BP Atomic Structure, Quantum Numbers, and Electron Configuration

Question 1

How many protons, neutrons, and electrons are in calcium-39²⁺?

Answer 1

20 protons, 19 neutrons, 18 electrons

Calcium-39²⁺ is a charged isotope of calcium. Its mass number is 39 and it contains 20 protons by definition; therefore, it has 19 neutrons. The number of electrons listed is correct. Neutral calcium has 20 electrons because there are 20 protons. Therefore, Ca²⁺ will have two fewer electrons than protons. Twenty minus 2 is 18 electrons.

Question 2

Which of the following molecular formulas match ferrous and cuprous ions, respectively?

• Fe²⁺ and Cu⁺
• Fe²⁺ and Cu²⁺
• Fe³⁺ and Cu⁺
• Fe³⁺ and Cu³⁺

Answer 2

Fe²⁺ and Cu⁺

If a given element can be found in the form of more than one different cation, its charge will be indicated by a superscript numeral, as in Fe2+ and Fe3+, or using Roman numerals, as in iron(II) oxide vs. iron(III) oxide. Another option is for the ion with the lesser charge may also be indicated with the suffix -ous, and the ion with the greater charge with the suffix -ic, as in ferrous ion and ferric ion. Copper has an oxidation state of +1 and +2 so the less charged ion, Cu+, would be cuprous ion and the more charged ion, Cu2+, would be cupric ion.

Question 3

What happens when an electron jumps from shell n = 2 to shell n = 1?

Answer 3

A photon is emitted; the electron moves to a more stable state

A photon is emitted when an electron jumps from a higher energy shell to a lower energy shell.

When an electron jumps from a higher shell to a lower shell it is moving to a lower energy, more stable state.

Question 4

Which of the following does an absorption spectrum depend on? Select all that apply.

• The specific element
• the movement of electrons from lower energy levels to higher energy levels
• the movement of electrons from higher energy levels to lower energy levels
• none of the above

Answer 4

• The specific element
• the movement of electrons from lower energy levels to higher energy levels

Atomic emission and absorption spectra are unique for each element.

When an electron absorbs energy from any source, it jumps to a higher energy level. An electron can only absorb those wavelengths of light whose photons carry the exact amount of energy to match the energy gap between two levels (these need not be adjacent levels).

As a result of this phenomenon, when a broad spectrum of visible light passes through a particular atom and its electron cloud, only certain wavelengths of this light are absorbed. An absorption spectrum is simply a list of those wavelengths that a particular element or material absorbs, usually presented graphically as the visible light spectrum with absorbed wavelengths denoted with black lines to indicate absorption.

Question 5

Which of the following is true about the energy associated with an electron moving from orbital n = 5 to n = 2? (Note R = 2.18 x 10^-18J)

• 4x10^-19J is absorbed
• 4x10^-20J is absorbed
• 4x10^-17J is emitted
• 4x10^-19J is emitted

Answer 5

4x10^-19J is emitted

Because the electron is moving to a lower energy shell we know energy is being emitted.

Using the Rydberg formula, we can plug in our given values:

2 for the final energy level and 5 for the initial energy level gives us R[1/2² – 1/5²] = R [1/4 – 1/25] = R [21/100]. Now we plug in the value for R (R = 2.18 x 10^-18 J), and solve to get approximately 4.2 x 10^-19 J.

Since the Balmer-Rydberg equation predicts the difference in energy between two energy levels in a hydrogen atom, the negative sign in our answer simply indicates that the n = 2 shell is 4.2 x 10^-19 J lower in energy than the n = 5 shell. We can from this draw the conclusion that this amount of energy must be emitted rather than absorbed.

Question 6

Which of the following situations would NOT occur under the Pauli exclusion principle?

• One electron in the 4s subshell and 5 electrons in the 3d subshell
• 4 electrons in the 2p subshell
• An electron in the 1s subshell with positive half spin and the other with a negative half spin
• 2 electrons in the 1s subshell with positive half spin

Answer 6

2 electrons in the 1s subshell with positive half spin

According to the Pauli exclusion principle, no two electrons in a given atom can have the EXACT same four quantum numbers because, metaphorically speaking, they can’t live in the exact same address. Two electrons in the 1s subshell with positive half spin would have the exact same four quantum numbers: 1, 0, 0, +1/2.

Question 7

Which of the following are possible values for angular momentum quantum numbers in the n = 4 shell?

• +1/2, -1/2
• 0, 1, 2, 3
• -3, -2, -1, 0, 1, 2, 3
• -1, 0, 1

Answer 7

0, 1, 2, 3

The angular momentum quantum number (l), also called the azimuthal quantum number, describes the shape of the orbital. This tells us what subshell the electron is located in WITHIN a shell, where L can range from 0 to n minus 1 for a given principal quantum number. A shell of n = 4 will have angular momentum quantum numbers 0 through 4-1, which includes 0, 1, 2, 3.

Question 8

How many electrons can the n = 2 shell hold?

Answer 8

8

The n = 2 shell is made up of the s subshell and p subshell. The s subshell has one orbital that can hold 2 electrons. The p subshell has 3 orbitals (-1, 0, +1) that can each hold two electrons for a total of 6 electrons in the p subshell. Two electrons from the s subshell plus 6 electrons from the p subshell equal 8 total electrons in the n = 2 shell.

Question 9

Which of the following orbitals CANNOT exist in an s subshell? Select all that apply.

• 0
• -1
• +1
• +2

Answer 9

-1, +1, +2

The question stem asks for orbitals that CANNOT exist. The magnetic quantum number, ML, value ranges from negative L to positive L for a given subshell. Thus, the s subshell (L = 0) has just 1 orbital, where ML is equal to 0, so there is only one possible orientation in space for this subshell.

Question 10

What is the electron configuration of beryllium (Be)?

Answer 10

1s²2s²

Beryllium has 4 protons, as indicated by its atomic number, so neutral beryllium should have 4 electrons. This is the only answer choice that accounts for 4 electrons and is written in proper notation.

Question 11

What is the electron configuration of Mg²⁺?

Answer 11

[Ne]

Mg2+ has two fewer electrons than neutral magnesium, giving the cation form the same electron configuration as neon (Ne). We can abbreviate the electron configuration by placing the last element of the prior row in brackets to represent its electron configuration, and then add the configuration of the valence electrons from the periodic table. Mg2+ has lost its two valence electrons so there is nothing to follow [Ne].

Question 12

How would you expect 5 electrons to be arranged in a p subshell?

Answer 12

Two electron pairs will occupy two orbitals, and the remaining electron will be in the third orbital

According to Hund’s rule, electrons like their personal space, and so one electron fills each orbital of a given subshell, with parallel spin to one another until each is half-filled, and then they begin sharing orbitals, or pairing, with another electron until the orbitals of that subshell are all filled. In this case, the first 3 electrons will each occupy their own orbital and the remaining two electrons will pair with an electron in the first two orbitals. This leaves two of the orbitals completely filled with two electrons each, and the third orbital half-filled with one electron.

Question 13

Which of the following elements have 10 electrons in their d subshell? Select all that apply.

• Zinc (Zn)
• Palladium (Pd)
• Silver (Ag)
• Roentgenium (Rg)

Answer 13

Zinc (Zn)

The electron configuration of zinc is [Ar]4s23d10.

Silver (Ag)

The electron configuration of silver is [Kr]5s14d10. Half-filled and fully-filled are more stable than subshells with any other number of electrons. So, a p subshell is especially happy with 3 or 6 electrons, and a d subshell is especially happy with 5 or 10 electrons, and so forth. What this means for us on a practical level is that this creates a few exceptions to the Aufbau principle for electron configuration, particularly in the chromium and copper columns of the periodic table. We might expect copper to have the electron configuration [Ar]5s24d9, but again that greedy 4d subshell steals an electron from the 5s orbital, resulting in the electron configuration [Ar]5s14d10. Silver is in the same column as copper so it will follow the same principle.

Roentgenium (Rg)

The electron configuration of roentgenium (Rg) is [Rn]7s15f146d10. Half-filled and fully-filled are more stable than subshells with any other number of electrons. So, a p subshell is especially happy with 3 or 6 electrons, and a d subshell is especially happy with 5 or 10 electrons, and so forth. What this means for us on a practical level is that this creates a few exceptions to the Aufbau principle for electron configuration, particularly in the chromium and copper columns of the periodic table. We might expect copper to have the electron configuration [Ar]5s24d9, but again that greedy 4d subshell steals an electron from the 5s orbital, resulting in the electron configuration [Ar]5s14d10. Roentgenium is in the same column as copper so it will follow the same principle.

Question 14

True or false: The highest-energy electrons can be found closest to the nucleus.

Answer 14

FALSE

This statement is false. Electrons in orbitals closest to the nucleus are the lowest in energy and the most stable. In contrast, the farther away an orbital is from the nucleus, the less its electrons experience the attractive forces from nuclear protons, and thus the more energy and less stability they have.

Question 15

List the following ions in order from smallest number of electrons to greatest number of electrons:

nitrate (NO3-), sulfite (SO32-), hypochlorite (ClO-), ferrous iron (Fe2+).

Answer 15

ferrous iron (Fe2+).

hypochlorite (ClO-)

nitrate (NO3-)

sulfite (SO32-)

The correct answer is ferrous iron, hypochlorite, nitrate, and then sulfite. Ferrous iron carries a +2 charge so it has 24 electrons, hypochlorite carries a -1 charge so it has 26 electrons, nitrate carries a -1 charge so it has 32 electrons, and sulfite carries a -2 charge so it has 42 electrons.

Question 16

Which of the following statements is true?

• Some elements in the periodic table have no protons
• Protons and electrons have the same mass and opposite charges
• Neutrons have no charge and essentially no mass
• An electron carries a charge that is equal in magnitude to a proton

Answer 16

An electron carries a charge that is equal in magnitude to a proton

An electron carries an opposite charge that is equal in magnitude to a proton.

Question 17

Which of the following statements are true? Select all that apply.

• Protons and neutrons are found in the nucleus
• Electrons reside with protons in the nucleus in order for the charges to cancel out
• It is unknown exactly where an atom’s electrons are located at any given time
• Electrons make up the bulk of an atom’s weight

Answer 17

Protons and neutrons are found in the nucleus;

It is unknown exactly where an atom’s electrons are located at any given time

Question 18

Which of the following statements accurately describes the relationship between orbital radius and electron energy?

• Electrons in lower energy levels are more tightly bound
• The closer to the nucleus, the lower an energy level an electron is occupying
• Electrons are at higher energy levels closer to the nucleus
• Valence electrons are at the lowest energy levels

Answer 18

The closer to the nucleus, the lower an energy level an electron is occupying

Question 19

Which of the following statements about valence electrons are FALSE? Select all that apply.

• Valence electrons tend to be the most tightly bound electrons in the atom
• Valence electrons interact with the electrons of other atoms
• Valence electrons are found farthest from the nucleus
• Chemical reactions are caused by the interactions of valence electrons between atoms

Answer 19

Valence electrons tend to be the most tightly bound electrons in the atom

The question stem asks us to identify a false statement about valence electrons. Valence electrons are found in the outermost orbital; as such, they are in higher-energy orbitals and tend to be less tightly bound than those at lower energy levels.

Question 20

What happens when an electron moves from orbital n = 1 to orbital n = 3?

Answer 20

The electron becomes excited.

The term n = 1 represents an orbital closest to the nucleus and n = 3 represents an orbital further away from the nucleus. An electron that moves to a higher orbital, which is higher in energy and less stable, is said to have become “excited”.

Question 21

What is the significance of electrons moving between orbitals of distinct quanta?

Answer 21

electrons can only move between orbitals when a precise amount of energy is absorbed or released; an atomic emission spectrum can be modeled using lines to represent the exact amount of energy decrease between orbitals.

When an electron jumps between energy levels, the energy of the photon is emitted or absorbed in discrete amounts called quanta. That means a photon can’t have any amount of energy it wants - it must be an exact amount that we can actually calculate and predict based on which energy level an electron is moving between.

When an electron jumps between energy levels, the energy of the photon is emitted or absorbed in discrete amounts called quanta. That means a photon can’t have any amount of energy it wants - it must be an exact amount that we can actually calculate and predict based on which energy level an electron is moving between. This results in an atomic emission spectrum and an atomic absorption spectrum that are unique for each element. Specifically, an atomic emission spectrum is a pattern of distinct lines that each represent the quantized amount of energy emitted when an electron falls to a lower energy level.

Question 22

Which of the following is true about the energy associated with an electron moving from orbital n = 3 to n = 6? (Note: R = 2.18 x 10^-18 J)

• 2x10^-19J is absorbed
• 5x10^-20J is absorbed
• -2x10^-19J is emitted
• 2x10^-19J is emitted

Answer 22

2x10^-19J is absorbed

According to the Rydberg formula, the energy change associated with electrons moving between orbitals is equal to the Rydberg constant R times one over the square of the initial energy level minus one over the square of the final energy level. Plugging in 3 for the initial energy level and 6 for the final energy level gives us R[1/3² – 1/6²] = R [1/9 – 1/36] = R [3/36]. Now we plug in the value for R (R = 2.18 x 10^-18J) and solve to get approximately 1.8 x 10^-19. Putting this value in proper scientific notation gives us approximately 2 x 10^-19 J. As a final logic check, we know that this process of moving to a higher energy orbital consumes energy so we should end up with a positive value, and we do!

Question 23

Which of the following statements could be true according to the Heisenberg uncertainty principle? Select all that apply.

• An electron is located at coordinates x, y, and z, travelling at 800 m/s
• An electron is located at coordinates x, y, and z, travelling at an unknown velocity
• An electron is traveling at 800 m/s from an unknown location
• An electron is most likely located in the d orbital as it travels at 800 m/s

Answer 23

• An electron is located at coordinates x, y, and z, travelling at an unknown velocity
• An electron is traveling at 800 m/s from an unknown location
• An electron is most likely located in the d orbital as it travels at 800 m/s

According to the Heisenberg uncertainty principle, it is impossible to precisely know both the exact location and the exact momentum of an electron at any given moment in time. In this case we know the precise location of the electron, represented by coordinates x, y and z, so we know very little about its velocity.

According to the Heisenberg uncertainty principle, it is impossible to precisely know both the exact location and the exact momentum of an electron at any given moment in time. In this case we know the precise velocity of the electron, so we cannot know its precise location. All we know is roughly which orbital the electron is in.

Question 24

Which of the following could NOT represent the locations of two electrons in an atom? Select all that apply.

• 1, 0, 0, +1/2 and 1, 0, 0, +1/2
• 1, 0, 2, -1/2 and 1, 0, 2, +1/2
• 3, 1,0, +1/2 and 3, 1,0, -1/2
• 1, 0, 0, +1/2 and 3, 1, 0, -1/2

Answer 24

• 1, 0, 0, +1/2 and 1, 0, 0, +1/2
• 1, 0, 2, -1/2 and 1, 0, 2, +1/2

The question stem asks us to identify a set of quantum numbers that could NOT possibly identify the location of two electrons in an atom. This answer choice specifies the exact same quantum number for both electrons. According to the Pauli exclusion principle, no two electrons in a given atom can have the EXACT same four quantum numbers because they can’t occupy the exact same space at the exact same time.

The question stem asks us to identify a set of quantum numbers that could NOT possibly identify the location of two electrons in an atom. While these numbers would satisfy the Pauli exclusion principle, it is not possible to have a magnetic quantum number that is larger than the azimuthal quantum number. Therefore, this set of quantum numbers does NOT describe a possible location for electrons in an atom.

Question 25

True or false: The number of electrons that a given subshell can accommodate is dependent on the orbital the electron is in.

Answer 25

False

This statement is false. Each subshell can hold up to two electrons, regardless of the energy level, subshell shape, or subshell orientation.

Question 26

How many orbitals exist for a shell of n = 2?

Answer 26

4

A shell of n = 2 is comprised of 0 to n-1 subshells (L). 2 - 1 = 1, so the subshells contained in an n = 2 shell are subshell L = 0 (s subshell) and L = 1 (p subshell). The s subshell contains a single orbital, 0. The p subshell contains 3 orbitals: -1, 0, +1. 1 orbital + 3 orbitals = 4 orbitals.

Question 27

Match the following sets of quantum numbers with their most accurate description in terms of shells and subshells.

Answer 27

Question 28

Which of the following depicts the electron configuration of neutral fluorine?

Answer 28

1s²2s²2p⁵

The atomic number of fluorine is 9, which means there are 9 protons. Because we are talking about a neutral fluorine atom, there must also be 9 electrons. This electron configuration describes two electrons in the 1s subshell, two electrons in the 2s subshell, and five electrons in the 2p subshell. We know that lower energy subshells are filled first, which aligns with both s subshells being filled here (s subshells only have one orbital that can hold a maximum of two electrons). That accounts for four electrons in the s subshells with the remaining five electrons filling the 2p subshell.

Question 29

Match the following elements with their correct electron configuration.

Answer 29

Question 30

Which atom correctly matches following electron configuration: [Ar]4s²3d¹⁰4p¹?

Answer 30

Ga

Question 31

What is the correct electron configuration for the transition metal molybdenum (Mo)?

Answer 31

[Kr]5s¹4d⁵

The correct electron configuration for molybdenum is [Kr] 5s14d5, not [Kr] 5s24d4. Exceptions to the classic rules of electron configuration stem from the idea that half-filled and fully-filled subshells are more stable than subshells filled with some other number of electrons. For a p subshell, then, it is energetically favorable to contain either three or six electrons; similarly, a d subshell will be especially stable if it contains either five or ten electrons. The exceptions to understand for the MCAT are the transition metals in the same groups, or columns, as chromium (Cr) and copper (Cu). Specifically, these constitute exceptions to the Aufbau principle. We would expect the shortened configuration of chromium to be [Ar] 4s23d4. However, if the 3d orbital “steals” one electron from the 4s orbital, it will achieve that coveted half-filled state, with five total 3d electrons. The electron configuration of chromium is thus [Ar] 4s13d5. This means that the correct electron configuration for molybdenum should be [Kr] 5s14d5, not [Kr] 5s24d4 because molybdenum is in the same group as chromium.

Question 32

How can the electron configuration of Na⁺ be described compared to Na?

Answer 32

The electron configuration of Na⁺ will have one fewer electron in the 3s orbital.

Neutral sodium has an electron configuration of [Ne] 3s¹, while Na⁺ has an electron configuration identical to that of neon (Ne). This means that there is one fewer electron in the 3s orbital. When forming an ion, we always add or remove electrons from the subshell with the highest energy level.

Question 33

True or false: When filling orbitals, electrons will first spread out to fill every orbital in the subshell with one electron per orbital.

Answer 33

TRUE

This statement is true. According to Hund’s rule, electrons like their personal space, and so one electron fills each orbital of a given subshell, with parallel spin to one another until each is half-filled, and then they begin sharing orbitals, or pairing, with another electron until the orbitals of that subshell are all filled.

Question 34

Which of the following will have a half empty s subshell in order to have a completely full or half full d subshell? Select all that apply.

• Molybdenum (Mo)
• Iron (Fe)
• Osmium (Os)
• Gold (Au)

Answer 34

Molybdenum (Mo); Gold (Au)

This is a correct answer because molybdenum is in the same group as chromium. Exceptions to the classic rules of electron configuration stem from the idea that half-filled and fully-filled subshells are more stable than subshells filled with some other number of electrons. For a p subshell, then, it is energetically favorable to contain either three or six electrons; similarly, a d subshell will be especially stable if it contains either five or ten electrons. The exceptions to understand for the MCAT are the transition metals in the same groups, or columns, as chromium (Cr) and copper (Cu). Specifically, these constitute exceptions to the Aufbau principle. We would expect the shortened configuration of chromium to be [Ar] 4s23d4. However, if the 3d orbital “steals” one electron from the 4s orbital, it will achieve that coveted half-filled state, with five total 3d electrons. The electron configuration of chromium is thus [Ar] 4s13d5.

This is a correct answer because gold is in the same group as copper. Exceptions to the classic rules of electron configuration stem from the idea that half-filled and fully-filled subshells are more stable than subshells filled with some other number of electrons. For a p subshell, then, it is energetically favorable to contain either three or six electrons; similarly, a d subshell will be especially stable if it contains either five or ten electrons. The exceptions to understand for the MCAT are the transition metals in the same groups, or columns, as chromium (Cr) and copper (Cu). Specifically, these constitute exceptions to the Aufbau principle. We would expect the shortened configuration of copper to be [Ar] 4s23d9. However, if the 3d orbital “steals” one electron from the 4s orbital, it will achieve that coveted filled state, with ten total 3d electrons. The electron configuration of copper is thus [Ar] 4s13d10.

Question 35

According to the periodic table, how many valence electrons does nitrogen have?

Answer 35

5

According to the electron configuration expected for nitrogen from its position in the periodic table, nitrogen will have two electrons in the 1s subshell, two electrons in the 2s subshell and three electrons in the 2p orbital. For elements with p subshells, the electrons in their s and p subshell with the highest principal quantum number are valence electrons. This means we should only count electrons from the 2s and 2p subshell as valence electrons for carbon. Two electrons from the 2s subshell and three electrons from the 2p subshell make for a total of five valence electrons.

Question 36

What happens to fluorine when it gains a proton?

Answer 36

It becomes a new element.

Question 37

How many protons, neutrons, and electrons are in calcium-38?

Answer 37

20 protons, 18 neutrons, 20 electrons

Calcium-38 is an isotope of calcium. Its mass number is 38 and it contains 20 protons by definition; therefore, it has 18 neutrons. The number of electrons listed is correct. Neutral calcium has 20 electrons because there are 20 protons. There is no charge on calcium-38, so there should be 20 electrons.

Question 38

What happens when an electron jumps from shell n = 1 to shell n = 2?

Answer 38

The electron becomes excited; energy is absorbed