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Topic 12Nuclear Chemistry
Courtesy I. Markic
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Nuclear Chemistry•The study of nuclear reactions and their uses in chemistry
•When nuclei change spontaneouslyspontaneously, emitting radiation, they are said to be radioactive
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Radioactive elements are used in medicine as…
i. diagnostic toolsii.treatment for canceriii.determine mechanisms for chemical
reactionsiv.trace movements of atoms in biological
systems
Radioactive elements are used in archeology…• To date important historical artifacts
Uses of nuclear reactions…i. generate electricityii.create WMDs of property and/or life
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Radioactivity•Radioactive nuclei are called radionuclides•Atoms containing radioactive nuclei are called radioisotopes
•Atoms with the same atomic number but different mass numbers are called isotopes
•Different isotopes of an element are distinguished by their mass numbers
•Different nuclei have different stabilities•The nuclear properties of an atom depend on the ratio of neutrons to protons in its nucleus
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Types of Radioactive Decay
•The most common types…
1. alpha decay α 2He4
2. beta decay β -1e0 or -1β 0
3. gamma radiation 00 or just
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+ Battery -
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Alpha Radiation
•α 2He4
•Contains 2 protons and 2 neutrons•Has heavy mass, does not travel far…|…stopped by a thin sheet of paper or skin
•Dangerous if ingested
• 92U238 90Th234 + 2He4 α
emission
• 92U238 + 2He4 94Pu242
α absorption
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Beta Radiation
• β -1e0 or -1 β 0
• Less charge than alpha particle• Extremely small mass• More penetrating, can pass through
paper… …stopped by aluminum foil or thin pieces of wood
• 6C14 7N14 + -1e0 β
emission• 19K42 + -1e0 18Ar42 β absorption
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Gamma Radiation• 00 or just • No mass, no charge• High energy photon• Often emitted with α or β particles
during decay• Can pass though paper, wood, and our
bodies• Can mostly be stopped by several meters
of concrete or several cm of lead•
90Th230 86Ra222 + 2 2He4 + 00
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Nuclear Transformations•There are ~115 different elements, but more than 1500 different nuclei are known (Wait a minute—how is this possible?)
•Only 264 nuclei are stable and do not decay over time
•Stability of a nucleus depends on it neutron-to-proton ratio
•The stable nuclei are in a region called the band of stability
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•As the at # increases, the 0n1:1p1 ratio of the stable nuclei increases
•The stable nuclei are located in the belt of stability (shaded )
•Radioactive nuclei occur outside this belt
2:1 neutron-to-proton ratio
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n/p too large
beta decay
X
n/p too small
positron decay or electron capture
Y
n/p too large => need more 1p1s &/or fewer 0n1s; so…turn 0n1s into 1p1s by…
n/p too small => need fewer 1p1s &/or more 0n1s; so…turn 1p1s into 0n1s by…
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16Nuclear Stability
• Certain numbers of 0n1s and 1p1s are extra stable•
0n1 or 1p1 count = 2, 8, 20, 50, 82, 126• Like extra stable numbers of -1e0s in noble
gases (2, 10, 18, 36, 54, 86)
• Nuclei with even numbers of both 1p1s and 0n1s are more stable than those with odd numbers
• All isotopes with at#s ≥ 84 (Po) are radioactive
• All isotopes of 43Tc and 61Pm are radioactive
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Transmutation Reactions•Transmutation - the conversion of an atom of one element into an atom of a different element
•Can occur by radioactive decay•Can occur when particles (1p1-s, 0n1-s, or 2He4-s) bombard the nucleus of an atom
• 7 N15 + 1H1 6C12 + 2He4
• 92U238 + 0n1 93Np239 + -1β0
• 4Be9 + 2He4 6C12 + 0n1
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Sample Exercise
•What product is formed when radium-226 undergoes alpha decay?
•What element undergoes alpha decay to form lead-208?
• 12Mg27 decays by beta emission to form…
• 1H3 __ + -1e0
• 3Li9 4Be9 + __
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Sample Exercise
• 6C14 -1e0 + ____
• 95Am241 2He4 + ____
• 7N16 6O16 + ____
•Alpha decay of francium-208…•Beta emission by argon-37…•Positron emission by fluorine-17…
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Sample Exercise
• 3Li6 + 0n1 2He4 + __
• 92U235 + 0n1 __ + 56Ba141 + 3 0n1
• 13Al27 + 2He4 __ + -1e0
• 92U235 38Sr90 + __ + 0n1 + 4 -1e0
• 83Bi214 2He4 + __
• 14Si27 -1e0 + __
• 29Cu66 30Zn66 + __
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Decay Series
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Half-Life (t½)
•The time required for one-half of the nuclei of a radioisotope sample to decay to products
•After each half-life, one-half of the atoms we had at the beginning of the half-life have decayed into atoms of a new element
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Fig. 23.3
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Decay of a 10.0 g Sample of Sr-90
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Decay of a 1.000 mg Sample of Mo-99
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Brigham Young Researcher Scott Woodward Taking a Bone Sample
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A Dendrochronologist Cutting a Section from a Dead Tree
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Sample Exercise
•Po-214 has a relatively short t½ of 164 sec. How many seconds would it take for 8.0 g of this isotope to decay to 0.25 g?
•How many days does it take for 16 g of Pa-107 to decay to 1.0g? t½ of Pa-107 is 17 days.
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Sample Exercise
•In 5.49 seconds, 1.20 g of Ar-35 decay to leave only 0.15 g. What is t½ of Ar-35?
•Na-24 has a t½ of 15 hours. How much Na-24 remains in an 18.0 g sample after 60 hours?
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•In 6.20 h, a 100. g sample of Ag-112 decays to 25.0 g. What is t½ of Ag-112?
•Mn-56 is a β emitter with t½ of 2.6 h. What is the mass of Mn-56 in a 1.0 mg sample of the isotope at the end of 10.4 h?
•C-14 emits beta radiation and decays with t½ of 5730 years. Assume you start with 2.00 x 10-12 g C-14.▫How long is 3 t½s?▫How many grams of the isotope remain at the end of 3 t½s?
Sample Exercise
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Nuclear Fission•Occurs when the nuclei of certain isotopes are bombarded with neutrons…
•…and the nucleus splits into smaller fragments
•In a chain reaction, some of the neutrons produced react with other fissionable atoms, producing more neutrons with will react with still more fissionable atoms
•Can release enormous amounts of energy (atomic bombs, nuclear reactors)
•Exothermic processes•Undergoes fission when struck by a slow-moving neutron
•U-235 or Pu-239
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Fig. 23.6
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Fission
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Uranium Oxide (refined uranium)
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A PHYSICIST WORKS WITH A SMALL CYCLOTRON AT THE UNIVERSITY OF CALIFORNIA BERKLEY
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An Aerial View of the Fermilab High Energy Particle Accelerator
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The Accelerator Tunnel at Fermilab
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Chain Reaction•If the fission process continues and the energy is unchecked, the result is a violent explosion
•The sample of fissionable material must have a minimum ‘critical mass’ – if not, 0n1-s escape from the sample before they strike another nucleus and cause additional fission
•The chain Rx stops if enough 0n1-s are lost
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Critical Mass
•The amount of fissionable material large enough to maintain the chain reaction with a constant rate of fission
•When critical mass is present, only one 0n1 from each fission is subsequently effective in producing another fission
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(a) (b)
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(a)
Subcritical Mass
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(a)
Subcritical Mass
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(a)
Subcritical Mass
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(a)
Subcritical Mass
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(a)
Subcritical Mass
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(a)
Subcritical Mass
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(a) (b)
Subcritical Mass Critical Mass
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(a) (b)
Subcritical Mass Critical Mass
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(a) (b)
Subcritical Mass Critical Mass
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(a) (b)
Subcritical Mass Critical Mass
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(a) (b)
Subcritical Mass Critical Mass
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Reactant Core
• Coolant fluid Na(l) or H2O
• ModeratorSlows 0n1s so they can be absorbed (D2O, graphite)
• Control Rods Decrease the # of slow moving 0n1s (B, Cd)
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To steam turbine
Pump
Shield
Water
Uranium fuel
Control rod
Steam
Shield
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Nuclear Waste• Fuel rods are one major source of waste• Fuel rods are made of fissionable isotope U-235 or Pu-239
• Isotope-depleted (spent) fuel rods are removed and replaced with new fuel rods
• All nuclear power plants have holding tanks, or “swimming pools,” for spent fuel rods
• Water cools the spent rods, and acts as a radiation shield to reduce the radiation levels (12 m deep)
• The rods continue to produce heat for years after their removal from the core
• Spent fuel rods spend a decade or more in a holding tank
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Annual Waste Production
35,000 tons SO2
4.5 x 106 tons CO2
1,000 MW coal-firedpower plant
3.5 x 106
ft3 ash
1,000 MW nuclearpower plant
70 ft3 vitrified waste
Nuclear Fission
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Fig. 23.p961bottom
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Nuclear Fission
Hazards of the
radioactivity in spent
fuel compared to that of
uranium ore
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Nuclear Fusion•Nuclei combine to produce a nucleus of greater mass
•Fusion Rxs release more energy than fission Rxs•Energy released by the sun comes from nuclear fission
•Only occurs at high temperatures, over 40,000,000 °C
•H nuclei (1p1s) fuse to make He nuclei (a combination of 2 low-mass nuclei to form a nucleus of larger mass)
•1H2 + 1H3 2He4 + 0n1 + energy
•More appealing because..▫ the greater availability of light isotopes▫ fusion products are generally not radioactive
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Detecting Radiation
•Radiation cannot be heard, heard, felt, nor smelled…
•…but, radiation produces ions, which can be detected by instruments…
•… such as Geiger counters, scintillation counters, and film badges
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Geiger Counter
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18–69
Geiger-Muller Counter
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Geiger-Muller Counter
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Scintillator
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Film Badge
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Using Radiation• Radioisotopes (tracers) are used in agriculture to test the effects of herbicides, pesticides, and fertilizers
• Radioisotopes are used to diagnose medical problems and to treat diseases
• Radiation (medical) in the body should…▫ have short t½
▫ be quickly eliminated from the body
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Household Smoke Detector•Contains a small amount of Am-241
•When smoke particles interfere with ionizing O2 in the air, smoke gets ionized instead
•The drop in current is detected by an electric circuit which soundsan alarm
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Radiation in our Lives
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Dose-dependent Effects of Radiation
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Radioisotopes in Medicine• 1 of 3 hospital patients has a nuclear medicine
procedure
• Na-24, t½ = 14.8 hr, emitter, blood-flow tracer
• I-131, t½ = 14.8 hr, emitter, thyroid gland activity
• I-123, t½ = 13.3 hr, ray emitter, brain imaging
• F-18, t½ = 1.8 hr, emitter, positron emission tomography
• Tc-99m, t½ = 6 hr, ray emitter, imaging agent
Brain images with I-123-labeled compound
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Chemistry In Action: Food Irradiation
Dosage Effect
Up to 100 kiloradInhibits sprouting of potatoes, onions, garlic; inactivates trichinae in pork; kills or prevents insects from reproducing in grains, fruits, and vegetables
100 – 1000 kilorads Delays spoilage of meat poultry and fish; reduces salmonella; extends shelf life of some fruit
1000 to 10,000 kiloradsSterilizes meat, poultry and fish; kills insects and microorganisms in spices and seasoning.
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Fig. 23.p969top
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Image of a portion of the Cygnus Loop supernova remnant
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A pellet containing radioactive I-131
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Effects of Short-Term Exposures to Radiation
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Typical radiation exposures for a person living in the United States (1 millirem =
10-3 rem)
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•End
