1. Chapter 25 Nuclear Chemistry 25.2 Nuclear Transformations
25.1 Nuclear Radiation
25.2 Nuclear Transformations
25.3 Fission and Fusion
25.4 Radiation in Your Life
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2. What is the source of radon in homes?
CHEMISTRY & YOU
What is the source of radon in homes?
Radon may accumulate in a basement that is not well ventilated. Test kits are available to measure the
levels of radon in a building.
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3. Nuclear Stability and Decay
What determines the type of decay a radioisotope undergoes?
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4. Nuclear Stability and Decay
The nuclear force is an attractive force that acts between all nuclear particles that are extremely close together, such as protons and neutrons in a nucleus.
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5. Nuclear Stability and Decay
The nuclear force is an attractive force that acts between all nuclear particles that are extremely close together, such as protons and neutrons in a nucleus.
At these short distances, the nuclear force dominates over electromagnetic repulsions and holds the nucleus together.
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6. Interpret Data
The stability of a nucleus depends on the ratio of neutrons to protons.
This graph shows the number of neutrons vs. the number of protons for all known stable nuclei.
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7. Interpret Data
The stability of a nucleus depends on the ratio of neutrons to protons.
The region of the graph in which these points are located is called the band of stability.
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8. Interpret Data
The stability of a nucleus depends on the ratio of neutrons to protons.
For elements of low atomic number (below about 20), this ratio is about 1.
Above atomic number 20, stable nuclei have more neutrons than protons.
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9. Nuclear Stability and Decay
A nucleus may be unstable and undergo spontaneous decay for different reasons.
The neutron-to-proton ratio in a radioisotope determines the type of decay that occurs.
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10. Nuclear Stability and Decay
Some nuclei are unstable because they have too many neutrons relative to the number of protons.
When one of these nuclei decays, a neutron emits a beta particle (fast-moving electron) from the nucleus.
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11. Nuclear Stability and Decay
Some nuclei are unstable because they have too many neutrons relative to the number of protons.
When one of these nuclei decays, a neutron emits a beta particle (fast-moving electron) from the nucleus.
A neutron that emits an electron becomes a proton. n 1
p + e –1
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12. Nuclear Stability and Decay
Some nuclei are unstable because they have too many neutrons relative to the number of protons.
When one of these nuclei decays, a neutron emits a beta particle (fast-moving electron) from the nucleus.
A neutron that emits an electron becomes a proton. n 1
p + e –1
This process is known as beta emission.
It increases the number of protons while decreasing the number of neutrons.
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13. Nuclear Stability and Decay
Radioisotopes that undergo beta emission include the following. Cu 66 29
Zn + 30 e –1 C 14 6
N + 7 e –1
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14. Nuclear Stability and Decay
Other nuclei are unstable because they have too few neutrons relative to the number of protons.
These nuclei increase their stability by converting a proton to a neutron.
An electron is captured by the nucleus during this process, which is called electron capture. Co 59 27
Ni e 28 –1 Cl 37 17
Ar e 18 –1
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15. Nuclear Stability and Decay
A positron is a particle with the mass of an electron but a positive charge.
Its symbol is e.
During positron emission, a proton changes to a neutron, just as in electron capture. +1 B 8 5
Be + 4 e +1 O 15 8
N + 7 e +1
When a proton is converted to a neutron, the atomic number decreases by 1 and the number of neutrons increases by 1.
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16. Nuclear Stability and Decay
Nuclei that have an atomic number greater than 83 are radioactive.
These nuclei have both too many neutrons and too many protons to be stable.
Therefore, they undergo radioactive decay.
Most of them emit alpha particles.
Alpha emission increases the neutron-to-proton ratio, which tends to increase the stability of the nucleus.
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17. Nuclear Stability and Decay
In alpha emission, the mass number decreases by four and the atomic number decreases by two. Ra
226 88
Rn He
222 86 4 2 Th
232 90
Ra He
228 88 4 2
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18. Nuclear Stability and Decay
Recall that conservation of mass is an important property of chemical reactions.
In contrast, mass is not conserved during nuclear reactions.
An extremely small quantity of mass is converted into energy released during radioactive decay.
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19. During nuclear decay, if the atomic number decreases by one but the mass number is unchanged, the radiation emitted is
A. a positron.
B. an alpha particle.
C. a beta particle.
D. a proton.
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20. During nuclear decay, if the atomic number decreases by one but the mass number is unchanged, the radiation emitted is
A. a positron.
B. an alpha particle.
C. a beta particle.
D. a proton.
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21. How much of a radioactive sample remains after each half-life?
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22. Interpret Graphs 1 2
A half-life (t ) is the time required for one-half of the nuclei in a radioisotope sample to decay to products.
After each half-life, half of the original radioactive atoms have decayed into atoms of a new element.
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23. Half-Lives of Some Naturally Occurring Radioisotopes
Half-Life
Comparing Half-Lives
Half-lives can be as short as a second or as long as billions of years.
Half-Lives of Some Naturally Occurring Radioisotopes
Isotope
Half-life
Radiation emitted
Carbon-14
5.73 × 103 years b
Potassium-40
1.25 × 109 years
b, g
Radon-222
3.8 days a
Radium-226
1.6 × 103 years
a, g
Thorium-234
24.1 days
Uranium-235
7.0 × 108 years
Uranium-238
4.5 × 109 years
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24. Half-Life
Comparing Half-Lives
Scientists use half-lives of some long-term radioisotopes to determine the age of ancient objects.
Many artificially produced radioisotopes have short half-lives, which makes them useful in nuclear medicine.
Short-lived isotopes are not a long-term radiation hazard for patients.
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25. Half-Life
Comparing Half-Lives
Uranium-238 decays through a complex series of unstable isotopes to the stable isotope lead-206.
The age of uranium-containing minerals can be estimated by measuring the ratio of uranium-238 to lead-206.
Because the half-life of uranium-238 is 4.5 × 109 years, it is possible to use its half-life to date rocks as old as the solar system.
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26. CHEMISTRY & YOU
Uranium compounds are found in rocks and in soils that form from these rocks. How can these uranium compounds lead to a buildup of radon in homes and other buildings?
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27. CHEMISTRY & YOU
Uranium compounds are found in rocks and in soils that form from these rocks. How can these uranium compounds lead to a buildup of radon in homes and other buildings?
Radon gas is a product of the decay of uranium. As the uranium compounds in the soil beneath homes and buildings decay, radon is produced and seeps into the structure.
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28. Half-Life
Radiocarbon Dating
Plants use carbon dioxide to produce carbon compounds, such as glucose.
The ratio of carbon-14 to other carbon isotopes is constant during an organism’s life.
When an organism dies, it stops exchanging carbon with the environment and its radioactive
C atoms decay without being replaced.
Archaeologists can use this data to estimate when an organism died. 14 6
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29. Half-Life
Exponential Decay Function
You can use the following equation to calculate how much of an isotope will remain after a given number of half-lives.
A = A0  1 2 n
A stands for the amount remaining.
A0 stands for the initial amount.
n stands for the number of half-lives.
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30. 1 A = A0  2 Exponential Decay Function n
Half-Life
Exponential Decay Function
A = A0  1 2 n
The exponent n indicates how many times A0 must be multiplied by to determine A. 1 2
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31. Decay of Initial Amount (A0) of Radioisotope
Half-Life
Exponential Decay Function
This table shows examples in which n = 1 and n = 2.
Decay of Initial Amount (A0) of Radioisotope
Half-Life
Amount Remaining
A0 × ( )0 = A0 1
A0 × ( )1 = A0 × 2
A0 × ( )2 = A0 × × 1 2
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32. Using Half-Lives in Calculations
Sample Problem 25.1
Using Half-Lives in Calculations
Carbon-14 emits beta radiation and decays with a half-life (t ) of 5730 years. Assume that you start with a mass of 2.00 × 10–12 g of carbon-14. 1 2
a. How long is three half-lives?
b. How many grams of the isotope remain at the end of three half-lives?
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33. Analyze List the knowns and the unknowns.
Sample Problem 25.1
Analyze List the knowns and the unknowns. 1
To calculate the length of three half-lives, multiply the half-life by three.
To find the mass of the radioisotope remaining, multiply the original mass by for each half-life that has elapsed. 1 2
KNOWNS
UNKNOWNS
t = 5730 years
initial mass (A0) = 2.00 × 10–12 g
number of half-lives (n) = 3 1 2
3 half-lives = ? years
mass remaining = ? g
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34. Calculate Solve for the unknowns.
Sample Problem 25.1
Calculate Solve for the unknowns. 2
a. Multiply the half-life of carbon-14 by the total number of half-lives.
t × n = 5730 years × 3 = 17,190 years 1 2
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35. Calculate Solve for the unknowns.
Sample Problem 25.1
Calculate Solve for the unknowns. 2
b. The initial mass of carbon-14 is reduced by one-half for each half-life. So, multiply by three times. 1 2
Remaining mass = 2.00 × 10–12 g × × × 1 2
= × 10–12 g
= 2.50 × 10–13 g
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36. ( ) Calculate Solve for the unknowns. 2
Sample Problem 25.1
Calculate Solve for the unknowns. 2
b. You can get the same answer by using the equation for an exponential decay function.
= (2.00 × 10–12 g)
= × 10–12 g
= 2.50 × 10–13 g
A = A = (2.00 × 10–12 g)
( ) 1 2 n 3 8
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37. Evaluate Do the results make sense?
Sample Problem 25.1
Evaluate Do the results make sense? 3
The mass of carbon-14 after three half-lives should be one-eighth of the original mass.
If you divide 2.5 × 10–13 g by 2.00 × 10–12 g, you will get 12.5%, or . 1 8
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38. The half-life of phosphorus-32 is 14. 3 days
The half-life of phosphorus-32 is 14.3 days. How many milligrams of phosphorus-32 remain after days if you begin with 2.5 mg of the radioisotope?
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39. The half-life of phosphorus-32 is 14. 3 days
The half-life of phosphorus-32 is 14.3 days. How many milligrams of phosphorus-32 remain after days if you begin with 2.5 mg of the radioisotope?
n = days × = 7 half-lives
1 half-life
14.3 days
= (2.5 mg) = 2.0 × 10–2 mg
A = A = (2.5 mg)
( ) 1 2 n 7
( )
128
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40. Transmutation Reactions
What are two ways in which transmutation can occur?
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41. Transmutation Reactions
For thousands of years, alchemists tried to change lead into gold.
What they wanted to achieve is transmutation, or the conversion of an atom of one element into an atom of another element.
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42. Transmutation Reactions
Transmutation can occur by radioactive decay, or when particles bombard the nucleus of an atom.
The particles may be protons, neutrons, alpha particles, or small atoms.
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43. Transmutation Reactions
Ernst Rutherford performed the earliest artificial transmutation in 1919.
He bombarded nitrogen gas with alpha particles.
The unstable fluorine atoms quickly decay to form a stable isotope of oxygen and a proton.
O + 17 8 p 1 F 18 9
Proton
Oxygen-17
Fluorine-18
N + 14 7 He 4 2
Nitrogen-14
Alpha particle
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44. Transmutation Reactions
Rutherford’s experiment eventually led to the discovery of the proton. He
Alpha particle N
Nitrogen atom F
Unstable fluorine atom O
Oxygen P
Proton 4 2 14 7 18 9 17 8 1
He and other scientists noticed a pattern as they did different transmutation experiments. Hydrogen nuclei were emitted.
Scientists realized that these hydrogen nuclei (protons) must have a fundamental role in atomic structure.
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45. Transmutation Reactions
James Chadwick’s discovery of the neutron in 1932 also involved a transmutation experiment.
Neutrons were produced when beryllium-9 was bombarded with alpha particles.
C + 12 6 n 1
Neutron
Carbon-12
Be + 9 4 He 2
Beryllium-9
Alpha particle
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46. Transmutation Reactions
Elements with atomic numbers above 92, the atomic number of uranium, are called transuranium elements.
None of these elements occurs in nature.
All of them are radioactive.
All transuranium elements undergo transmutation.
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47. Transmutation Reactions
Transuranium elements are synthesized in nuclear reactors and nuclear accelerators.
Reactors produce beams of low-energy particles.
Accelerators are used to increase the speed of bombarding particles to very high speeds.
The European Organization for Nuclear Research, CERN, has a number of accelerators. The figure at right shows CERN’s largest accelerator.
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48. Transmutation Reactions
When uranium-238 is bombarded with the relatively slow neutrons from a nuclear reactor, some uranium nuclei capture these neutrons. The product is uranium-239.
Uranium-239 is radioactive and emits a beta particle. The other product is an isotope of the artificial radioactive element neptunium (atomic number 93).
Neptunium is unstable and decays, emitting a beta particle and a second artificial element, plutonium (atomic number 94).
Np +
239 93 e –1 U 92
U +
238 n 1
Pu + 94 Np
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49. Transmutation Reactions
Scientists in Berkeley, California, synthesized the first two artificial elements in 1940.
Since that time, more than 20 additional transuranium elements have been produced artificially.
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50. Which of the following always changes when transmutation occurs?
A. The number of electrons
B. The mass number
C. The atomic number
D. The number of neutrons
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51. Which of the following always changes when transmutation occurs?
A. The number of electrons
B. The mass number
C. The atomic number
D. The number of neutrons
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52. Key Concepts
The neutron-to-proton ratio in a radioisotope determines the type of decay that occurs.
After each half-life, half of the original radioactive atoms have decayed into atoms of a new element.
Transmutation can occur by radioactive decay, or when particles bombard the nucleus of an atom.
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53. Key Equation
A = A0  1 2 n
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54. Glossary Terms
nuclear force: an attractive force that acts between all nuclear particles that are extremely close together, like protons and neutrons in a nucleus
band of stability: the location of stable nuclei on a neutron-vs.-proton plot
positron: a particle with the mass of an electron but a positive charge
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55. Glossary Terms
half-life: the time required for one-half of the nuclei of a radioisotope sample to decay to products
transmutation: the conversion of an atom of one element to an atom of another element
transuranium elements: any elements in the periodic table with atomic number above 92, the atomic number of uranium
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56. Electrons and the Structure of Atoms
BIG IDEA
Electrons and the Structure of Atoms
Unstable atomic nuclei decay by emitting alpha or beta particles.
Often gamma rays are emitted.
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57. END OF 25.2
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