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CHAPTER 10:
NUCLEAR CHEMISTRY
# Protons
Defines an Atom
•
Atomic number (Z) = # protons in the nucleus
•
Every atom of a given element has
the same
number of protons
in the nucleus.
•
Different
elements have
different
atomic numbers.
Mass Number (A) =
protons (Z)
+
neutrons
The mass number is NOT on the periodic table!
Always a whole number
Nuclear Symbol and
Hyphen Notation
are Interchangeable
Carbon-12
Hydrogen-2
ISOTOPES are Atoms of the Same Element That
Have Different # of Neutrons
Isotopes
of
elements have the
same # of protons
but different # of
neutrons.
Magnesium-24
Magnesium-25
Magnesium-26
An
isotope
of an element is
identified by the mass number
1
2
3
4
2
Radioisotopes Used in Medicine
A radioactive isotope, called a radioisotope, is unstable and spontaneously emits energy to forma more stable nucleus.
I
131 53 Used to
detect thyroid problems
Co
60 27Used in cancer therapy to
kill cancerous
tissue
P
32 15Used in leukemia
therapy
Cs
13755 Used to irradiate food to kill bacteria and other organisms
Radioactivity is the nuclear radiation emitted by
a radioactive isotope.
Radioactive decayis the process whereby an unstable radioactive element (radionuclide) is transformed into another element as a result of the
emission of radiation from its nucleus.
Types of Radioactive Decay:
Alpha particle decay Beta particle decay Gamma ray emission Positron emission Electron capture
Radioactive Materials Give Off Radiation
There are three different types of emissions from
naturally radioactive materials.
a
– Particle (Helium Nucleus)
An
a
particle has
2 protons
and
2 neutrons
with a +2 charge;
in other words, it is a
helium nucleus
.
2
4
a
2
4
He
or
b
Particle (High Energy Electron)
A bparticle has a charge and mass identical to an electron.
-
1
0
b
-
1
0
e
or
A βparticle is formed when a neutron(n) is
converted to a proton(p) and an electron(e).
1
0n 11p
neutron proton + −1e0
bparticle
g
Ray
A gray is a form of high-energy radiation without mass or charge.
0
0
g
g
or
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8
9
10
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The Effects of Radioactivity
•Radioactivity cannot be detected by the senses, yet it can have a powerful effect.•Nuclear radiation will damage or kill rapidly dividing cells such as bone marrow, skin, and the reproductive and intestinal systems.
•Cancer cells divide rapidly as well, making radiation an effective treatment for cancer.
•Food is irradiated, exposed to gamma radiation, to kill any living organism in the food.
•Afterwards, the food is not radioactive, and has a considerably longer shelf life.
A Nuclear Equation Can Be Written for the
Process of Radioactive Decay
originalnucleus nucleusnew + radiationemitted
The following must be equalon both sides of a nuclear equation :
•The sum of the mass numbers (A)
•The sum of the atomic numbers (Z)
Alpha Particle Decay
Alpha emission is the decay of a nucleus by emitting an a particle.
Example:
2 4
a
92 238
U
90 234
Th
+
How to Balance an Equation
for a Nuclear Reaction
Write a balanced nuclear equation showing howamericium-241 decays to form an a-particle.
Step [1] Write an incomplete equation with the
original nucleus on the left and the particle emitted on the right.
241
95Am 42He + ?
How to Balance an Equation
for a Nuclear Reaction
Write a balanced nuclear equation showing howamericium-241 decays to form an a-particle.
Step [2] Calculate the mass number and atomic
number of the newly formed nucleus on the right.
241
95Am 42He +
mass number
241 − 4 = 237 atomic number95 − 2 = 93 237 93
Step [3] Use the atomic number to identify the new nucleus and complete the equation.
Np
Beta Particle Decay
Beta emission is the decay of a nucleus by emitting a βparticle; 1 neutron is lost and 1 proton is gained.
Example:
-1 0
b
6 14
C
7 14
N
+
13
14
15
16
Gamma Ray Decay (Energy Only)
Gamma emission is the decay of a nucleus by emitting gradiation. Commonly,
g
emission accompaniesa
or βemission.Example:
2 4
a
88 226
Ra
86 222
Rn
+
0 0
g
+Gamma rays are often left out of the balanced nuclear equation because they don’t affect the balancing of the equation or the identity of the daughter nuclide.
2 4
a
88 226
Ra
86 222
Rn
+
Writing Balanced Nuclear Equations
Write the nuclear equation for the
beta decay of Co-60.
60 60 0
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Co
®
Ni +
28 -1e
The
half-life
(
t
1/2) of a radioactive isotope is the time it
takes for
one-half of the sample to decay
.
A decay curve shows the decay of radioactive atoms and the remaining
radioactive sample.
Half-Life Calculations
•In one half-life, 40 mg of a radioisotope decays to 20 mg.
•After two half-lives, 10 mg of radioisotope remains.
Initial 40 mg
20 mg
10 mg
How To Use a Half-Life to Determine the
Amount of Radioisotope Present
If the half-life of iodine-131 is 8.0 days, how much of a 100. mg sample remains after 32 days?
Step [1] Determine how many half-lives occur in the given amount of time.
32 days 1 half-life 8.0 days
x = 4.0 half-lives
How To Use a Half-Life to Determine the
Amount of Radioisotope Present
If the half-life of iodine-131 is 8.0 days, how much of a 100. mg sample remains after 32 days?
Step [2] For each half-life, multiply the initial massby one-half to obtain the final mass.
100. mg initial mass
x 12 x 12 x 12 x 12
The mass is halved four times.
= 6.25 mg final mass
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Using Half-Life to Determine the Amount of
Radioisotope Present
The half life of I-123 is 13 hours.
How much of a 64 mg sample of I-123 remains active after 26 hours?
1) 32 mg
2) 16 mg
3) 8 mg
Detecting and Measuring Radioactivity:
Geiger Counter
The amount of radioactivity in a sample is measured by the number of nuclei that decay per unit time—
disintegrations per second. A Geiger counter:
Detects beta and gamma radiation and uses ions produced by radiation to create an electrical current
Detecting and Measuring Radioactivity:
Film Badge
The darkening of the film badge indicates the extent
of radiation exposure. Different filters are present
in the badge, so various parts of the badge register
exposures to different types of radiation (alpha, beta, gamma, and X-rays)
Several units are used to measure the amount
of radiation
absorbed
by an organism.
•The rad—radiation absorbed dose—is the amount of radiation absorbed by one gram of a substance.
•The rem—radiation equivalent for man—is the amount of radiation that also factors in its energy and potential to damage tissue.
•1 rem of any type of radiation produces the
same amount of tissue damage.
Radiation Dose
The average radiation dose per year for a person is about 0.27 rem.
Generally, no detectable biological effects are noticed for a radiation dose less than 25 rem.
A single dose of 25–100 rem causes a temporary decrease in white blood cell count.
A dose of more than 100 rem causes radiation sickness—nausea, vomiting, fatigue, etc.
The LD50—the lethal dose that kills 50% of a
population—is 500 rem in humans, while 600 rem is fatal for an entire population.
Radioisotopes can be
injected
or
ingested
to determine if an
organ is functioning
properly
or to detect the
presence of a tumor
.
Technetium-99m is used to evaluate the gall bladder and bile ducts and to detect internal bleeding. Thallium-201 is used in stress tests to diagnose
coronary artery disease.
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26
27
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Which Cells are Sensitive to Radiation?
The cells in the body most sensitive to radiation are the ones undergoing rapid division: those of the bone marrow, skin, reproductive organs, andintestinal lining.
Cancer cells are also rapidly dividing cells and they are sensitive to radiation as well. Large doses of radiation are used to kill cancer cells.
I
131 53
Used to detect and kill thyroid cancer
Co
60 27
Used in cancer therapy to kill cancerous tissue
P
32 15
Used in leukemia therapy
Scans With Radioisotopes
Certain radioisotopes are known to concentrate in particular organs. For example, iodine is taken up by the thyroid gland. Iodine-131 can be given orally. Twenty-four hours later, the amount of radioactive iodine present in the thyroid is determined. On the scan, an area of decreased or increased radiation can indicate a disease, a tumor, a blood clot,
or edema.
Positron emission tomography
(PET) scans
use radioisotopes which emit positrons which
enable
scanning of an organ
.
PET scans can detect tumors, coronary artery disease, Alzheimer’s disease, and track the progress of cancer.
Shielding for Radiation Protection
Radiation protection requires
▪paper and clothing for alpha particles
▪a lab coat or gloves for beta particles
▪a lead shield or a thick concrete wall for gamma rays
▪limiting the amount of time spent near a radioactive source
▪increasing the distance from the source
Penetrating Ability of Radiation
Tissue depth 0.05 mm 4–5 mm 50 cm or more
Heavy clothing, lab coats, gloves Paper,
clothing thick concreteLead, Shielding
a
b
g
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32
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Radiation Exposure by Source
Radon-222
Optional: Nuclear Fission
Nuclear fission is the splitting apart of a heavy nucleus into lighter nuclei and neutrons. It can begin when a neutron bombards a uranium-235 nucleus:
235
92U + 10n 9136Kr + 14256Ba+ 301n •The bombarded U-235 nucleus splits apart into
krypton-91, barium-142, and three high-energy neutrons, while releasing a great deal of energy.
•The released neutrons can then bombard other uranium nuclei, creating a chain reaction.
•Critical mass: The minimum amount of U-235 needed to sustain a chain reaction.
Optional: Nuclear Fission
Nuclear fission is the splitting apart of a heavy nucleus into lighter nuclei and neutrons. It can begin when a neutron bombards a uranium-235 nucleus:
235
92U + 10n 9136Kr + 14256Ba+ 310n
Optional: Nuclear Power
•A nuclear power plant uses the large amount of energy released in fission.
•This energy is used to boil water and create steam, which turns a turbine and generates electricity. •The dangers of generating nuclear power are
possible radiation leaks and the disposal of
nuclear waste.
•Radiation leaks can be minimized by containment facilities within the power plant itself.
•Nuclear waste is currently buried, but it is unclear whether this is the best method.
Optional: Nuclear Fusion
Nuclear fusion is the joining together of two light nuclei to form a larger nucleus.
2 1H +
3
1H 42He 1 0n +
•A neutron and a large amount of energy are also produced.
Fusion is not currently useable as an energy source because it can only occur at extremely high temperatures and pressures.
•Hydrogen-2 (deuterium) and hydrogen-3 (tritium) undergo fusion to create a helium nucleus:
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