unit five part b
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Unit Five Part B. Nuclear Chemistry. Radioactivity. There are two main types of radioactivity: Natural and Induced. Natural Radioactivity. Occurs in nature Usually large, unstable nuclei Occurs in three ways: Decay (alpha particle) Decay (beta particle) Decay (gamma ray). - PowerPoint PPT PresentationTRANSCRIPT
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Unit Five Part B
Nuclear Chemistry
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Radioactivity
There are two main types of radioactivity: Natural and Induced
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Natural Radioactivity
Occurs in natureUsually large, unstable nucleiOccurs in three ways:
Decay (alpha particle)
Decay (beta particle)
Decay (gamma ray)
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Alpha Decay
A helium nucleus is released from the nucleus. ( )
The mass decreases by 4The atomic number decreases by 2
(Because the He nucleus has 2p+ and 2no)
Alpha radiation can be stopped by a piece of paper. Cannot penetrate skin. Not dangerous.
He4
2
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Alpha Decay Example
Notice that the uranium has changed into a new element, thorium.
HeThU 4
2
234
90
238
92
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Beta Decay
An electron is released from the nucleus when a neutron becomes a proton.
The mass is unaffected. (the mass of a neutron is roughly equal to the mass of a proton)The atomic number is increased by 1.Harder to stop and more dangerous.
e01
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Beta Decay Example
eNC 0
1
14
7
14
6
Notice that carbon has changed into nitrogen.
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Gamma Decay
Pure energy is released from the nucleus.The mass and atomic number are unaffected.Stopped by lead. The most harmful to living tissue.
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Gamma Decay Example
SrSr 87
38
87
38*
No new element formed. Gamma radiation (energy) released.
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Induced Radioactivity
Particles are slammed together to cause transmutation of stable elements. (Nuclear Bombardment)Discovered by Rutherford in 1919.
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Uranium-238 Decay Series
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Radioactive Decay of U-238
HeThU 4
2
234
90
238
92
•Uranium-238 becomes Thorium-234
•Transmutation by Alpha Decay
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Radioactive Decay of U-238
ePaTh 0
1
234
91
234
90
Thorium-234 becomes Protactinium-234
Transmutation by Beta Decay
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Radioactive Decay of U-238
eUPa 0
1
234
92
234
91
Protactinium-234 becomes Uranium-234
Transmutation by Beta Decay
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Radioactive Decay of U-238
HeThU 4
2
230
90
234
92
Uranium-234 becomes Thorium-230
Transmutation by Alpha Decay
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Radioactive Decay of U-238
HeRaTh 4
2
226
88
230
90
Thorium-230 becomes Radium-226
Transmutation by Alpha Decay
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Radioactive Decay of U-238
HeRnRa 4
2
222
86
226
88
Radium –226 becomes
Radon-222
Transmutation by Alpha Decay
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Radioactive Decay of U-238
HePoRn 4
2
218
84
222
86
Radon-222 becomes
Polonium-218
Transmutation by Alpha Decay
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Radioactive Decay of U-238
HePbPo 4
2
214
82
218
84
Polonium-218 becomes Lead-214
Transmutation by Alpha Decay
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Radioactive Decay of U-238
eBiPb 0
1
214
83
214
82
Lead-214 becomes Bismuth-214
Transmutation by Beta Decay
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Radioactive Decay of U-238
Bismuth-214 becomes Polonium-214
Transmutation by Beta Decay
ePoBi 0
1
214
84
214
83
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Radioactive Decay of U-238
HePbPo 4
2
210
82
214
84
Polonium-214 becomes Lead-210
Transmutation by Alpha Decay
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Radioactive Decay of U-238
eBiPb 0
1
210
83
210
82
Lead-210 becomes Bismuth-210
Transmutation by Beta Decay
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Radioactive Decay of U-238
ePoBi 0
1
210
84
210
83
Bismuth-210 becomes Polonium-210
Transmutation by Beta Decay
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Radioactive Decay of U-238
Polonium-210 becomes Lead-206
Transmutation by Alpha Decay
Lead-206 is stable. (phew!)
HePbPo 4
2
206
82
210
84
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Half-Life
The time it takes for half of the atoms in a given radioactive sample to decay into a more stable isotope. This number is different for each kind of isotope of any kind of element.Can be calculated because atoms decay at a predictable rate.Half lives can range from fractions of a second to millions of years.
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Half-Life
Two formulas will help you solve half life problems:
1. Half-Life = Total timen
2. Final Mass = Total Mass 2 n
(n = # decay cycles)
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Example ProblemsThe half-life of technetium is 6.00 hours. What mass of Tc-99 remains from a 10.0 gram sample after 24.0 hours.
First, calculate the number of half-life cycles that have occurred in the time given.
n = Total time n = 24.0 hrs n = 4Half-Life 6.00 hrs
Next, use the value of n to calculate the remaining mass of the sample.
Final Mass = Total Mass = 10.0g = 10.0g
2n 24 16
= 0.625g Tc-99
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How about another one???
A 50.0g sample of N-16 decays to 12.5g in 14.4s. What is its half-life?
Final mass = Initial mass 2n
2n = Initial mass 2n = 50.0g n = log(4 ) n = 2
Final mass 12.5g log(2)
Half-Life = Total Time n
Half-Life = 14.4s 2Half-Life = 7.20s
50.0g 1 half = 25.0g 2 half = 12.5 g
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Sure, one more… why not?
There are 5.0g of I-131 left after 40.35 days. How many grams were in the original sample if its half-life is 8.07 days?Final Mass = Total Mass 2 n
1st : How many cycles have occurred?40.35 / 8.07 = 5 cycles.
2nd: Rearrange the formula to solve for the original total mass. Total Mass = Final Mass x 2n
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So, solve it already!!!
3rd: SolveTotal Mass = 5.0g x 25
Total Mass = 5.0g x 32
Total Mass = 160g
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Shall we check it???(of course)160.0g Total MassAt the end of one half life = 80.0g (8.07days)At the end of two cycles = 40.0g (16.14 days)At the end of three cycles = 20.0g(24.21 days)At the end of four cycles = 10.0g(32.28 days)At the end of five cycles = 5.0g(40.35 days)
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Since decay occurs at a predictable rate, we can use the ratio of decayed to undecayed isotopes to…Determine the age of Organic Matter with Carbon – 14 (Up to 30,000 yrs)Determine the age of Rocks (and therefore other earth structures) with Uranium – 238 (Millions of yrs.)
Using Radioisotopes
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More uses for radioisotopes…
Tracers used to detect structure and function of organs (thyroid, gall bladder, GI tract, etc…)Can also be used to track movement of silt in rivers and nutrient uptake in plants.Cancer treatmentFood preservationSensors in Smoke DetectorsStarters in Fluorescent lampsNuclear fuel for power plants
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Detection of Radioactivity
Detected with a Geiger Counter. (When ions strike the cylinder of the Geiger counter, it emits an audible click.)
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Detection of Radiation
Dosimeter – measures the total amount of radiation that a person has received. Works because photographic film is sensitive to radiation. Usually is worn like a badge. The film is later developed and the exposure to radiation can be measured.Unit used to measure radiation exposure in humans is the rem. (Stands for Roentgen Equivalent for Man) (Roentgen discovered X-rays.)
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Biological Effects of Radiation
o Destruction of tissue especially blood and lymph which cells multiply rapidly.
o Causes various cancers.o Direct damage to an organism is
called Somatic Damage.o Damage that affects reproductive
cells is called Genetic Damage. This leads to birth defects in offspring.
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Nuclear Fission
A large nucleus is split into two smaller nuclei of approximately equal mass.The fission of 4.5g of U-235 will satisfy the average person’s energy needs for an entire year. (Equal to about 15 tons of coal.)
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Nuclear Fission
The total mass of the products in a fission reaction is slightly less than the mass of the starting materials.Law of Conservation of Matter does not apply to fission reactions!This small amount of “missing” mass is converted into a huge amount of energy. (E = mc2) c=300,000,000m/s
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Nuclear Fission
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Nuclear Fission
A fission reaction can produce a Chain Reaction because each reaction releases high speed neutrons, each capable of starting another fission reaction.Chain reactions make the fission process sustainable for use in Nuclear Power Plants.
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Chain Reaction
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Nuclear Fusion
Two small nuclei join to form a large nucleus.Some mass is converted into energy (even more than fission reactions)Difficult to produce and control. To overcome the repulsion between nuclei, they must be heated to 40 million kelvins. For this reason, they are sometimes called Thermonuclear Reactions.
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Nuclear Fusion
Thermonuclear reactions create the energy produced by the sun and other stars.Thermonuclear reactions are the source of the destructive power of a hydrogen bomb.Not (yet?) sustainable for use in nuclear power plants.
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Nuclear Fusion