiation
Non-ionizing radiation (long-wavelength) Ionizing radiation (shorter wavelength)Both transfer energy to matter!
UV radiation damage
Isotopes
• Atoms of the same element that contain different numbers of neutrons.
• What chemical symbol do these atoms have? What is their atomic number?
What is the mass number of the atom on the left?
Isotope Tables
• http://en.wikipedia.org/wiki/Isotope_table_%
28divided%29
NOTATION
Uranium – 238
238
mass number ( p + n )
U
92
atomic number ( # p )
Nuclear Radiation
• It is a form of ionizing radiation that is caused
by changes in the nucleus of an atom.
– Remember:
unstable nucleus = radioactive atom
 change in identity of atom as the nucleus
breakdown (radioactive decay).
How is this dangerous?
- Unstable nuclei can emit high-speed particles
from the nucleus thus releasing energy (nuclear
radiation).
Stable vs. Unstable isotopes
• Radioactive Isotopes: unstable atoms, due to a nucleus with too
many or too few neutrons relative to the number of protons.
• No amount of neutrons can hold a nucleus together once it has
more that 82 protons. All of the elements with an atomic number
greater than 82 have only unstable isotopes.
• Unstable atoms emit energy in the form of radiation when they
break down (decay)
• Large nucleus (unstable)  nucleus + energy
• Reaction is EXTREMELY exothermic; called nuclear energy; mass is
lost in the process
Nuclear Radiation
Alpha radiation, α
Beta Radiation, β
Gamma Radiation, γ
low penetrating
power
Moderate penetrating
power
Very high penetrating
power
2+
-1
0
Ex: Radium-226
Ex: carbon-14
Cobalt -60
3 types of nuclear radiation:
Alpha & beta particles and gamma rays
 Charge of each
 Gamma rays – highest
energy – similar to x-rays
frequency.
 UVA, UVB, UVC rays from
the sun
 UVC (100-290 nm –
wavelength)
 UVA (320 -400nm –
wavelengths)
ALPHA PARTICLE
• The same as a helium nucleus
• 2 protons and 2 neutrons
4
mass of 4 amu
He
2
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 Emission
• gamma emission usually occurs in concert with other forms of decay that
produce nuclei in ‘nuclear excited states’
ignoring the alpha decay event,
gamma emission looks like this:
GAMMA RADIATION
• Pure electromagnetic energy with high
frequency and short wavelength
• No mass or charge associated with it
MOST PENETRATING POWER
Strength of alpha, beta, and gamma
radiation
PENETRATING POWER OF NUCLEAR
RADIATION
Background radiation
• Background radiation: a constant level of
radioactivity that results from natural sources and
from sources related to human activity.
o You can’t completely eliminate background radiation
it’s essentially everywhere: soil, water, food, rocks,
etc…
o Currently, 500 mrem (millirem) is the established US
background radiation limit for the general public
• EPA_Cosmic Radiation
– ~8% of our annual radiation exposure comes from outer space. The closer we get
to outer space the more we are exposed to cosmic radiation (such as during
flights).
• Full Body Scanners at Airports
Fyi’s…..
• BED (Banana Equivalent Dose) The equivalent dose for
365 bananas (one per day for a year) is 3.6
millirems(36 μSv).
• Bananas are radioactive enough to regularly cause
false alarms on radiation sensors used to detect
possible illegal smuggling of nuclear material at US
ports.
• Other foods that are above avg. for radioactivity in
food include: potatoes, kidney beans, nuts, and
sunflower seeds.
Ionizing radiation in you!
UNITS OF RADIATION DOSAGE
• GRAY= SI unit that is equivalent to the dose
absorbed: 1 gray = 1 Joule per 1 kilogram of
body tissue
• SIEVERT: SI unit that is used to express the
ability of radiation to cause ionization in human
tissue
• RAD: abosrbed dose similar to gray
• REM: absorbed dose similar to Sievert
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24
Radiation
Doses
Causing
Death
Effect of Ionizing Radiation
Ionizing radiation worksheet
•
•
Units
How much is safe?
– Look at 3 main factors:
•
•
•
•
1. Radiation Density – (how much per given volume)
2. Dose – (how much total received)
3. Energy associated with that type of radiation
Biological effects of ionizing radiation:
– Large doses (but less than 200 rems) – can causes changes in DNA or proteins in the body than
can result in mutations leading malfunctioning proteins that are unable to do their job in the
body, to cancer , or birth defects, etc..
–
•
•
•
Low level radiation – does not produce as unique effects or obvious mutations attributable to
the radiation exposure that is easy to distinguish from other injuries or illnesses – it can take
years for symptoms to even show (making it nearly impossible in some cases to attribute to
radiation damage)
Chemotherapy vs. radiation therapy
Radiation Risk from Medical Imaging
Cancer and Radiation
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
NUCLEAR TRANSMUTATION
(natural)
Original
Radioisotope
238
U
92
New
Isotope + Radiation
234
Th
90
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
228
Ac
atomic number increases by 1
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
4
He
2
Th-234 is a beta emitter:
234
234
Th
Pa +
90
91
0
e
-1
U-238 decay series
U-238
Th-234
Th-230
Pa-234
Ra-226
Pb-214
Bi-210
U-234
Rn-222 Po-218
Bi-214 Po-214
Po-210
Pb-210
Pb-206 (stable)
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
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42
One very special isotope…
U-238
Th-234 Pa-234
Th-230
Pb-214
Bi-210
U-234
Ra-226 Rn-222
Bi-214 Po-214
Po-218
Pb-210
Po-210 Pb-206 (stable)
Rn-222 is a gas!
Figure 6.34 – Page 626
Figure 6.35 – Page 626
Figure 6.36 – Page 627
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47
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
Transmutation
• 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 and can happen 2 ways:
1)by radioactive decay (natural) OR
2) when particles bombard the nucleus of the
atom. (nuclear reactor)
Figure 19.5: Representation of a
fission process.
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52
are a large fraction of potassium
atoms radioactive?
are a large fraction of tellurium
atoms radioactive?
no, a tiny fraction
is radioactive!
yes, more than 1/2
of tellurium atoms
in nature
are radioactive.
are a large fraction of uranium
atoms radioactive?
ALL uranium atoms
are radioactive!
Why is there still uranium on earth?
It takes a long time to decay.
(need to consider how long it takes for a
radionuclide to decay)
HALF LIVES
Half Lives
• Half-life - is the time required for one half of
original atoms to decay
• The half-life is unique to that isotope.
• Knowing the half-life of a radioisotope can
determine how harmful or useful that
substance can be.
Half-Life: A Radioactive Clock
the time during which half of a sample of radioactive atoms
will decay into their daughter atoms.
after one half-life 50% of the
radioisotopic nuclei will
have decayed into their
daughter nuclei
Common Radionuclide
Half-Lives
Common Radionuclide
Half-Lives
Parent Isotope Half-Life (yr)
Lead-210
22
Carbon-14
5730
Uranium-235
704 million
Daughter Product
Bismuth 210
Nitrogen-14
Lead-207
Potassium-40
Uranium-238
Thorium-232
1.25 billion
4.5 billion
14.0 billion
Argon-40
Lead-206
Lead-208
Rubidium-87
48.8 billion
Strontium-87
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62
Decay Chain of Uranium
Half-Life: A Radioactive Clock
Rutherford says: half-life is a time during which there
is a 50-50 chance a radioactive atom will
undergo nuclear decay
If the half life of a radioactive atom is a small amount of time
will it be more of less likely to decay than
a different radioactive atom with a long half life?
MORE LIKELY!
Radiometric Dating
Radiocarbon Dating
Why do living things maintain a constant concentration of
carbon-14? Shouldn’t it be consistently decaying in all material? Wh
Radiocarbon
Dating
Radiometric Dating
Example Calculations
• How long will it take for a sample containing 1
gram of radium 223 to reach a point where it
only contains 0.25 grams of radium 223 if a halflife is 12 days?
• Answer:
– 1 gram  0.5 grams = 12days
– 0.5 grams  0.25 grams = 12 days
– 12 days + 12 days = 24 days
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70
Storing High Level Radioactive Waste
• Store for 10 half lives!
• Store in special canisters.
• Store in special, secured
containment site
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71
Calculations with half-life
• A particular plant is 50,000 year old, how
many carbon-14 half-lives have occurred?
– A: 8.7 half-lives
• If the plant original had 30o counts of C-14
atoms, roughly how many radioactive C-14
atoms are left in the plant after 50,000 yrs?
– ~0.7 counts
Diagnostic & Therapeutic Uses
NUCLEAR CHEMISTRY IN MEDICINE
Nuclear Chemistry in Medical Field
•
•
•
•
X-rays
CT or CAT scan
MRI – no radiation
PET scan
• Radiation Therapy
Therapeutic
• http://www.cancer.gov/cancertopics/factshee
t/Therapy/radiation
NUCLEAR REACTORS
Fission vs. Fusion
• Fission: Splitting a heavy nucleus into two
nuclei with smaller mass numbers.
= Nuclear transmutation!
• Fusion: combining two light nuclei to form
a heavier nucleus.
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78
Nuclear Power Plant
http://www.nrc.gov/reactors/power.html