MCAT Physical : Atoms, Elements, and the Periodic Table
Study concepts, example questions & explanations for MCAT Physical
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Example Questions
Example Question #1 : Subatomic Particles And Elemental Properties
Which given element has the highest electronegativity?
Titanium
Arsenic
Phosphorus
Oxygen
Sodium
Oxygen
Electronegativity increases from the bottom to the top, and from the left to the right of the periodic table. Of the choices given, oxygen is closest to the top right corner.
Example Question #1 : Subatomic Particles And Elemental Properties
Which of the following has the largest radius?
The answer is 
Example Question #2 : Subatomic Particles And Elemental Properties
Which of these elements has an atomic radius that is smaller than its ionic radius?
Chlorine is the only nonmetal listed which would gain electrons to ionize. For a nonmetal, becoming an ion usually means gaining electron(s), and therefore means an increase in radius. Therefore, since 
Example Question #1 : Subatomic Particles And Elemental Properties
Which of these pairs of elements are most chemically similar?
Chlorine and xenon
Hydrogen and helium
Copper and cobalt
Calcium and barium
Magnesium and potassium
Calcium and barium
The most chemically similar elements can are in the same family on the periodic table. Calcium and barium are both in group II of the periodic table. In all other answer choices, the two elements listed are in different families and are therefore chemically dissimilar.
Example Question #1 : Ions And Isotopes
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 studying the reaction above is surprised to find traces of Xe-124 in his analysis of the reaction. How is Xe-124 different from Xe-135?
Xe-124 has 11 fewer electrons
Xe-124 has 11 fewer positrons
Xe-135 is a more massive ion than Xe-124
Xe-124 has 11 fewer protons
Xe-135 is a more massive isotope than Xe-124
Xe-135 is a more massive isotope than Xe-124
Xe-135 and Xe-124 are isotopes of xenon, not ions. Ions have charges, while isotopes have varying atomic mass numbers; thus, Xe-135 is a more massive isotope of xenon than is Xe-124.
Note that this change in mass can only be attributed to neutron numbers. Changing proton number would alter the elemental identity. Changing electron number would not alter the mass, and would create a charge discrepancy.
Example Question #2 : Ions And Isotopes
The isotope 
78 . . . 78 . . . 53
53 . . . 131 . . . 53
131 . . . 53 . . . 131
53 . . . 78 . . . 53
53 . . . 78 . . . 78
53 . . . 78 . . . 53
The lower number, the atomic number, indicates the number of protons. The upper number, the mass number, indicates the sum of protons and neutrons. The number of neutrons would be equal to the number of protons subtracted from the total mass number. In a neutral atom, the number of protons and electrons is equal in order to balance charge.
The atomic number of iodine is 53, meaning there must be 53 protons. The total mass number is 131.
The remaining mass must come from neutrons; thus there are 78 neutrons. Since the atom is neutral, the electrons will balance the protons. In total, there are 53 protons, 78 neutrons, and 53 electrons.
Example Question #1 : Ions And Isotopes
Which of the following combination of particles accurately describes the above element?
29 protons, 64 neutrons, 35 electrons
29 protons, 35 neutrons, 35 electrons
29 protons, 64 neutrons, 29 electrons
29 protons, 35 neutrons, 29 electrons
29 protons, 93 neutrons, 29 electrons
29 protons, 35 neutrons, 29 electrons
The lower number next to the atomic symbol gives the number of protons, 29. The upper number gives the number of protons plus neutrons, so the number of neutrons is given by the difference between upper and lower numbers: 64 - 29 = 35. Finally, this is a neutral isotope since no net charge is noted, so the number of electrons must equal the number of protons, 29.
Example Question #4 : Ions And Isotopes
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 a neutron to flourine
Adding a proton to flourine
Adding an electron to flourine
Removing an electron from flourine
Removing a neutron from flourine
Adding an electron to flourine
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.
Example Question #1 : Ions And Isotopes
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 is a nuclear reaction, as described in the passage, different from a chemcial reaction?
Nuclear reactions frequently involve changes in atomic number; chemical reactions do not
Nuclear reactions involve the release and absorption of energy; chemical reactions do not
Chemical reactions result in an increase in the entropy of the universe; nuclear reactions do not
Nuclear reactions frequently involve changes in electric charge; chemical reactions do not
Chemical reactions frequently involve changes in atomic number; nuclear reactions do not
Nuclear reactions frequently involve changes in atomic number; chemical reactions do not
Nuclear reactions are unique because they can involve the change of one atom to another, by changing the atomic number. Chemical reactions, by and large, keep atomic number constant, but often vary the ionic state of atoms. In this way, chemical reactions can be thought of as generally involving electrons, while nuclear reactions generally involve protons and neutrons.
Example Question #1 : Ions And Isotopes
An isotope of an element __________.
- has all the chemical characteristics of the element
- can be stable, or non-radioactive
- is designated by the atomic number of the element
- contributes to the atomic mass of the element, as shown on the periodic table, in proportion to its abundance
- all of the above
4
1
2
5
3
5
The correct answer is 5. An isotope is a form of an element with a different number of neutrons in the nucleus. Because the proton number (atomic number) is the same, the electron configuration is the same and therefore the atom behaves chemically in an identical manner to other atoms of the same element. Many isotopes are stable, and indeed that is how elements such as chlorine come to have average atomic masses in between integers; chlorine at 35.45 g/mole is a weighted average of the two stable isotopes, chlorine-35 and chlorine-37. Unstable isotopes of any element undergo decay at various rates. Unless they persist in some quantifiable amount, they aren’t really “there” to participate in the weighted average of an atomic mass.
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