Atoms, Elements, and the Periodic Table
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MCAT Physical › Atoms, Elements, and the Periodic Table
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.
A scientist is studying atomic radius within a sample of radioactive flourine sourced from a nuclear reactor similar to the one described in the passage. Which of the following changes is likely to result in the largest atomic radius found in the sample?
Adding an electron to flourine
Adding a neutron to flourine
Removing an electron from flourine
Removing a neutron from flourine
Adding a proton to flourine
Explanation
Adding an electron to flourine results in a flourine anion. Anions generally have larger atomic radii than do their original atomic counterparts, and relative to their cation counterparts. Addition or substraction of neutrons has less of an impact due to their neutral charges and location in the nucelus. Adding a proton would increase the charge in the nucleus, convert flourine to neon, and pull the existing electrons in more tightly, decreasing radius.
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.
A scientist is studying atomic radius within a sample of radioactive flourine sourced from a nuclear reactor similar to the one described in the passage. Which of the following changes is likely to result in the largest atomic radius found in the sample?
Adding an electron to flourine
Adding a neutron to flourine
Removing an electron from flourine
Removing a neutron from flourine
Adding a proton to flourine
Explanation
Adding an electron to flourine results in a flourine anion. Anions generally have larger atomic radii than do their original atomic counterparts, and relative to their cation counterparts. Addition or substraction of neutrons has less of an impact due to their neutral charges and location in the nucelus. Adding a proton would increase the charge in the nucleus, convert flourine to neon, and pull the existing electrons in more tightly, decreasing radius.
Place the following atoms in decreasing order of atomic radius.
Cl, Ar, K, Ca
K, Ca, Cl, Ar
Ca, K, Ar, Cl
Cl, Ar, K, Ca
Ca, K, Cl, Ar
Explanation
Atomic radius has two general trends which you should remember:
1. Atomic radius will decrease when moving left to right along a period.
2. Atomic radius will increase when moving down a group.
Since potassium (K) and calcium (Ca) are farther down the group than argon (Ar) and chlorine (Cl), we conclude that they are the largest atoms in the set. Because Ca is to the right of K, it is slightly smaller than K. As a result, K has the largest atomic radius in the set followed by Ca. Since Ar is to the right of Cl, Cl has a larger atomic radius than Ar.
The decreasing order is K, Ca, Cl, Ar.
Place the following atoms in decreasing order of atomic radius.
Cl, Ar, K, Ca
K, Ca, Cl, Ar
Ca, K, Ar, Cl
Cl, Ar, K, Ca
Ca, K, Cl, Ar
Explanation
Atomic radius has two general trends which you should remember:
1. Atomic radius will decrease when moving left to right along a period.
2. Atomic radius will increase when moving down a group.
Since potassium (K) and calcium (Ca) are farther down the group than argon (Ar) and chlorine (Cl), we conclude that they are the largest atoms in the set. Because Ca is to the right of K, it is slightly smaller than K. As a result, K has the largest atomic radius in the set followed by Ca. Since Ar is to the right of Cl, Cl has a larger atomic radius than Ar.
The decreasing order is K, Ca, Cl, Ar.
Rank the following ions in order of decreasing atomic radius.
Explanation
First, you should see that all four of these ions have the same amount of electrons, resulting in a stable \[Kr\] electron shell. These ions differ, however, by the number of protons in their nuclei. Since the heavier ions have more protons pulling on the same amount of electrons, the atomic radius will be smaller, as the negative electrons are drawn inward toward the positive nucleus.
Sr2+ has the most protons in its nucleus out of this set, so it will have the smallest atomic radius. The atomic radius will increase with decreasing atomic number.
The correct order from largest to smallest is Se2-, Br-, Rb+, and Sr2+.
Rank the following ions in order of decreasing atomic radius.
Explanation
First, you should see that all four of these ions have the same amount of electrons, resulting in a stable \[Kr\] electron shell. These ions differ, however, by the number of protons in their nuclei. Since the heavier ions have more protons pulling on the same amount of electrons, the atomic radius will be smaller, as the negative electrons are drawn inward toward the positive nucleus.
Sr2+ has the most protons in its nucleus out of this set, so it will have the smallest atomic radius. The atomic radius will increase with decreasing atomic number.
The correct order from largest to smallest is Se2-, Br-, Rb+, and Sr2+.
Ionization energy __________ and atomic radius __________ down a group of the periodic table.
decreases . . . increases
increases . . . decreases
increases . . . increases
stays the same . . . decreases
decreases . . . decreases
Explanation
Elements within a group have the same number of valence electrons, but in increasing energy levels. Elements toward the bottom of a group have valence electrons with higher energies in larger orbitals. This results in a larger radius and a weaker attractive force between the nucleus and outer electrons. The ionization energy decreases as the electrons are more removed from the attraction of the nucleus.
When moving down a group, atomic radius increases and ionization energy decreases.
Ionization energy __________ and atomic radius __________ down a group of the periodic table.
decreases . . . increases
increases . . . decreases
increases . . . increases
stays the same . . . decreases
decreases . . . decreases
Explanation
Elements within a group have the same number of valence electrons, but in increasing energy levels. Elements toward the bottom of a group have valence electrons with higher energies in larger orbitals. This results in a larger radius and a weaker attractive force between the nucleus and outer electrons. The ionization energy decreases as the electrons are more removed from the attraction of the nucleus.
When moving down a group, atomic radius increases and ionization energy decreases.
Which of the following correctly describes the trend for atomic radius in the periodic table of elements?
Atomic radius increases with increasing energy level. Atomic radius decreases as new electrons are added within the same orbital.
Atomic radius increases with increasing energy level. Atomic radius increases as new electrons are added within the same orbital.
Atomic radius decreases with increasing energy level. Atomic radius increases as new electrons are added within the same orbital.
Atomic radius increases with increasing energy level. Atomic radius remains constant as new electrons are added within the same orbital.
Atomic radius decreases with increasing energy level. Atomic radius remains constant as new electrons are added within the same orbital.
Explanation
Energy level increases moving down a group of the periodic table. As energy level increases, the outer valence shell becomes more distant from the nucleus, causing atomic radius to increase.
Energy level remains constant across a period, but electrons are added within the same orbitals. When new electrons are added within the same orbital, additional protons are also added to the nucleus. This increases the effective nuclear charge, pulling the electrons closer to the nucleus. The trend for atomic radius is to decrease as we move right along a row.
This means that the general trend for atomic radius is to increase as one moves to the left and downward on the periodic table.
Which of the following correctly describes the trend for atomic radius in the periodic table of elements?
Atomic radius increases with increasing energy level. Atomic radius decreases as new electrons are added within the same orbital.
Atomic radius increases with increasing energy level. Atomic radius increases as new electrons are added within the same orbital.
Atomic radius decreases with increasing energy level. Atomic radius increases as new electrons are added within the same orbital.
Atomic radius increases with increasing energy level. Atomic radius remains constant as new electrons are added within the same orbital.
Atomic radius decreases with increasing energy level. Atomic radius remains constant as new electrons are added within the same orbital.
Explanation
Energy level increases moving down a group of the periodic table. As energy level increases, the outer valence shell becomes more distant from the nucleus, causing atomic radius to increase.
Energy level remains constant across a period, but electrons are added within the same orbitals. When new electrons are added within the same orbital, additional protons are also added to the nucleus. This increases the effective nuclear charge, pulling the electrons closer to the nucleus. The trend for atomic radius is to decrease as we move right along a row.
This means that the general trend for atomic radius is to increase as one moves to the left and downward on the periodic table.