1. (Number of protons is an atom’s ID number)
Nuclear chemistry
Nuclide: Name for an atom in nuclear chemistry
Nucleons: Protons and neutrons
Nuclear reaction: Reaction that affects the nucleus of an atom
Transmutation: The change in the identity of a nucleus due to change in number of protons
(Number of protons is an atom’s ID number)
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2. Isotopes in Nature
Most elements in nature occur as mixtures of isotopes.
All naturally occurring isotopes of elements with atomic numbers > 83 are radioactive.
Beyond 83 amu, the repulsive force of protons is so great that no stable nuclides exist.
However, some natural radioactive isotopes are also found among lighter elements.
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3. Band of Stability Unstable if neutron: proton ratio > 1.5:1
Source:
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4. Types of Radioactive Decay
Symbol
Charge
Mass (amu)
Alpha particle He 2+
Beta particle β 1-
Positron 1+
Gamma ray γ
4 2
- 1 4
+ 1
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5. Nuclear equations Atomic numbers and mass numbers are conserved.
Use the proper type of particle to balance.
If atomic number is 0, it is a neutron.
If atomic number is -1, it is an electron.
If atomic number is 1 or more it is an element, except for a positron.
A positron increases the number of protons by 1 and does not change mass. ( β ) +1
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6. Example nuclear equations
238 92
234 90
U ____ Th
U He Th
Ar + ____ Cl
Ar e Cl
Be + He C + n
234 90
238 92 4 2 37 18 37 17 37 17 37 18 -1 9 4 4 2 12 6 1
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7. Half-life
Time required for half the atoms of a RADIOACTIVE nuclide to decay.
How many ½-lives have passed after 60 days if a nuclide has a given ½-life of 12 days?
Number of ½-lives = time elapsed x 1 half-life
given time
Number of ½-lives = 60 days x 1 half-life = 5 half-lives
12 days
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8. Using half-life data
Multiply the amount of a substance by (1/2)y to find remaining substance after y half-lives have passed.
Ex.: Polonium-210 decays with a half-life of days. How many milligrams remain after days if you start with 2.0 mg of Po-210?
y = days x 1 half-life = 3 half-lives
138.4 days
2.0 mg (1/2)(1/2)(1/2) = 0.25 mg
2.0 mg (1/2) = 0.25 mg
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9. y = 104.7 min x 1 half-life = 10 half-lives 10.47 min
Cobalt-60 decays with a half-life of min. How many milligrams remain after min if you start with 10.0 mg of Co-60?
y = min x 1 half-life = 10 half-lives
10.47 min
10.0 mg (1/2) = mg
The half-life of carbon-14 is 5715 years. How long will it be until only half of the carbon-14 in a sample remains?
y = number of ½-lives 1/2 sample remains.
1 sample x (1/2)y = 1/2 sample
y = 1 half-life 1 half-life = 5715 years
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10. Another half-life example
Uranium-238 decays with a half-life of 4.46 x 109 years. How long would it take for 7/8 of a sample of U-238 to decay?
y = number of ½-lives
1/8 sample remains.
1 sample(1/2)y = 1/8 sample
yln(1/2) = ln(1/8)
Or….
(1/2)(1/2)(1/2) = 1/8 y = 3 half-lives
3(4.46 x 109 years) = 1.34 x 1010 years
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11. How did Becquerel discover radioactivity?
Fluorescence: Certain materials emit light when struck by radiant energy such as UV rays.
He wrapped a photographic plate in black paper and left it beside a fluorescent mineral in a drawer during a period of several cloudy days. He had planned to place the mineral and the plate in the sun so that the minerals could fluoresce (emit X-rays).
When the film was removed, it was exposed, just as if the mineral had been exposed to radiation and fluoresced.
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12. Ionizing Radiation Ionizing radiation Non-ionizing radiation
Higher energy waves that include UV, X-ray and gamma rays.
Can eject electrons from atoms and molecules, forming fragments and ions. These fragments can cause disruptions of normal cellular chemistry.
Non-ionizing radiation
Electromagnetic radiation in the visible and lower energy region of the spectrum.
Transfers energy to matter, causing molecules to vibrate or move their electrons to a higher energy level.
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13. Exposure to Ionizing Radiation
Background radiation from natural sources:
Decay of radioactive elements(U-238, U-235 and Th-232) found in soil and water
Very high-energy particles from outer space
Radioactive isotopes in the atmosphere (Rn-222) and its decay products such as polonium.
Radioactive isotopes in foods such as carbon-14 and potassium-40.
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14. Units of Radiation exposure
gray (Gy): The SI unit used to measure the quantity of ionizing radiation delivered to a sample such as human tissue.
1 Gy = 1 J/kg
Sievert (Sv): The SI unit that expresses the ability of any radiation to cause ionization in human tissue.
1 Sv exposure = same effect as 1 Gy of gamma rays
rad measures absorbed dose of radiation
1/100th of 1 gray
rem measures the ionizing effect on living organisms.
1/100th of 1 Sv
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15. Tissue Damage Due to Ionizing Radiation
Two factors:
Radiation density (# radiations within a given volume)
Dose (the quantity of radiation received)
Gamma rays and X-rays break bonds in molecules—at low levels, the body can repair these molecules.
Damage to nucleic acid and proteins are big concerns with high levels of radiation.
Proteins make up enzymes, molecules that control the rates of chemical reactions in the body.
Nucleic acids in DNA may incur mutations, or changes in DNA structure which may result in production of altered proteins.
Cancer: cell growth and metabolism are out of control.
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16. Geiger-Mueller Counter
Used to detect radiation
Detector tube is filled with gas such as argon.
If ionizing radiation enters the end of tube, it will ionize the gas particles.
When ions are formed, (-) ions are drawn to (+) charged center, and the (+) ions go to the (-) outer wall.
Moving of charged particles makes a pulse of electric current.
Each pulse is counted to indicate the INTENSITY of radiation.
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17. Scintillation Counter
When radiation strikes the detector, flashes of light are emitted.
These flashes are detected electronically by a photomultiplier tube.
The photomultiplier tube converts light to an electron pulse and increases the pulse many times.
The detector is a solid whose atoms are excited by ionizing radiation. An example is NaI.
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18. Solid State Detector
Monitors change in movement of electrons when they are exposed to radiation.
Counts the changes electronically.
Electron movement is monitored through silicon or another semiconductor.
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19. Types of particles in radioactive decay
Alpha particle 2
equal to He atom
Easy to stop from penetrating (large mass)
Very damaging to living tissue if not stopped (large mass and high energy,charge)
Beta particle 0 -1
A neutron decays to a proton and an electron.
Net effect to the nuclide is that neutron changes to proton. The electron is ejected at high speed = beta emission.
Lighter and faster than alpha
Not as damaging to living tissue, but harder to block than alpha. He β
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20. Gamma rays (γ) No charge, no mass
Most penetrating (need barrier such as lead to block)
Result of alpha or beta decay leaving nuclei in an excited and metastable state
Energy released from this state is high energy electromagnetic radiation.
Least damage to tissue over comparable distances: Damage is related to the extent of ionization created by the radiation.
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21. Nuclear Fission
Nucleus of one heavy atom splits into 2 or more nuclei.
Bombardment usually used to start fission.
Products are nuclei and nuclides from decay.
Used for powering nuclear reactors, submarines and missiles
Chain reaction: Product of one reaction starts another
Produces more nuclear waste than fusion
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22. Light mass nuclei combine to form a heavier, more stable nucleus.
Nuclear Fusion
Light mass nuclei combine to form a heavier, more stable nucleus.
Releases more energy per gram of fuel than fission.
Hydrogen is usually the fuel for fusion.
Fusion reaction control is limited by the high initial temperature.
The sun is a good example of fusion:
4 H nuclei combine to form He nucleus.
Loss of mass = huge energy release
4 H nuclei He nucleus β particles 1 4 2 +1
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23. Nuclear Reactor Power Plants
Fuel rods: Contain uranium dioxide pellets about the size of chalk sticks. These contain ~3% fissionable U-235—enough to sustain a chain reaction, but too little for a nuclear explosion.
Shielding: Radiation-absorbing material used to contain radiation
Control rods: Neutron-absorbing rods that control reaction by limiting free neutrons (boron and cadmium)
Moderator: Slows down fast neutrons produced by fission (water and graphite)
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