2.7 - Ground-state & Excited-state Conversions

1.

An atom in the ground state has 39 protons and 50 neutrons. How many electrons are in each shell in this atom in the ground state?

A.

2 electrons in the first shell, 8 electrons in the second shell, 18 electrons in the third shell, 32 electrons in the fourth shell, 50 electrons in the fifth shell
B.

2 electrons in the first shell, 8 electrons in the second shell, 18 electrons in the third shell, 9 electrons in the fourth shell, 2 electrons in the fifth shell
C.

2 electrons in the first shell, 8 electrons in the second shell, 18 electrons in the third shell, 32 electrons in the fourth shell, 2 electrons in the fifth shell
D.

2 electrons in the first shell, 8 electrons in the second shell, 18 electrons in the third shell, 9 electrons in the fourth shell

Correct Answer
B. 2 electrons in the first shell, 8 electrons in the second shell, 18 electrons in the third shell, 9 electrons in the fourth shell, 2 electrons in the fifth shell

Explanation
Based on the atomic number, be sure to remember that the number of electrons in each shell is found from the ground-state electron configuration on the Periodic Table. Notice the question does NOT say anything about filled electron shells.

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2.

Which electron configuration represents an atom of selenium in an excited state?

A.

2-7-18-7
B.

2-7-17-4
C.

2-8-18-6
D.

2-7-17-7

Correct Answer
A. 2-7-18-7

Explanation
Excited-state configurations have the same number of electrons as the ground-state electron configuration shown on the Periodic Table, but you DO NOT have the same number of electrons in each level as the ground-state electron configuration does. The most useful approach for this question is to add up the number of electrons in the configuration to find if it matches the element of interest, based on the atomic number. If it DOES match the atomic number, then compare it to the ground-state configuration on the Periodic Table to see if it matches. If it does NOT match the ground-state configuration, but has the same total number of electrons, it is an excited-state configuration. Also, note that the 2n^2 rule can NEVER be exceeded. Choices like "2-7-19-13-1" and "2-6-19-14-1" exceed the 2n^2 rule for n=3, where the maximum number of electrons in the n=3 level is 2(3)^2=18.

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3.

An atom has an electron configuration of 2-6-18-18-5. How do the energy and charge of the electrons in the first and fourth shells of this atom compare to one another?

A.

The energy of the electrons in the first shell are less than that of the electrons in the fourth shell. The charge of the electrons in the first shell is less than that of electrons in the fourth shell.
B.

The energy of the electrons in the first shell are less than that of the electrons in the fourth shell. The charge of the electrons in the first shell and the fourth shell are the same.
C.

The energy of the electrons in the first shell are the same as that of the electrons in the fourth shell. The charge of the electrons in the first shell and the fourth shell are also the same.
D.

The energy of the electrons in the first shell are the same as that of the electrons in the fourth shell. The charge of the electrons in the first shell is less than that of electrons in the fourth shell.

Correct Answer
B. The energy of the electrons in the first shell are less than that of the electrons in the fourth shell. The charge of the electrons in the first shell and the fourth shell are the same.

Explanation
Remember: Higher electron shells (greater value of n)=greater distance from the nucleus AND greater amount of energy. However, electrons will ALWAYS have the same charge of -1, regardless of their location.

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4.

What happens to electron energy and electron location when an atom changes from the ground state to the excited state?

A.

The electron energy increases in moving from the ground state to the excited state, while the electron moves closer to the nucleus in moving to the excited state.
B.

The electron energy decreases in moving from the ground state to the excited state, while the electron moves further from the nucleus in moving to the excited state.
C.

The electron energy increases in moving from the ground state to the excited state, while the electron moves further from the nucleus in moving to the excited state.
D.

The electron energy decreases in moving from the ground state to the excited state, while the electron moves closer to the nucleus in moving to the excited state.

Correct Answer
C. The electron energy increases in moving from the ground state to the excited state, while the electron moves further from the nucleus in moving to the excited state.

Explanation
Remember: Excited states are higher than ground states are, in terms of energy. When an electron moves to a higher electron shell, it moves further from the nucleus.

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5.

An atom in an excited state has an electron configuration of 2-7-14-3. What is the electron configuration of this atom in the ground state?

A.

2-8-14-3
B.

2-8-14-2
C.

2-7-15-2
D.

2-8-15-1

Correct Answer
B. 2-8-14-2

Explanation
Remember: To answer this type of question, add up the number of electrons. This total number of electrons equals the number of protons and the atomic number. Based on the atomic number, find the element and find its ground-state electron configuration in the element box.

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6.

An atom in an excited state has an electron configuration of 2-7-15-4. Why is this atom electrically neutral?

A.

Because the maximum number of electrons are present in each energy level.
B.

Because the number of protons and the number of electrons are equivalent.
C.

Because the number of electrons in each energy level is less than the maximum amount possible.
D.

Because the number of electrons in the electron shells do not match the ground-state electron configuration.

Correct Answer
B. Because the number of protons and the number of electrons are equivalent.

Explanation
Remember: Electrically neutral means that the atom has an overall charge of zero. To get this overall charge of zero, the number of protons and the number of electrons must be equal. The fact that the atom is in an excited state is unrelated to electrical neutrality of an atom.

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7.

An atom in an excited state has an electron configuration of 2-7-17-16-3. How do the second, third, and fifth shells demonstrate that this atom is in an excited state?

A.

This configuration does NOT match 2-8-18-15-2, which is the ground-state configuration for an atom with an atomic number of 45.
B.

This configuration does NOT match 2-8-18-15-1, which is the ground-state configuration for an atom with an atomic number of 44.
C.

This configuration does NOT match 2-8-18-16-1, which is the ground-state configuration for an atom with an atomic number of 45.
D.

This configuration does NOT match 2-8-18-18, which is the ground-state configuration for an atom with an atomic number of 46.

Correct Answer
C. This configuration does NOT match 2-8-18-16-1, which is the ground-state configuration for an atom with an atomic number of 45.

Explanation
Remember: To solve this kind of problem, you must first add up the electrons to find the total number of electrons. From the total number of electrons, you can find the number of protons, the atomic number, and the ground-state electron configuration from the element box. To prove that parts of an electron configuration demonstrate that the atom is in an excited state, the answer must include the ground-state configuration, the atomic number, and the fact that the configuration written in the answer is the ground-state electron configuration to begin with.

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8.

Which of the following represents an atom of molybdenum in an excited state?

A.

2-8-18-12-2
B.

2-7-19-13-1
C.

2-6-19-14-1
D.

2-7-18-13-1

Correct Answer
A. 2-8-18-12-2

Explanation
Excited-state configurations have the same number of electrons as the ground-state electron configuration shown on the Periodic Table, but you DO NOT have the same number of electrons in each level as the ground-state electron configuration does. The most useful approach for this question is to add up the number of electrons in the configuration to find if it matches the element of interest, based on the atomic number. If it DOES match the atomic number, then compare it to the ground-state configuration on the Periodic Table to see if it matches. If it does NOT match the ground-state configuration, but has the same total number of electrons, it is an excited-state configuration. Also, note that the 2n^2 rule can NEVER be exceeded. Choices like "2-7-19-13-1" and "2-6-19-14-1" exceed the 2n^2 rule for n=3, where the maximum number of electrons in the n=3 level is 2(3)^2=18.

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9.

An atom in an excited state has an electron configuration of 2-8-17-2. What is the electron configuration of this atom in the ground state?

A.

2-8-18-1
B.

2-8-18-2
C.

2-8-18-1-1
D.

2-8-18-3

Correct Answer
A. 2-8-18-1

Explanation
Remember: To answer this type of question, add up the number of electrons. This total number of electrons equals the number of protons and the atomic number. Based on the atomic number, find the element and find its ground-state electron configuration in the element box.

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10.

An atom in an excited state has an electron configuration of 2-7-12-2. What is the number of protons and the identity of this element?

A.

22, the identity of the element is V
B.

23, the identity of the element is Ti
C.

23, the identity of the element is V
D.

22, the identity of the element is Ti

Correct Answer
C. 23, the identity of the element is V

Explanation
Remember: To answer this type of question, add up the number of electrons. This total number of electrons equals the number of protons and the atomic number. Based on the atomic number, find the element and find its ground-state electron configuration in the element box.

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