Nuclear Chemistry and Electrons

Example Question #1 : Radioactive Decay

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 * 10⁶barns, while another common poison, Sm-149, has a neutron absorption capacity of 74,500 barns.

Nuclear reactions, such as the one described above with uranium, emit energy in the form of radiation. A scientist is considering three different forms of radiation to generate power. Which of the following types of radiation is highest in energy?

Possible Answers:

Gamma radiation

Beta radiation

Positron emission

Alpha radiation

Electron capture

Correct answer:

Gamma radiation

Explanation:

Gamma radiation is a highly energetic form of radiation, and is the type of radiation emitted when antimatter and matter annhilate each other upon contact.

Example Question #2 : Radioactive Decay

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 * 10⁶barns, while another common poison, Sm-149, has a neutron absorption capacity of 74,500 barns.

Xe-135 has a half life of around 9.2 hours. A reaction generated 70g of Xe-135, as measured by a scientist on a Friday afternoon. If he returned 46 hours later to study the sample, how much would remain?

Possible Answers:

4g

2g

70g

3g

14g

Correct answer:

2g

Explanation:

The scientist allowed the substance to sit for a duration that was equal to five of its half life. The original sample decayed from 70g, to 35g, to 17.5g, to 8.75g, to 4.35g, to ~2 g.

$\frac{46\ hours}{9.2\ hours}=5$

$m_{remaining}=m_{original}(0.5)^n$

n is equal to the number of half lives that have passed. For us, n is equal to 5.

$m_{remaining}=(70g)(0.5)^5=(70g)(\frac{1}{32})=2.2g$

Example Question #3 : Radioactive Decay

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 * 10⁶barns, while another common poison, Sm-149, has a neutron absorption capacity of 74,500 barns.

A scientist is studying a nuclear reaction similar to the one described in the passage. He realizes that the product atom has a mass number four less than the reactant, and an atomic number two less than the reactant. What is true of this type of radiation?

Possible Answers:

It results in the emission of a heavy hydrogen nucleus, which is strictly a type of non-ionizing radiation

It results in the emission of either a helium nucleus or a heavy hydrogen nucleus, depending on conditions

It results in the emission of a helium nucleus, which is strictly a type of non-ionizing radiation

It results in the emission of a heavy hydrogen nucleus, a type of ionizing radiation

It results in the emission of a helium nucleus, a type of ionizing radiation

Correct answer:

It results in the emission of a helium nucleus, a type of ionizing radiation

Explanation:

This is an example of alpha radiation, which is the emission of a helium nucleus, $_{2}^{4}\textrm{He}$ , thus explaining the loss of two protons and four mass units from the original species. Because this is a nucleus, it can interact with other atoms in the environment to generate ionic species. This is in contrast to non-ionizing radiation, such as infrared or radio waves.

Example Question #11 : Radioactive Decay

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 * 10⁶barns, while another common poison, Sm-149, has a neutron absorption capacity of 74,500 barns.

When a U-235 atom breaks down __________.

Possible Answers:

the reaction absorbs an amount of energy, and then a lesser amount is released when the new species form

the reaction releases energy with no absorption

The reaction absorbs an amount of energy, and then an equal amount is released when the new species form

the reaction absorbs an amount of energy, and then a greater amount is released when the new species form

the reaction absorbs energy with no release

Correct answer:

the reaction absorbs an amount of energy, and then a greater amount is released when the new species form

Explanation:

Like any system, energy is absorbed to break bonds and released when bonds are formed. Energy is absorbed to get U-235 to an unstable state. It then breaks apart, and new species form. These new species release more energy than was originally absorbed when they form their bonds, thus leading to a net release of energy from the system.

Example Question #21 : Nuclear Chemistry And Electrons

An isotope of uranium initially contains 92 protons and 146 neutrons. This isotope then undergoes two alpha decays, one beta ( $\beta^{-}$ ) particle decay, and two gamma decays. How many neutrons does the resulting nucleus contain?

Possible Answers:

140

142

147

150

141

Correct answer:

141

Explanation:

To solve this question, you must be familiar with each type of radioactive decay.

Alpha decay emits two protons and two neutrons: $_{2}^{4}\alpha$ .

Beta particle decay emits an electron, which has been converted from a neutron: $_{-1}^{\ 0}{\beta}$ .

Gamma decay is purely electromagnetic, with no attached particles: $_{0}^{0}\gamma$ .

We start with uranium: $_{92}^{238}\textrm{U}$ .

$_{92}^{238}\textrm{U}\rightarrow 2_{2}^{4}\alpha+_{-1}^{0}\beta+2_{0}^{0}\gamma+_{89}^{230}\textrm{Ac}$

The number of neutrons will be equal to the total mass number, minus the number of protons.

230-89=141

There will be 141 neutrons in the final product nucleus.

Example Question #13 : Radioactive Decay

What is the decay constant of krypton-96, if half of a sample decays to rubidium-96 in $\small 80ms$ ?

Possible Answers:

$12.5s^{-1}$

$3.64s^{-1}$

$6.25s^{-1}$

$0.12s^{-1}$

$8.66 s^{-1}$

Correct answer:

$8.66 s^{-1}$

Explanation:

The general equation for exponential decay is:

$N(t)=N_{o}e^{-\lambda t}$

$N_{o}$ represents the initial amount of the compound, $\lambda$ is the decay constant, and is the time.

If half of the initial amount of krypton has decayed after $\small 80ms$ , we can set up the equation as a proportion:

$\frac{1}{2}=(1)e^{\lambda(-0.08s)}$

To get the variable out from the exponent, take the natural log of both sides.

$ln(\frac{1}{2})=ln(e^{\lambda(-0.08s) })$

$ln(\frac{1}{2})=(-0.08s)\lambda$

$\lambda = 8.66 s^{-1}$

Example Question #14 : Radioactive Decay

The activity of a radioactive element is $\small 240 Bq$ . After $\small 24min$ , the activity is $\small 60Bq$ . What is the half-life of this element?

Possible Answers:

12min

24min

48min

4min

Correct answer:

12min

Explanation:

Radioactive elements spontaneously release particles. The half life is determined by the amount of time it takes half of a sample to release particles, thereby decreasing in mass and radioactive activity.

The formula for radioactive decay is:

$A_E=A_0(0.5)^{\frac{t}{t_{0.5}}}$

We can solve for the half life by using the formula. We are given the initial and final activity values and the time period. Use these to solve for the half life.

$60Bq=(240Bq)(0.5)^{\frac{24}{x}}$

$0.25=(0.5)^{\frac{24}{x}}$

$2=\frac{24}{x}$

x=12min

Example Question #15 : Radioactive Decay

What is the daughter particle produced from $\alpha$ -decay, followed by a single round of $\beta^-$ -decay, of $_{64}^{157}\textrm{Gd}$ ?

Possible Answers:

$_{63}^{153}\textrm{Gd}$

$_{62}^{153}\textrm{Sm}$

$_{63}^{153}\textrm{Eu}$

$_{61}^{153}\textrm{Pm}$

Correct answer:

$_{63}^{153}\textrm{Eu}$

Explanation:

Start with the parent particle, $_{64}^{157}\textrm{Gd}$ . Alpha decay will result in the loss of a helium nucleus: two protons and two neutrons.

$_{64}^{157}\textrm{Gd}\rightarrow _{2}^{4}\textrm{He}+_{62}^{153}\text{Sm}$

Remember that a change in proton number must be associated with a change in elemental identity, changing gadolinium to samarium.

Next, the particle will undergo beta-decay. In this process, a neutron is converted to a proton and an electron. The result is no net change in mass, but an increase in proton number and the release of an electron.

$_{62}^{153}\text{Sm}\rightarrow _{-1}^{\ 0}\textrm{e}\ +_{63}^{153}\textrm{Eu}$

The final result is a europium atom.

Example Question #12 : Radioactive Decay

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 * 10⁶barns, while another common poison, Sm-149, has a neutron absorption capacity of 74,500 barns.

A scientist is studying a nuclear reaction similar to the one in the passage. He finds that the atomic mass of the reactant does not change, but its atomic number decreases by one. This type of nuclear reaction is best described as __________.

Possible Answers:

Positron emission

Beta decay

Alpha decay

Gamma decay

Electron capture

Correct answer:

Positron emission

Explanation:

If the atomic number decreases by one, but the atomic mass is unchanged, the reactant must be converting one proton to a neutron. The resulting byproduct is a positron.

$_{10}^{10}\textrm{Reactant}\rightarrow _{+1}^{\ \ 0}\textrm{e}+\ _{9}^{10}\textrm{Reactant}$

Notice that the positron on the right is released upon the rectant's conversion. This type of emission is known as positron emission.

Example Question #17 : Radioactive Decay

Which of the following choices incorrectly represents a radioactive decay process?

Possible Answers:

$_{92}^{238}U\rightarrow _{90}^{236}Th+2H$

$_{6}^{14}\textrm{C}\rightarrow\ _{5}^{14}\textrm{B}+\ _{1}^{0}\textrm{e}$

$_{27}^{60}Co\rightarrow_{27}^{60}Co+\gamma$

$_6^{14}C\rightarrow\ _7^{14}N+\ _{-1}^0e$

$_{92}^{238}U\rightarrow _{90}^{234}Th+_2^4He$

Correct answer:

$_{92}^{238}U\rightarrow _{90}^{236}Th+2H$

Explanation:

In general, there are three types of radioactive decay.

Alpha decay: An atomic nucleus emits a helium nucleus, composed of two protons and two neutrons. The net result is mass reduction of four (from the two neutrons and two protons) and an atomic number reduction of two (from the loss of two protons).

Beta decay: An atomic nucleus emits an electron, while a neutron splits into a proton, an electron, and anti-electron neutrino. The net result is an increase in atomic number (due to the added proton). Note, the atomic mass remains constant because although a neutron is lost, a proton is formed.

Possitron emmision: An atomic nucleus emits an anti-electron neutrino and electron, after a proton splits into a neutron. The net result is an decrease in atomic number (due to the tranformed proton). Note, the atomic mass remains constant because although a proton is lost, a neutron is formed.

Gamma Decay: The energy level of atomic nucleus drops, without losing protons or neutrons.

Each one of these decay processes are reflected individually in the answer choices, other than the decay process of $_{92}^{238}U\rightarrow _{90}^{236}Th+2H$ . No such decay process emitting hydrogen atoms exists.

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MCAT Physical : Nuclear Chemistry and Electrons

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Example Question #1 : Radioactive Decay

Example Question #2 : Radioactive Decay

Example Question #3 : Radioactive Decay

Example Question #11 : Radioactive Decay

Example Question #21 : Nuclear Chemistry And Electrons

Example Question #13 : Radioactive Decay

Example Question #14 : Radioactive Decay

Example Question #15 : Radioactive Decay

Example Question #12 : Radioactive Decay

Example Question #17 : Radioactive Decay

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