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AP Chemistry Help: Atomic Structure And Electron Configuration

Review real example questions for Atomic Structure And Electron Configuration in AP Chemistry.

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A neutral atom has atomic number 24. Which of the following is the correct ground-state electron configuration for this atom?

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Question 1

A neutral atom has atomic number 24. Which of the following is the correct ground-state electron configuration for this atom?

  1. [Ar] 4s2 3d4[\mathrm{Ar}]\,4s^2\,3d^4
  2. [Ar] 4s1 3d5[\mathrm{Ar}]\,4s^1\,3d^5 (correct answer)
  3. [Ar] 4s2 3d5[\mathrm{Ar}]\,4s^2\,3d^5
  4. [Ar] 4s0 3d6[\mathrm{Ar}]\,4s^0\,3d^6
  5. [Ar] 4s1 3d4 4p1[\mathrm{Ar}]\,4s^1\,3d^4\,4p^1

Explanation: This question tests the skill of atomic structure and electron configuration. Atomic number 24 means chromium with 24 electrons, but it is an exception to the Aufbau principle for stability. Normally, electrons would fill 4s² 3d⁴, but chromium prefers half-filled d subshell, so it is 4s¹ 3d⁵. This configuration provides extra stability due to the half-filled subshells. A tempting distractor like A, [Ar] 4s² 3d⁴, is incorrect because it ignores the exception for chromium and copper in the periodic table. Remember exceptions for elements like Cr and Cu where half-filled or fully filled d subshells are preferred for stability.

Question 2

A neutral atom has the electron configuration [He] 2s2 2p4[\mathrm{He}]\,2s^2\,2p^4. How many unpaired electrons are present in the atom's ground state?

  1. 0
  2. 1
  3. 2 (correct answer)
  4. 3
  5. 4

Explanation: This question tests the skill of atomic structure and electron configuration. The configuration [He] 2s² 2p⁴ is oxygen with 8 electrons. In 2p⁴, Hund's rule places electrons in three orbitals: two paired in one, and two unpaired in others. This results in two unpaired electrons total. The pairing minimizes energy while following rules. A tempting distractor like A, 0, is incorrect as it assumes all paired, ignoring Hund's distribution. Use orbital diagrams to visualize electron pairing and count unpaired electrons in p subshells.

Question 3

An ion Q3+Q^{3+} is formed from an element with atomic number 13. Which electron configuration corresponds to Q3+Q^{3+} in the ground state?

  1. 1s2 2s2 2p6 3s21s^2\,2s^2\,2p^6\,3s^2
  2. 1s2 2s2 2p6 3s2 3p11s^2\,2s^2\,2p^6\,3s^2\,3p^1
  3. 1s2 2s2 2p61s^2\,2s^2\,2p^6 (correct answer)
  4. 1s2 2s2 2p5 3s21s^2\,2s^2\,2p^5\,3s^2
  5. 1s2 2s2 2p6 3s11s^2\,2s^2\,2p^6\,3s^1

Explanation: This question tests the skill of atomic structure and electron configuration. Atomic number 13 is aluminum with 13 electrons neutrally, but Q³⁺ loses 3, leaving 10. Ions form by removing electrons from outermost orbitals, here 3s and 3p. The resulting configuration is 1s² 2s² 2p⁶, matching neon's. A tempting distractor like E, 1s² 2s² 2p⁶ 3s¹, is incorrect as it retains an electron in 3s, not fully ionizing to 3+. For cations, remove electrons from valence shells first and verify total matches atomic number minus charge.

Question 4

A neutral atom has atomic number 12. Which of the following is the correct ground-state electron configuration for this atom?

  1. 1s2 2s2 2p6 3s21s^2\,2s^2\,2p^6\,3s^2 (correct answer)
  2. 1s2 2s2 2p6 3s1 3p11s^2\,2s^2\,2p^6\,3s^1\,3p^1
  3. 1s2 2s2 2p5 3s2 3p11s^2\,2s^2\,2p^5\,3s^2\,3p^1
  4. 1s2 2s2 2p6 3p21s^2\,2s^2\,2p^6\,3p^2
  5. 1s2 2s2 2p6 4s21s^2\,2s^2\,2p^6\,4s^2

Explanation: This question tests your understanding of atomic structure and electron configuration. A neutral atom with atomic number 12 has 12 electrons. Electrons fill orbitals in order: 1s² (2), 2s² (4), 2p⁶ (10), and 3s² (12 total). This configuration for magnesium follows the Aufbau principle without exceptions. A tempting distractor is choice E (1s² 2s² 2p⁶ 4s²), which skips the 3s orbital, but 3s fills before 4s. Count the total electrons and fill subshells in sequence: 1s, 2s, 2p, 3s, 3p, 4s, etc.

Question 5

A neutral atom has atomic number 20. Which of the following is the correct ground-state electron configuration for this atom?

  1. [Ar] 4s2[\mathrm{Ar}]\,4s^2 (correct answer)
  2. [Ar] 3d2[\mathrm{Ar}]\,3d^2
  3. [Ne] 3s2 3p6 4s2[\mathrm{Ne}]\,3s^2\,3p^6\,4s^2
  4. [Ar] 4s2 4p2[\mathrm{Ar}]\,4s^2\,4p^2
  5. [Ar] 4s1 3d1[\mathrm{Ar}]\,4s^1\,3d^1

Explanation: This question tests your understanding of atomic structure and electron configuration. A neutral atom with atomic number 20 has 20 electrons. Electrons fill up to [Ar][\mathrm{Ar}] (18 electrons), then 4s24s^2 for the remaining 2, resulting in [Ar] 4s2[\mathrm{Ar}]\,4s^2 for calcium. This follows the standard order without exceptions. A tempting distractor is choice C ([Ne] 3s2 3p6 4s2[\mathrm{Ne}]\,3s^2\,3p^6\,4s^2), which expands Ar correctly but is less concise than using [Ar]. Start with the nearest noble gas core and add valence electrons for simplified configurations.

Question 6

A neutral atom has the electron configuration [Kr] 5s2 4d5[\text{Kr}]\,5s^2\,4d^5. How many unpaired electrons are present in the atom's ground state?​​

  1. 1
  2. 3
  3. 5 (correct answer)
  4. 7
  5. 0

Explanation: This question involves atomic structure and electron configuration, specifically counting unpaired electrons. The configuration [Kr]5s²4d⁵ shows a filled 5s subshell and a half-filled 4d subshell. The d subshell has 5 orbitals, and with 5 electrons following Hund's rule, each electron occupies a separate orbital with parallel spins. This results in 5 unpaired electrons in the 4d subshell (the 5s electrons are paired). The tempting distractor B (3 unpaired) might come from incorrectly pairing some 4d electrons, but Hund's rule requires maximum unpaired electrons before pairing begins. To count unpaired electrons, examine each partially filled subshell and apply Hund's rule to determine electron distribution.

Question 7

A neutral atom has electron configuration [Ne] 3s2 3p4[\text{Ne}]\,3s^2\,3p^4. Which subshell contains the highest-energy electron in the ground state of this atom?

  1. 2p
  2. 3s
  3. 3p (correct answer)
  4. 2s
  5. 1s

Explanation: This question requires understanding atomic structure and electron configuration to identify the highest-energy occupied orbital. The configuration [Ne]3s²3p⁴ shows electrons filling through the 3p subshell. In multi-electron atoms, energy increases with both principal quantum number (n) and orbital type, following the order: 1s < 2s < 2p < 3s < 3p < 4s < 3d, etc. Since 3p is the last (highest) occupied subshell in this configuration, it contains the highest-energy electrons. The tempting distractor B (3s) is incorrect because 3s electrons are lower in energy than 3p electrons within the same principal energy level. To find the highest-energy electrons, identify the last subshell written in the electron configuration.

Question 8

A neutral atom has the electron configuration [Ar] 4s23d104p5[\mathrm{Ar}]\,4s^23d^{10}4p^5. Which of the following is the electron configuration of the ion formed when this atom gains one electron to form a $1-$ ion?

  1. [Ar] 4s23d104p6[\mathrm{Ar}]\,4s^23d^{10}4p^6 (correct answer)
  2. [Ar] 4s23d104p54d1[\mathrm{Ar}]\,4s^23d^{10}4p^54d^1
  3. [Ar] 4s13d104p6[\mathrm{Ar}]\,4s^13d^{10}4p^6
  4. [Ar] 4s23d114p5[\mathrm{Ar}]\,4s^23d^{11}4p^5
  5. [Ar] 4s23d94p6[\mathrm{Ar}]\,4s^23d^94p^6

Explanation: This question involves atomic structure and electron configuration of ions. The neutral atom has configuration [Ar]4s²3d¹⁰4p⁵, which means it has 35 electrons (18 from Ar + 2 + 10 + 5). When this atom gains one electron to form a 1- ion, the electron enters the lowest available orbital, which is the 4p subshell. The 4p subshell can hold 6 electrons total, so 4p⁵ becomes 4p⁶, giving [Ar]4s²3d¹⁰4p⁶. Option B incorrectly places the extra electron in the 4d orbital, which is higher in energy than completing the 4p subshell. Remember that electrons always fill the lowest energy orbital available first.

Question 9

A neutral atom has atomic number 29. Which of the following is the correct ground-state electron configuration for the atom?

  1. [Ar] 4s1 3d10[\mathrm{Ar}]\,4s^1\,3d^{10} (correct answer)
  2. [Ar] 4s2 3d10[\mathrm{Ar}]\,4s^2\,3d^{10}
  3. [Ar] 4s2 3d9[\mathrm{Ar}]\,4s^2\,3d^9
  4. [Ar] 4s1 3d9 4p1[\mathrm{Ar}]\,4s^1\,3d^9\,4p^1
  5. [Ar] 4s0 3d11[\mathrm{Ar}]\,4s^0\,3d^{11}

Explanation: This question tests the skill of atomic structure and electron configuration. Atomic number 29 is copper with 29 electrons, an exception for stability. Normally [Ar] 4s² 3d⁹, but it prefers full d subshell, so [Ar] 4s¹ 3d¹⁰. This half-filled s and full d provide extra stability. A tempting distractor like A, [Ar] 4s² 3d⁹, is incorrect because it follows strict Aufbau without the exception. Recall exceptions for Cu and Cr where electron promotion stabilizes the configuration.

Question 10

A neutral atom has atomic number 12. Which of the following is the correct ground-state electron configuration?

  1. 1s2 2s2 2p6 3s21s^2\,2s^2\,2p^6\,3s^2 (correct answer)
  2. 1s2 2s2 2p6 3p21s^2\,2s^2\,2p^6\,3p^2
  3. 1s2 2s2 2p5 3s2 3p11s^2\,2s^2\,2p^5\,3s^2\,3p^1
  4. 1s2 2s2 2p6 3s1 3p11s^2\,2s^2\,2p^6\,3s^1\,3p^1
  5. 1s2 2s2 2p6 4s21s^2\,2s^2\,2p^6\,4s^2

Explanation: This question tests the skill of atomic structure and electron configuration. Atomic number 12 is magnesium with 12 electrons, filling up to 3s23s^2 after [Ne]. Orbitals are filled in order: 1s, 2s, 2p, 3s, with s subshells holding 2 electrons each. The configuration is 1s22s22p63s21s^2 2s^2 2p^6 3s^2, totaling 12 electrons. A tempting distractor like D, 1s22s22p63p21s^2 2s^2 2p^6 3p^2, is incorrect as it places electrons in 3p before fully filling 3s, violating Aufbau. Count total electrons and ensure orbitals are filled in the correct energy order for ground state.