Atomic Nucleus
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MCAT Physical › Atomic Nucleus
Which of the following is not accurate regarding spontaneous fission reactions?
The products have smaller binding energies per nucleon than the original atom
The two resulting nuclei are approximately equal to each other in mass and atomic number
The total kinetic energy of the products is less than the kinetic energy of the original atom
The potential energy of the system is greater after the reaction than before
The sum of the atomic weights of the products is less than the atomic weight of the original atom
Explanation
In fission a large atom, with low binding energy per nucleon, splits into two approximately equal smaller atoms, which are more stable due to greater binding energy per nucleon. All the other properties mentioned are accurate.
In the operation of nuclear reactors, engineers make use of substances known as neutron poisons. These are used to help store nuclear waste and slow nuclear reactions, but are also generated naturally in nuclear chain reactions as a by-product. This natural by-product can stop the desirable chain reaction present in a nuclear reactor used for power generation.
For example, in nuclear power plants, U-235 is used as a fuel. U-235 absorbs a neutron, and subsequently generates neutrons (which power the chain reaction) and Xe-135. Xe-135 is a well-known neutron poison, and thus can impact the continued chain reaction of a nuclear power plant if it becomes over abundant during power generation.
To help account for this, engineers have developed measurements to quantify the impact of Xe-135 on nuclear operations. For instance, the time during which there is an inability to start a reactor due to the buildup of Xe-135 is referred to as the precluded start-up time. Also, the amount of time that the reactor cannot override the effects of built up Xe-135 is called poison outage time. Perhaps the most important measure that engineers have developed is the _neutron absorption capacity (_σ), which is measured in units of barns and is a function of microscopic cross section. Xe-135 has a neutron absorption capacity of 2.00 * 106 barns, while another common poison, Sm-149, has a neutron absorption capacity of 74,500 barns.
What is the force responsible for attracting neutrons to the nuclei of neutron poisons?
Strong nuclear force
Electrostatic force
Electroweak force
Gravitational force
Covalent force
Explanation
The subatomic particles in an atom's nucleus, protons and neutrons, are held together by their own force called the strong nuclear force. Strong nuclear forces act in a somewhat counterintuitive way, and only over very short distances. The strong force is what allows protons to overcome their electrical repulsion to one another and exist together in the nucleus.
In the operation of nuclear reactors, engineers make use of substances known as neutron poisons. These are used to help store nuclear waste and slow nuclear reactions, but are also generated naturally in nuclear chain reactions as a by-product. This natural by-product can stop the desirable chain reaction present in a nuclear reactor used for power generation.
For example, in nuclear power plants, U-235 is used as a fuel. U-235 absorbs a neutron, and subsequently generates neutrons (which power the chain reaction) and Xe-135. Xe-135 is a well-known neutron poison, and thus can impact the continued chain reaction of a nuclear power plant if it becomes over abundant during power generation.
To help account for this, engineers have developed measurements to quantify the impact of Xe-135 on nuclear operations. For instance, the time during which there is an inability to start a reactor due to the buildup of Xe-135 is referred to as the precluded start-up time. Also, the amount of time that the reactor cannot override the effects of built up Xe-135 is called poison outage time. Perhaps the most important measure that engineers have developed is the _neutron absorption capacity (_σ), which is measured in units of barns and is a function of microscopic cross section. Xe-135 has a neutron absorption capacity of 2.00 * 106 barns, while another common poison, Sm-149, has a neutron absorption capacity of 74,500 barns.
Xe-135 is a fission product of U-235 after it absorbs a neutron. An alternative fission reaction allows U-235 to produce Ba-141 and three neutrons. If all that is produced in this fission reaction is Ba-141, three neutrons, and one other species, which of the following could be that one other species?
Kr-92
Xe-124
Xe-134
Kr-91
Kr-93
Explanation
This is a tricky question, because it requires two steps. The first step is to recognize that the question specifies that U-235 absorbs a neutron via fission, so it becomes U-236. U-236 then breaks down into three neutrons, Ba-141, and Kr-92, because the total mass must not change.
Uranium has 92 protons, meaning that U-236 has 144 neutrons (236 – 92 = 144). Barium has 56 protons, meaning that Ba-141 must have 85 neutrons.
The identity of the unknown compound is given by the total protons (92) minus the protons in barium (56).
92 – 56 = 36
The unknown compound has 36 protons, meaning it must be Kr. The remaining neutrons must make up the remaining mass of the krypton isotope. There are 144 neutrons total, 85 given to barium, and 3 lost during reaction.
144 – (85 + 3) = 56
Krypton has 56 neutrons and 36 protons, giving it a total mass of 92.
In the nucleus, the accumulation of positive charges creates an electrostatic repulsion, known as the electromagnetic force. With the repulsive force that is generated, what is responsible for binding atoms together?
The binding energy of the strong force is greater than the electrostatic repulsion of the electromagnetic force
The presence of neutrons are able to neutralize the repulsive force
The gravitational force on Earth binds nucleons together
The nucleus is constantly disintegrating and reforming, giving off the appearance that it is bound together
Explanation
The binding energy of atoms is due in large part to the nuclear force, or strong force, which is the most powerful of the four universal forces.
Because the magnitude of the strong force is so large, it is able to overcome the electromagnetic force that is generated by the repulsion of like charges. Atoms exist in other parts of the universe beyond Earth, so the gravitational force is clearly not responsible. Neutrons have "neutral" charge, but that does not mean they are able to "neutralize" positive or negative charges.
The mass of a certain nucleus is 
Explanation
When protons and neutrons combine to form the nucleus of an atom, they generally have less total energy than the energies of the individual nucleons combined. This difference is known as the mass defect. In this case, the nucleus has 24 protons, each with a mass of 
The mass defect will be the difference between this predicted mass and the actual measured mass.
Choose the best answer for the following question.
You have a 
Explanation
Since the half-life of the sample is 3 days long, throughout the course of 9 days, the sample will undergo a half-life cycle 3 times. This means we have to half the mass of the original sample 3 times.
We originally start with
After 3 days:
After 6 days:
After 9 days:
Another way to solve these types of problems is using this formula: 
thus 
Therefore after 9 days, there will be 
In the operation of nuclear reactors, engineers make use of substances known as neutron poisons. These are used to help store nuclear waste and slow nuclear reactions, but are also generated naturally in nuclear chain reactions as a by-product. This natural by-product can stop the desirable chain reaction present in a nuclear reactor used for power generation.
For example, in nuclear power plants, U-235 is used as a fuel. U-235 absorbs a neutron, and subsequently generates neutrons (which power the chain reaction) and Xe-135. Xe-135 is a well-known neutron poison, and thus can impact the continued chain reaction of a nuclear power plant if it becomes over abundant during power generation.
To help account for this, engineers have developed measurements to quantify the impact of Xe-135 on nuclear operations. For instance, the time during which there is an inability to start a reactor due to the buildup of Xe-135 is referred to as the precluded start-up time. Also, the amount of time that the reactor cannot override the effects of built up Xe-135 is called poison outage time. Perhaps the most important measure that engineers have developed is the _neutron absorption capacity (_σ), which is measured in units of barns and is a function of microscopic cross section. Xe-135 has a neutron absorption capacity of 2.00 * 106 barns, while another common poison, Sm-149, has a neutron absorption capacity of 74,500 barns.
How does a nuclear fusion reaction compare to the nuclear fission reaction described in the passage?
Both have unstable intermediates
Fission reactions generate energy, while fusion reactions do not
Fission reactions use less massive species as reactants
Fusion reactions generate gamma radiation, while fission reactions only generate alpha radiation
Fission reactions generate gamma radiation, while fusion reactions only generate alpha radiation
Explanation
Both reactions have the potential to produce energy, and both reactions have unstable intermediates. Fission reactions have unstable intermediates in the form of species like Xe-135, which the passage describes as being reactive with neutrons to become a different chemical species. Similarly, fusion reactions fuse smaller nuclei into unstable larger nuclei that subsequently give off energy upon their decay.
Which of the following are correct statements about binding energy?
I. Light nuclei experience an increase in binding energy by fusion, and heavy nuclei by fission
II. Binding energy is the amount of energy required to separate a nucleus into its individual nucleons
III. Radioactive decay processes tend to produce nuclei which have lower binding energies than the original nucleus
I and II only
I, II, and III
I and III only
II and III only
III only
Explanation
The first two statements are true. Statement III is false becuse radioactive decays tend to produce nuclei with higher binding energy per nucleon than the parent nucleus, since nuclei with higher binding energy are held together more tightly and thus are more stable.
Which of the following is not a characteristic of nuclear fission?
Fission of light elements is exothermic
Some isotopes of a particular element may perform spontaneous fission, while other isotopes do not
The combined mass of the resulting nuclei is less than the mass of the original nucleus
Fission is caused when the electromagnetic force dominates the strong nuclear force
Fission can release neutrons as well as new nuclei
Explanation
Fission of heavy elements is exothermic; it releases energy because the products are closer to the peak of the binding energy curve than the original nucleus was. In other words, the products are more tightly bound and have more negative binding energies.
In contrast, fission is endothermic for lighter elements because their fission products are farther from the peak of the binding energy curve (the products are less tightly bound). So, fission of lighter elements would require an energy input in order to occur, making light-element fission endothermic.
All other listed answers are true statements.
Nuclear attraction is a force between which two subatomic particles?
Neutrons and protons
Neutrons and electrons
Protons and electrons
Neutrons and neutrons
Explanation
Nuclear attraction is a force that holds together the molecules in the nucleus of an atom. Remember that there are a total of three subatomic particles in an atom: protons, neutrons, and electrons. Protons are positively charged, neutrons are neutral, and electrons are negatively charged. Of the three subatomic particles, only protons and neutrons are found inside the nucleus. Nuclear attraction must occur between a neutron and a proton. This force counteracts repulsion between protons to hold the nucleus together. Electrons are found in electron shells outside the nucleus and do not participate in nuclear attraction.