nuclear chemistry. chemical reactionsnuclear reactions - occur when bonds are broken or formed...
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Nuclear Chemistry
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Chemical Reactions Nuclear Reactions
- Occur when bonds are broken or formed
-Occur when the nucleus emits particles or rays
-Atoms remain unchanged, just rearranged
-Atoms are often converted into atoms of another element
- Involve only valence electrons
- May involve protons, neutrons, or electrons
- Small energy changes may occur
- Very large energy changes may occur
Chemical Reactions vs. Nuclear Reactions
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The Nucleus
• Remember that the nucleus is comprised of the two nucleons, protons and neutrons.
• The number of protons is the atomic number.• The number of protons and neutrons together is
effectively the mass of the atom.
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Isotopes
• Not all atoms of the same element have the same mass because of different numbers of neutrons in those atoms.
• For example, there are three naturally occurring isotopes of uranium:– Uranium-234– Uranium-235– Uranium-238
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Radioactivity
• It is not uncommon for some isotopes of an element to be unstable, or radioactive.
• We refer to these as radioisotopes.
• There are several ways a radioactive nucleus can decay into a more stable nucleus.
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Types of Radioactive Decay Alpha Decay
Loss of an -particle (a helium nucleus)
He42
U23892
Th23490 He4
2+
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Types of Radioactive Decay Beta Decay
Loss of a -particle (a high energy electron)
0−1 e0
−1or
I13153 Xe131
54 + e0
−1
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Types of Radioactive Decay Gamma Emission
Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)
00
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Penetrating Ability
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Types of Radioactive Decay Positron Emission
Loss of a positron (a particle that has the same mass as but opposite charge than an electron)
e01
C116
B115 + e0
1
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Types of Radioactive Decay Electron Capture
Addition of an electron to a proton in the nucleus– As a result, a proton is transformed into a
neutron.
C116 + e0
−1 B11
5
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Balancing Nuclear Reactions•In the reactants (starting materials – on the left side of an equation) and products (final products – on the right side of an equation)
Atomic numbers must balanceand
Mass numbers must balance
•Use a particle or isotope to fill in the missing protons and neutrons
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Nuclear Reactions
• Alpha decay example
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Nuclear Reactions
• Beta decay example
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Other Types of Nuclear Reactions
Positron (0+1): a positive electron
Electron capture: Electron capture: the capture of an electron
207 207
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Learning Check
What radioactive isotope is produced in the following bombardment of boron?
10B + 4He ? + 1n
5 2 0
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Radioactive DecayRadioactive elements are unstable. They decay, or change into differentelements over time. Here are some facts to remember:
The half-life of an element is the time it takes for half of the material you started with to decay. Remember, it doesn’t matter howmuch you start with. After 1 half-life, half of it will have decayed.Each element has its own half-life
Each element decays into a new element C14 decays into N14 while U238 decays into Pb206 (lead)The half-life of each element is constant. It’s like a clock keeping perfect time.
Now let’s see how we can use half-life to determine theage of a rock or other artifact.
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The grid below represents a quantity of C14. Each time you click,one half-life goes by. Try it! C14 – blue N14 - red
As we begin notice that no timehas gone by and that 100% of thematerial is C14
Halflives
% C14 %N14 Ratio of C14 to N14
0 100% 0% no ratio
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The grid below represents a quantity of C14. Each time you click,one half-life goes by. Try it! C14 – blue N14 - red
Half
lives
% C14 %N14 Ratio of
C14 to N14
0 100% 0% no ratio
1 50% 50% 1:1
After 1 half-life (5700 years), 50% ofthe C14 has decayed into N14. The ratioof C14 to N14 is 1:1. There are equalamounts of the 2 elements.
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The grid below represents a quantity of C14. Each time you click,one half-life goes by. Try it! C14 – blue N14 - red
Halflives
% C14 %N14 Ratio of C14 to N14
0 100% 0% no ratio
1 50% 50% 1:1
2 25% 75% 1:3
Now 2 half-lives have gone by for a totalof 11,400 years. Half of the C14 that waspresent at the end of half-life #1 has nowdecayed to N14. Notice the C:N ratio. Itwill be useful later.
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The grid below represents a quantity of C14. Each time you click,one half-life goes by. Try it! C14 – blue N14 - red
Halflives
% C14 %N14 Ratio of C14 to N14
0 100% 0% no ratio
1 50% 50% 1:1
2 25% 75% 1:3
3 12.5% 87.5% 1:7
After 3 half-lives (17,100 years) only12.5% of the original C14 remains. Foreach half-life period half of the materialpresent decays. And again, notice the ratio, 1:7
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Practice question
• If the half-life of an element is 10 days, how much of a 80 gram sample will remain after 50 days?
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Practice question 2
• If you start with a 200 gram sample, and 25 grams remains after 75 years, what was the half-life of the sample?
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Test question may involvegraphs like this one. The mostcommon questions are:"What is the half-life of this element?"
Just remember that at the endof one half-life, 50% of theelement will remain. Find 50%on the vertical axis, Follow theblue line over to the red curveand drop straight down to findthe answer:
The half-life of this element is 1 million years.
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Neutron-Proton Ratios• Any element with more than
one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus.
• A strong nuclear force helps keep the nucleus from flying apart.
• Neutrons play a key role stabilizing the nucleus.
• Therefore, the ratio of neutrons to protons is an important factor.
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Neutron-Proton Ratios
For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.
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Neutron-Proton Ratios
As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.
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Stable Nuclei
The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.
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Stable Nuclei
• Nuclei above this belt have too many neutrons.
• They tend to decay by emitting beta particles.
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Stable Nuclei
• Nuclei below the belt have too many protons.
• They tend to become more stable by positron emission or electron capture.
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Stable Nuclei
• There are no stable nuclei with an atomic number greater than 83.
• These nuclei tend to decay by alpha emission.
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Practice Questions:
• Predict the type of decay of carbon-14 and plutonium-239.