Atomic Structure and Electron Configuration - AP Chemistry

A neutral atom has atomic number 29. Which of the following is the correct ground-state electron configuration for the atom?
Answer: $[\mathrm{Ar}]\,4s^1\,3d^{10}$ 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.
A neutral atom has atomic number 24. Which of the following is the correct ground-state electron configuration for this atom?
Answer: $[\mathrm{Ar}]\,4s^1\,3d^5$ 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.
An ion $X^{2+}$ has atomic number 30. Which of the following is the correct electron configuration for $X^{2+}$ in its ground state?
Answer: $[\mathrm{Ar}]\,4s^03d^{10}$ This problem requires understanding atomic structure and electron configuration of cations. An atom with atomic number 30 has 30 electrons when neutral, but X²⁺ has lost 2 electrons, leaving 28 electrons. The neutral atom's configuration is [Ar]4s²3d¹⁰. When forming cations, electrons are removed from the highest energy orbital first, which for transition metals means 4s electrons are removed before 3d electrons. Therefore, X²⁺ has configuration [Ar]4s⁰3d¹⁰ or simply [Ar]3d¹⁰. Option C incorrectly removes electrons from the 3d orbital instead of 4s, giving only 26 electrons total. Always remove electrons from the outermost shell (highest n value) first when forming cations.
A neutral atom has atomic number 12. Which of the following is the correct ground-state electron configuration?
Answer: $1s^2\,2s^2\,2p^6\,3s^2$ This question tests the skill of atomic structure and electron configuration. Atomic number 12 is magnesium with 12 electrons, filling up to $3s^2$ after [Ne]. Orbitals are filled in order: 1s, 2s, 2p, 3s, with s subshells holding 2 electrons each. The configuration is $1s^2 2s^2 2p^6 3s^2$ , totaling 12 electrons. A tempting distractor like D, $1s^2 2s^2 2p^6 3p^2$ , 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.
A neutral atom has atomic number 9. Which of the following is the correct ground-state electron configuration?
Answer: $1s^2\,2s^2\,2p^5$ This question tests the skill of atomic structure and electron configuration. Atomic number 9 is fluorine with 9 electrons, filling up to $2p^5$ . Orbitals fill: $1s^2$ , $2s^2$ , then $2p^5$ , as p holds up to 6 but has only 5 here. Total is $2 + 2 + 5 = 9$ . A tempting distractor like B, $1s^2 2s^2 2p^6$ , is incorrect as it has 10 electrons, representing neon. Always verify the total electron count matches the atomic number for neutral atoms before assigning configurations.
An ion $X^{2+}$ is formed from an element with atomic number 26. Which electron configuration corresponds to $X^{2+}$ in the ground state?
Answer: $[\mathrm{Ar}]\,4s^0\,3d^6$ This question tests the skill of atomic structure and electron configuration. The element with atomic number 26 is iron, with 26 electrons in its neutral state, but the X²⁺ ion has lost 2 electrons, leaving 24. For transition metals, electrons are removed first from the 4s orbital before the 3d, following the ion configuration rules. Thus, from neutral [Ar] 4s² 3d⁶, the ion becomes [Ar] 3d⁶. A tempting distractor like A, [Ar] 4s² 3d⁴, is incorrect as it assumes removal from 3d first, ignoring the actual ionization order. To determine ion configurations, subtract electrons from the outermost s orbital first for transition metals.
A neutral atom has the electron configuration $[\mathrm{Ar}]\,4s^23d^{10}4p^5$ . Which of the following is the electron configuration of the ion formed when this atom gains one electron to form a $1-$ ion?
Answer: $[\mathrm{Ar}]\,4s^23d^{10}4p^6$ 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.
An ion $T^{+}$ is formed from a neutral atom with electron configuration $[\mathrm{Kr}]\,5s^2\,4d^{10}\,5p^1$ . Which electron configuration corresponds to $T^{+}$ in the ground state?
Answer: $[\mathrm{Kr}]\,5s^2\,4d^{10}$ This question tests the skill of atomic structure and electron configuration. The neutral configuration [Kr] 5s² 4d¹⁰ 5p¹ is indium with 49 electrons, and T⁺ loses 1, leaving 48. For p-block metals, the electron is removed from the 5p orbital first. Thus, it becomes [Kr] 5s² 4d¹⁰. A tempting distractor like A, [Kr] 5s² 4d¹⁰ 5p², is incorrect as it adds rather than removes an electron. Determine ion configurations by removing electrons from the highest n or outermost subshell in the neutral atom.
An ion of element Y has atomic number 30 and a $2+$ charge. Which of the following is the correct ground-state electron configuration for $\mathrm{Y^{2+}}$ ?
Answer: $[\mathrm{Ar}]\,3d^{10}$ This question tests your understanding of atomic structure and electron configuration. For an ion with atomic number 30 and 2+ charge, the neutral atom has 30 electrons, but the ion has 28 after losing 2. Electrons are removed from the highest energy orbitals first, so for zinc (atomic number 30), neutral is [Ar] 4s² 3d¹⁰, and the ion loses the 4s electrons, resulting in [Ar] 3d¹⁰. This configuration reflects the stability of a filled d subshell. A tempting distractor is choice B ([Ar] 4s⁰ 3d¹⁰), which is similar but unnecessarily specifies 4s⁰, while the correct shorthand omits empty orbitals. When determining ion configurations, subtract electrons from the neutral setup starting with outermost orbitals.
A neutral atom has the electron configuration $[\mathrm{Kr}]\,5s^24d^{10}5p^2$ . Which subshell contains the highest-energy electron in the ground state of this atom?
Answer: $5p$ This question tests understanding of atomic structure and electron configuration energy levels. The configuration [Kr]5s²4d¹⁰5p² shows electrons in three subshells beyond krypton. Among occupied orbitals, energy increases in the order: 5s < 4d < 5p. The 5p subshell is highest in energy because it has both a higher principal quantum number (n=5) and higher angular momentum quantum number (l=1) compared to 4d. The highest-energy electrons are the 5p² electrons. Option B (4d) is incorrect because 4d orbitals are lower in energy than 5p orbitals, despite having filled after 5s. To identify highest-energy electrons, consider both n and l values, with higher values indicating higher energy.
A neutral atom has atomic number 12. Which of the following is the correct ground-state electron configuration for this atom?
Answer: $1s^2\,2s^2\,2p^6\,3s^2$ 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.
An ion $R^{2+}$ is formed from a neutral atom with atomic number 30. Which electron configuration corresponds to $R^{2+}$ in the ground state?
Answer: $[\mathrm{Ar}]\,4s^0\,3d^{10}$ This question tests the skill of atomic structure and electron configuration. Atomic number 30 is zinc with 30 electrons neutrally, but R²⁺ loses 2, leaving 28. For zinc, electrons are removed from 4s, as it's higher energy in ions. From [Ar] 4s² 3d¹⁰, it becomes [Ar] 3d¹⁰. A tempting distractor like E, [Ar] 4s⁰ 3d⁸, is incorrect as it removes from d instead of s. For transition metal ions, remove from ns orbital before (n-1)d to get the correct configuration.
A neutral atom has the partial electron configuration $[\text{Ar}]\,4s^2\,3d^{10}\,4p^3$ . How many unpaired electrons are present in the atom's ground state?
Answer: 3 This problem tests your understanding of atomic structure and electron configuration, specifically determining unpaired electrons. The given configuration [Ar]4s²3d¹⁰4p³ shows the atom has filled 4s and 3d subshells, with 3 electrons in the 4p subshell. The 4p subshell has three orbitals (px, py, pz), and according to Hund's rule, electrons occupy orbitals singly before pairing. With 3 electrons in 3 orbitals, each electron occupies a separate orbital with parallel spins, resulting in 3 unpaired electrons. A tempting distractor might be 1 (choice B), thinking only one electron is unpaired, but this ignores Hund's rule. To find unpaired electrons, identify partially filled subshells and apply Hund's rule to distribute electrons among available orbitals.
A neutral atom has the electron configuration $1s^2\,2s^2\,2p^6\,3s^2\,3p^4$ . Which subshell contains the highest-energy electron in this atom?
Answer: $3p$ This question tests your understanding of atomic structure and electron configuration. The given configuration 1s² 2s² 2p⁶ 3s² 3p⁴ shows electrons filled up to the third energy level. Orbital energies increase as 1s < 2s < 2p < 3s < 3p, so the last electrons added are in 3p. Thus, the highest-energy electrons are in the 3p subshell. A tempting distractor is choice A (2p), which might confuse inner shells with higher energy, but 3p is higher due to principal quantum number. Identify the highest-energy subshell by looking at the last occupied orbital in the configuration.
A neutral atom has the electron configuration $[\mathrm{He}]\,2s^2\,2p^4$ . How many unpaired electrons are present in the atom's ground state?
Answer: 2 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.
A neutral atom has the electron configuration $[\mathrm{Ne}]\,3s^2\,3p^3$ . How many unpaired electrons are present in the atom's ground state?
Answer: 3 This question tests the skill of atomic structure and electron configuration. The given configuration [Ne] 3s² 3p³ indicates 15 electrons, corresponding to phosphorus. Electrons fill orbitals following Hund's rule, which states that electrons occupy separate orbitals with parallel spins before pairing. In the 3p subshell with 3 electrons, each occupies a separate orbital, resulting in 3 unpaired electrons. A tempting distractor like C, 2, is incorrect because it might assume pairing starts early, violating Hund's rule for maximum multiplicity. Always apply Hund's rule by distributing electrons singly in degenerate orbitals before pairing to count unpaired electrons accurately.
A neutral atom has the electron configuration $[\mathrm{Ar}]\,4s^2\,3d^6$ . Based on Hund's rule, how many unpaired electrons are present in the ground state of this atom?
Answer: 4 This question tests your understanding of atomic structure and electron configuration. The given configuration [Ar] 4s² 3d⁶ indicates 18 electrons from Ar, plus 2 in 4s and 6 in 3d, totaling 26 electrons. Electrons fill orbitals following Hund's rule, which states that electrons occupy orbitals singly before pairing to maximize unpaired spins. For 3d⁶, the five 3d orbitals each get one electron (5 unpaired), then the sixth pairs one, leaving 4 unpaired electrons total, with the 4s² being paired. A tempting distractor is choice B (2), which might come from mistakenly thinking all d electrons pair up early, but Hund's rule requires single occupancy first. To find unpaired electrons, draw orbital diagrams and apply Hund's rule after determining the configuration.
A neutral atom of element Y has atomic number $Z=17$ . Which of the following is the correct ground-state electron configuration for Y?
Answer: $1s^2\,2s^2\,2p^6\,3s^2\,3p^5$ This question requires knowledge of atomic structure and electron configuration for neutral atoms. Element Y has atomic number 17 (chlorine), meaning it has 17 electrons in its neutral state. Following the Aufbau principle, electrons fill orbitals in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 3d, 4s, etc. Counting electrons as we fill: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁵ (17 total). The tempting distractor B shows 18 electrons with a complete 3p⁶, which would be the configuration for argon or a chloride ion (Cl⁻), not neutral chlorine. When writing electron configurations, always count electrons carefully and fill orbitals according to the standard order until you reach the correct total.
A neutral atom has atomic number 20. Which of the following is the correct condensed electron configuration for this atom?
Answer: $[\mathrm{Ar}]\,4s^2$ This question requires writing atomic structure and electron configuration in condensed form. Element 20 (calcium) has 20 electrons. The full configuration is 1s²2s²2p⁶3s²3p⁶4s². In condensed notation, we use the nearest noble gas core: argon ([Ar]) accounts for the first 18 electrons (1s²2s²2p⁶3s²3p⁶). The remaining 2 electrons go in the 4s orbital, giving [Ar]4s². Option A incorrectly writes out the entire third shell instead of using noble gas notation, though it represents the same configuration. When using condensed notation, always identify the preceding noble gas and add only the remaining electrons.
Element Q forms a $3+$ ion. A neutral atom of Q has atomic number 13. Which of the following is the correct electron configuration for $\mathrm{Q^{3+}}$ ?
Answer: $1s^2\,2s^2\,2p^6$ This question tests your understanding of atomic structure and electron configuration. Element Q with atomic number 13 is aluminum, neutral configuration 1s² 2s² 2p⁶ 3s² 3p¹, with 13 electrons. For the 3+ ion, remove 3 electrons from the outermost orbitals: first 3p¹, then 3s², leaving 1s² 2s² 2p⁶. This matches the configuration of neon. A tempting distractor is choice E (1s² 2s² 2p⁶ 3s²), which is the configuration after losing only one electron, not three. For cations, remove electrons starting from the highest principal quantum number orbitals.

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