AP Chemistry Quiz: Atomic Structure And Electron Configuration

What this quiz covers

This quiz focuses on Atomic Structure And Electron Configuration, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Chemistry.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. $1s^22s^22p^63s^23p^63d^64s^2$
2. $1s^22s^22p^63s^23p^64s^23d^5$
3. $1s^22s^22p^63s^23p^64s^13d^7$
4. $1s^22s^22p^63s^23p^64s^23d^8$
5. $1s^22s^22p^63s^23p^64s^23d^6$ (correct answer)

Explanation: This question tests understanding of atomic structure and electron configuration. An element with atomic number 26 has 26 electrons in its neutral state. Electrons fill orbitals following the Aufbau principle in the order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc. For 26 electrons: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁶ (18), 4s² (20), 3d⁶ (26 total). Option E incorrectly shows 3d⁵ instead of 3d⁶, which would only account for 25 electrons. When writing electron configurations, always count electrons systematically and remember that 4s fills before 3d.

Question 2

1. $[\mathrm{Ar}]\,4s^2\,3d^4$
2. $[\mathrm{Ar}]\,4s^1\,3d^5$ (correct answer)
3. $[\mathrm{Ar}]\,4s^2\,3d^5$
4. $[\mathrm{Ar}]\,4s^0\,3d^6$
5. $[\mathrm{Ar}]\,4s^1\,3d^4\,4p^1$

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 3

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 4

1. $1s^2\,2s^2\,2p^6\,3s^2$
2. $1s^2\,2s^2\,2p^6\,3s^2\,3p^1$
3. $1s^2\,2s^2\,2p^6$ (correct answer)
4. $1s^2\,2s^2\,2p^5\,3s^2$
5. $1s^2\,2s^2\,2p^6\,3s^1$

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 5

1. $1s^2\,2s^2\,2p^6\,3s^2$ (correct answer)
2. $1s^2\,2s^2\,2p^6\,3s^1\,3p^1$
3. $1s^2\,2s^2\,2p^5\,3s^2\,3p^1$
4. $1s^2\,2s^2\,2p^6\,3p^2$
5. $1s^2\,2s^2\,2p^6\,4s^2$

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 6

1. $[\mathrm{Ar}]\,4s^2$ (correct answer)
2. $[\mathrm{Ar}]\,3d^2$
3. $[\mathrm{Ne}]\,3s^2\,3p^6\,4s^2$
4. $[\mathrm{Ar}]\,4s^2\,4p^2$
5. $[\mathrm{Ar}]\,4s^1\,3d^1$

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 $[\mathrm{Ar}]$ (18 electrons), then $4s^2$ for the remaining 2, resulting in $[\mathrm{Ar}]\,4s^2$ for calcium. This follows the standard order without exceptions. A tempting distractor is choice C ($[\mathrm{Ne}]\,3s^2\,3p^6\,4s^2$), 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 7

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 8

1. $1s^2\,2s^2\,2p^6\,3s^2\,3p^3$
2. $1s^2\,2s^2\,2p^6\,3s^2\,3p^6$ (correct answer)
3. $1s^2\,2s^2\,2p^6\,3s^2\,3p^5$
4. $1s^2\,2s^2\,2p^6\,3s^2\,3p^4$
5. $1s^2\,2s^2\,2p^6\,3s^1\,3p^6$

Explanation: This question involves atomic structure and electron configuration for anions. Element Z has atomic number 15 (phosphorus), so a neutral atom has 15 electrons. The Z³⁻ ion has gained 3 electrons, giving it 18 electrons total. Following the Aufbau principle: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁶ (18 total). This configuration is identical to argon, a noble gas, which makes sense as ions often achieve noble gas configurations for stability. The tempting distractor A shows only 17 electrons (3p⁵), which would be the configuration for a neutral chlorine atom, not P³⁻. When dealing with anions, add the charge magnitude to the atomic number to find the total electron count, then fill orbitals accordingly.

Question 9

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 10

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

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 11

1. $[\mathrm{Ar}]\,4s^1\,3d^{10}$ (correct answer)
2. $[\mathrm{Ar}]\,4s^2\,3d^{10}$
3. $[\mathrm{Ar}]\,4s^2\,3d^9$
4. $[\mathrm{Ar}]\,4s^1\,3d^9\,4p^1$
5. $[\mathrm{Ar}]\,4s^0\,3d^{11}$

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 12

1. $1s^2\,2s^2\,2p^6\,3s^2$ (correct answer)
2. $1s^2\,2s^2\,2p^6\,3p^2$
3. $1s^2\,2s^2\,2p^5\,3s^2\,3p^1$
4. $1s^2\,2s^2\,2p^6\,3s^1\,3p^1$
5. $1s^2\,2s^2\,2p^6\,4s^2$

Explanation: This question tests the skill of atomic structure and electron configuration. Atomic number 12 is magnesium with 12 electrons, filling up to $3s^2$ 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.

Question 13

1. $[\mathrm{Ar}]\,4s^0\,3d^8$
2. $[\mathrm{Ar}]\,4s^0\,3d^{10}$ (correct answer)
3. $[\mathrm{Ar}]\,4s^2\,3d^8$
4. $[\mathrm{Ar}]\,4s^2\,3d^{10}$
5. $[\mathrm{Ar}]\,4s^1\,3d^9$

Explanation: 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.

Question 14

1. $1s^2\,2s^2\,2p^5$ (correct answer)
2. $1s^2\,2s^1\,2p^6$
3. $1s^2\,2s^2\,2p^6$
4. $1s^2\,2s^2\,2p^3\,3s^2$
5. $1s^2\,2s^2\,2p^4\,3s^1$

Explanation: 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.

Question 15

1. $[\mathrm{Ar}]\,4s^2\,3d^4$
2. $[\mathrm{Ar}]\,4s^0\,3d^6$ (correct answer)
3. $[\mathrm{Ar}]\,4s^1\,3d^5$
4. $[\mathrm{Ar}]\,4s^2\,3d^6$
5. $[\mathrm{Ar}]\,4s^0\,3d^4$

Explanation: 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.

Question 16

1. 0
2. 2
3. 4 (correct answer)
4. 5
5. 6

Explanation: 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.

Question 17

1. $[\mathrm{Ar}]\,4s^2\,3d^9\,4p^6$
2. $[\mathrm{Ar}]\,4s^2\,3d^{10}\,4p^4\,5s^2$
3. $[\mathrm{Ar}]\,4s^2\,3d^{10}\,4p^5$
4. $[\mathrm{Ar}]\,4s^2\,3d^{10}\,4p^6$ (correct answer)
5. $[\mathrm{Kr}]\,5s^1$

Explanation: This question tests your understanding of atomic structure and electron configuration. Element Z with atomic number 35 is bromine, neutral configuration [Ar] 4s² 3d¹⁰ 4p⁵, with 35 electrons. For the Z⁻ anion, add one electron to reach 36, filling the 4p subshell to 4p⁶, resulting in [Ar] 4s² 3d¹⁰ 4p⁶. This is determined by adding the extra electron to the next available orbital following the Aufbau principle. A tempting distractor is choice A, which is the neutral configuration, but it lacks the extra electron for the anion. For ions, adjust the electron count from the neutral atom and fill or remove from the appropriate subshells.

Question 18

1. $1s^2\,2s^2\,2p^6\,3s^2\,3p^5$ (correct answer)
2. $1s^2\,2s^2\,2p^6\,3s^2\,3p^6$
3. $1s^2\,2s^2\,2p^6\,3s^2\,3p^4\,4s^1$
4. $1s^2\,2s^2\,2p^5\,3s^2\,3p^6$
5. $1s^2\,2s^2\,2p^6\,3s^1\,3p^6$

Explanation: This question tests your understanding of atomic structure and electron configuration. For a neutral atom with atomic number 17, there are 17 protons and thus 17 electrons to place in orbitals. Electrons fill orbitals starting from the lowest energy level, following the Aufbau principle: first 1s (2 electrons), then 2s (2), 2p (6), 3s (2), and finally 3p with the remaining 5 electrons. This results in the configuration 1s² 2s² 2p⁶ 3s² 3p⁵, which matches choice A. A tempting distractor is choice B, which has 3p⁶, but that would be for 18 electrons, like argon, not 17. Always count the total electrons based on the atomic number for neutral atoms and fill orbitals in order of increasing energy.

Question 19

1. $[\mathrm{Ar}]\,4s^2\,3d^4$
2. $[\mathrm{Ar}]\,4s^1\,3d^5$ (correct answer)
3. $[\mathrm{Ar}]\,4s^2\,3d^5$
4. $[\mathrm{Ar}]\,4s^0\,3d^6$
5. $[\mathrm{Ar}]\,4s^1\,3d^4$

Explanation: This question tests your understanding of atomic structure and electron configuration. An atom with atomic number 24 has 24 electrons to distribute. Normally, electrons fill 4s before 3d, but chromium is an exception where [Ar] 4s¹ 3d⁵ provides extra stability due to half-filled subshells. This configuration is determined by recognizing known exceptions for elements like Cr and Cu. A tempting distractor is choice A ([Ar] 4s² 3d⁴), which follows the standard order but ignores the exception for stability. Memorize exceptions for transition metals and prioritize half-filled or fully filled subshells when applicable.

Question 20

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

Explanation: This question tests your understanding of atomic structure and electron configuration. The configuration [Ne] 3s² 3p³ indicates 10 electrons from Ne, plus 2 in 3s and 3 in 3p, totaling 15 electrons. Hund's rule dictates that the 3p electrons occupy separate orbitals singly, resulting in three unpaired electrons. The 3s² is fully paired, so only the 3p contributes unpaired electrons. A tempting distractor is choice E (5), which might assume all electrons are unpaired, but pairing occurs in filled subshells. Use orbital diagrams to visualize electron pairing and count unpaired ones per Hund's rule.