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The fact that nuclei exist is a manifestation of mass–energy equivalence. A nucleus exists by converting mass into energy, which is in turn used to hold the nucleus together. This energy is stored in the nucleus and can be released when the nucleus is altered. Changes of a nucleus occur via nuclear reactions, which can be represented by nuclear reaction equations. Nuclear reactions that split nuclei or reactions that combine nuclei release energy. Fission is the splitting of a heavy nucleus, whereas fusion occurs when two light nuclei are combined. Commercial nuclear reactors generate energy by fission whereas the Sun generates energy with fusion reactions. 
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nuclear reaction, fission, chain reaction, fusion
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Review problems and questions |
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- How is mass–energy equivalence related to nuclear fission and fusion?
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When a nucleus undergoes a nuclear fission or fusion reaction, energy is released. The energy comes from the change in the rest mass of the nuclei in the reaction. In other words, the rest mass of a nucleus (and the neutron that starts the reaction) before a fission reaction is greater than the sum of the rest masses of the two nuclei (and the two or three resultant neutrons) after the reaction. The loss in rest mass is converted into energy through mass–energy equivalence. 
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- Distinguish between atomic physics and nuclear physics.
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Atomic physics is the study of physics at the atomic level, which can include the properties of neutral atoms, such as bonding, covalence, conduction, etc. Nuclear physics studies properties of the nucleus itself, which has high potential energies, and is related to radioactivity, fission, and fusion.
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- Distinguish between chemical reactions and nuclear reactions.
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Chemical reactions combine atoms to form chemical compounds by sharing and exchanging electrons. Nuclear reactions change the nucleus, which means that the result is a new element or a new isotope of an element.
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- Balance the following reaction equation by providing the missing product:
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Answer: —an alpha particle
First we balance the the charge, which will give us the unknown atomic number Z: 1 + 2 = Z + 1 ⇒ Z = 2 Therefore the unknown element X is helium (He). Now we can find A, which will give us the specific isotope of helium by balancing mass: 2 + 3 = A + 1 ⇒ A = 4 The unknown element is .
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- For the reaction what is the unknown ?
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The answer is b. To find the answer, first equate the atomic numbers on both sides of the reaction to find that Z = 11, which corresponds to sodium. Then equate the mass on both sides of the reaction, which gives A = 22.
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- Determine the name of the unknown element, the atomic number Z, and the atomic mass number A in this nuclear reaction equation:
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Answer: krypton, Z = 36, A = 92 ()
The number of protons must be the same on both sides of the reaction equation, which means that the sum of the atomic numbers must be equal on both sides: Atomic number of 36 is the element krypton (Kr).
The total number of nucleons (protons plus neutrons) must also be equal on both sides of the reaction equation: The missing element is therefore krypton-92, which is represented as .
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- In this nuclear reaction equation:
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determine the atomic mass number A, atomic number Z, and the name of the unknown element.
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Answer: lanthanum, Z = 57, A = 146 ()
The number of protons must be the same on both sides of the reaction equation, which means that the sum of the atomic numbers must be equal on both sides: Atomic number of 57 is the element lanthanum (La).
The total number of nucleons (protons plus neutrons) must also be equal on both sides of the reaction equation: The missing element is therefore lanthium-146, which is represented as .
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- Smoke detectors contain radioactive isotope americium-235 and emit alpha particles, yet manufacturers claim that the products are safe. Evaluate the validity of this claim by researching natural exposure levels, medically-recommended limits, or exposure from medical imaging.
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Answers will vary but the claim is reasonable. Smoke detectors produce low-energy alpha particles, which can be blocked by a single sheet of paper. Exposure from smoke detectors is roughly 0.01 mrem per year, while the average annual exposure due to natural sources of radiation is 300 millirems. Health experts generally agree that a person's exposure to radiation above these natural background levels should be limited to about 100 mrem per year. Comparison with these guidelines confirms that radiation from smoke detectors is within safety limits.
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