- Nitrogen (N) has atomic number 7. What is the correct electron configuration for a neutral nitrogen atom?
Answer: 1s² 2s² 2p³
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! For nitrogen with 7 electrons, fill: 1s² (2), 2s² (4 total), and the remaining 3 go into 2p³, giving 1s² 2s² 2p³ with 5 valence electrons in shell 2 (2 in 2s + 3 in 2p). Choice B correctly constructs the electron configuration following Aufbau filling order and properly accounts for total 7 electrons. Choice A underfills 2p with only 2 electrons, but after 2s², the next 3 electrons go into 2p—count carefully to reach exactly 7! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell! You're nailing these configurations—keep the momentum!
- Oxygen (O) has atomic number 8. What is the electron configuration of a neutral oxygen atom?
Answer: 1s² 2s² 2p⁴
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! For oxygen with 8 electrons, fill: 1s² (2), 2s² (4 total), and the remaining 4 go into 2p⁴, giving 1s² 2s² 2p⁴ with 6 valence electrons in shell 2 (2 in 2s + 4 in 2p). Choice B correctly constructs the electron configuration following Aufbau filling order and properly accounts for total 8 electrons. Choice A shows 10 electrons, which would be for neon, not oxygen—always double-check the total electron count matches the atomic number! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell! You're making excellent progress—stay confident!
- The magnesium ion is written as Mg²⁺. Magnesium has atomic number 12. What is the electron configuration of Mg²⁺?
Answer: 1s² 2s² 2p⁶
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! For Mg²⁺, neutral Mg has 12 electrons (1s² 2s² 2p⁶ 3s²), but the +2 charge means it loses 2 electrons from the outermost 3s subshell, leaving 10 electrons with configuration 1s² 2s² 2p⁶, and now shell 2 is the outermost with 8 valence electrons (like neon). Choice B correctly constructs the electron configuration for the ion by removing electrons from the valence shell and accounting for the total of 10 electrons. Choice A fails to account for the ion charge, showing the neutral Mg configuration instead; always adjust electron count for ions! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell! Great job tackling ions—you've got this!
- An aluminum ion, Al³⁺, forms when aluminum (atomic number 13) loses three electrons from its highest energy level. What is the electron configuration of Al³⁺?
Answer: 1s² 2s² 2p⁶
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Neutral aluminum (atomic number 13) has configuration 1s² 2s² 2p⁶ 3s² 3p¹, but when it loses three electrons to form Al³⁺, it loses all electrons from the third shell (3s² 3p¹), leaving 10 electrons with configuration 1s² 2s² 2p⁶. Choice B correctly shows this configuration, which matches the noble gas neon. Choice A shows neutral aluminum, Choice C shows magnesium's configuration, and Choice D shows only 7 electrons. When forming cations, electrons are removed from the highest energy level first - aluminum loses all three of its third-shell electrons to achieve a stable noble gas configuration!
- Sodium ion is written as Na⁺. Sodium has atomic number 11. How does the electron configuration change when Na becomes Na⁺?
Answer: Na: 1s² 2s² 2p⁶ 3s¹; Na⁺: 1s² 2s² 2p⁶
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! For Na becoming Na⁺, neutral Na has 11 electrons (1s² 2s² 2p⁶ 3s¹ with 1 valence electron), and the +1 charge means it loses the 3s¹ electron, resulting in Na⁺ with 10 electrons (1s² 2s² 2p⁶) and now 8 valence electrons in shell 2. Choice A correctly shows the change by removing the valence electron from 3s and adjusting to 10 total electrons for the ion. Choice B incorrectly adds an electron to 3s for Na⁺, but cations lose electrons—remember positive charge means fewer electrons! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell! You're mastering ion changes—awesome!
- Which full electron configuration correctly represents fluorine (F), atomic number 9?
Answer: 1s² 2s² 2p⁵
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, then 3d, etc. The filling order for the first 20 elements goes: 1s, 2s, 2p, 3s, 3p, 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! For fluorine with atomic number 9, fill: 1s² (2), 2s² (4), 2p⁵ (9), so configuration 1s² 2s² 2p⁵, with 7 valence electrons in shell 2 (2+5). Choice A correctly constructs the electron configuration following Aufbau filling order and properly accounts for total electrons (atomic number 9). A distractor like B has 2p⁶, which is 10 electrons for neon—fluorine has only 9, so stop at 2p⁵ after adding exactly 9! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell!
- Using the Aufbau filling order (1s, 2s, 2p, 3s, 3p...), what is the full electron configuration of sulfur (S), atomic number 16?
Answer: 1s² 2s² 2p⁶ 3s² 3p⁴
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, then 3d, etc. The filling order for the first 20 elements goes: 1s, 2s, 2p, 3s, 3p, 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! For sulfur with atomic number 16, we fill: 1s² (2 electrons), 2s² (4 total), 2p⁶ (10 total), 3s² (12 total), and 3p⁴ (16 total), so the configuration is 1s² 2s² 2p⁶ 3s² 3p⁴, with 6 valence electrons in shell 3 (2 in 3s + 4 in 3p). Choice A correctly constructs the electron configuration following Aufbau filling order and properly accounts for total electrons (atomic number 16). A common distractor like D overfills 3p to 6 electrons, but sulfur has only 4 electrons in 3p since 12 electrons fill up to 3s², leaving 4 for 3p—remember to count precisely! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell!
- Which element (atomic number 1-20) has the electron configuration 1s² 2s² 2p⁶ 3s² 3p¹?
Answer: Aluminum (Al)
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, then 3d, etc. The filling order for the first 20 elements goes: 1s, 2s, 2p, 3s, 3p, 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! The given configuration 1s² 2s² 2p⁶ 3s² 3p¹ totals 13 electrons (2+2+6+2+1), with valence in shell 3: 2+1=3, matching aluminum (atomic number 13) in group 13. Choice C correctly identifies the element as aluminum based on total electrons equaling atomic number 13. A distractor like A is magnesium with 12 electrons (ends in 3s²), but adding one more electron goes to 3p¹, not stopping at 3s²—always sum the electrons to find the atomic number! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell!
- Calcium (Ca) has atomic number 20. What is the electron configuration of a neutral calcium atom (do not go beyond 4s)?
Answer: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! For calcium with 20 electrons, fill up to 1s² 2s² 2p⁶ 3s² 3p⁶ (18 total), then the last two go into 4s², giving 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² with 2 valence electrons in shell 4. Choice A correctly constructs the electron configuration following Aufbau by filling 4s before 3d and accounting for total 20 electrons. Choice B incorrectly fills 3d before 4s, but remember the order: 4s comes before 3d in filling sequence for these elements! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell! Impressive handling of higher atomic numbers—keep shining!
- Magnesium has atomic number 12. When magnesium forms Mg²⁺, which electron configuration represents Mg²⁺?
Answer: 1s² 2s² 2p⁶
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! For Mg²⁺ (magnesium atomic number 12, ion has 10 electrons), start with neutral Mg 1s² 2s² 2p⁶ 3s² and remove 2 electrons from valence 3s, resulting in 1s² 2s² 2p⁶ (matching neon). Choice B correctly constructs the electron configuration following Aufbau filling order and properly accounts for total electrons adjusted for ion charge (12 - 2 = 10). For example, choice A is neutral Mg, not accounting for electron loss—cations lose from the outer shell first! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell!
- Oxygen has atomic number 8. What is the electron configuration of the oxide ion, O²⁻?
Answer: 1s² 2s² 2p⁶
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! For O²⁻ (oxygen atomic number 8, but ion has 10 electrons), start with neutral O 1s² 2s² 2p⁴ and add 2 electrons to the valence 2p, resulting in 1s² 2s² 2p⁶, completing the shell with 8 valence electrons (matching neon). Choice B correctly constructs the electron configuration following Aufbau filling order and properly accounts for total electrons adjusted for ion charge (8 + 2 = 10). For example, choice A is neutral oxygen, forgetting to add electrons for the anion—remember anions gain electrons to fill shells! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell!
- Which electron configuration correctly represents oxygen (O), atomic number 8?
Answer: 1s² 2s² 2p⁴
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. For oxygen with atomic number 8, we place 8 electrons following the filling order: 1s² (2 electrons) + 2s² (2 electrons) + 2p⁴ (4 electrons) = 8 total electrons, giving configuration 1s² 2s² 2p⁴. Choice B correctly shows this configuration with all 8 electrons properly distributed. Choice A (1s² 2s² 2p⁶) represents neon with 10 electrons, Choice C incorrectly puts only 1 electron in 2s before filling 2p, and Choice D spreads electrons into the third shell prematurely. Remember the filling order: completely fill lower energy subshells before moving to higher ones - 2s must have 2 electrons before any go into 2p!
- Which element (atomic number 1-20) has the electron configuration 1s² 2s² 2p⁶ 3s² 3p¹?
Answer: Aluminum (Al)
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. The given configuration 1s² 2s² 2p⁶ 3s² 3p¹ has 2+2+6+2+1 = 13 total electrons, which means atomic number 13, corresponding to aluminum (Al). Choice B correctly identifies aluminum as the element with 13 electrons and this configuration. Choice A (Mg) has 12 electrons with configuration ending in 3s², Choice C (Si) has 14 electrons ending in 3p², and Choice D (Na) has only 11 electrons ending in 3s¹. To identify an element from its configuration, count the total electrons - this equals the atomic number for neutral atoms!
- Phosphorus (P) has atomic number 15. What is the electron configuration of a neutral phosphorus atom?
Answer: 1s² 2s² 2p⁶ 3s² 3p³
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. For phosphorus with atomic number 15, we place 15 electrons following the filling order: 1s² (2) + 2s² (2) + 2p⁶ (6) + 3s² (2) + 3p³ (3) = 15 total electrons, giving configuration 1s² 2s² 2p⁶ 3s² 3p³. Choice A correctly shows this configuration with all 15 electrons properly distributed. Choice B has 17 electrons (chlorine's configuration), Choice C incorrectly puts only 1 electron in 3s before filling 3p, and Choice D impossibly puts 8 electrons in 2p (max is 6). Phosphorus has 5 valence electrons (3s² 3p³) and is in group 15, confirming our configuration is correct!
- Chlorine (Cl) has atomic number 17. How many valence electrons does a neutral chlorine atom have?
Answer: 7
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. For chlorine with atomic number 17, we place 17 electrons: 1s² (2) + 2s² (2) + 2p⁶ (6) + 3s² (2) + 3p⁵ (5) = 17 total, and the valence electrons are those in the outermost shell (shell 3), which contains 2 + 5 = 7 electrons. Choice B correctly identifies 7 valence electrons (the 3s² and 3p⁵ electrons in the third shell). Choice A (5) only counts the 3p electrons but misses the 3s electrons, Choice C (17) incorrectly gives the total number of electrons instead of just valence, and Choice D (2) only counts the 3s electrons. Quick valence shortcut: chlorine is in group 17 (or VIIA), and for main group elements, the group number often indicates valence electrons - group 17 has 7 valence electrons!
- A calcium ion is written as Ca²⁺. What is the full electron configuration of Ca²⁺? (Calcium has atomic number 20.)
Answer: 1s² 2s² 2p⁶ 3s² 3p⁶
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, then 3d, etc. The filling order for the first 20 elements goes: 1s, 2s, 2p, 3s, 3p, 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! For Ca²⁺, calcium has atomic number 20 but loses 2 electrons, so 18 electrons: fill up to 1s² 2s² 2p⁶ 3s² 3p⁶ (18), with no electrons in shell 4, resembling argon's configuration. Choice C correctly constructs the electron configuration following Aufbau filling order and properly accounts for total electrons (atomic number 20 adjusted for +2 charge to 18 electrons). A distractor like A includes 4s², but that's for neutral Ca—cations lose from outermost 4s first, removing both to form Ca²⁺! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell!
- The fluoride ion, F⁻, forms when fluorine (atomic number 9) gains one electron. What is the electron configuration of F⁻?
Answer: 1s² 2s² 2p⁶
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Neutral fluorine (atomic number 9) has configuration 1s² 2s² 2p⁵, but when it gains one electron to form F⁻, it adds the electron to the 2p subshell, giving 10 electrons total with configuration 1s² 2s² 2p⁶. Choice B correctly shows this configuration, which matches the noble gas neon's stable arrangement. Choice A shows neutral fluorine's configuration, Choice C impossibly puts 7 electrons in 2p (max is 6), and Choice D incorrectly distributes electrons. For anions (negative ions), electrons are GAINED into the valence shell - fluorine completes its outer shell to achieve the stable noble gas configuration!
- An oxide ion is written as O²⁻. What is the full electron configuration of O²⁻?
Answer: 1s² 2s² 2p⁶
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, then 3d, etc. The filling order for the first 20 elements goes: 1s, 2s, 2p, 3s, 3p, 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! For O²⁻, oxygen has atomic number 8 but gains 2 electrons (negative charge means more electrons), so 10 electrons: fill 1s² (2), 2s² (4), 2p⁶ (10), with 8 valence electrons in shell 2, matching neon's stable configuration. Choice B correctly constructs the electron configuration following Aufbau filling order and properly accounts for total electrons (atomic number 8 adjusted for -2 charge to 10 electrons). A distractor like A is for neutral oxygen—remember, anions gain electrons to fill the valence shell, so O²⁻ adds 2 to 2p⁴ making 2p⁶! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell!
- A neutral atom has 2 electrons in the first shell, 8 electrons in the second shell, and 3 electrons in the third shell (shell model: 2-8-3). Which electron configuration matches this atom?
Answer: 1s² 2s² 2p⁶ 3s² 3p¹
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! The shell model 2-8-3 means shell 1: 1s² (2), shell 2: 2s² 2p⁶ (8), shell 3: 3s² 3p¹ (3), totaling 13 electrons, which is 1s² 2s² 2p⁶ 3s² 3p¹ for aluminum. Choice A correctly constructs the electron configuration following Aufbau filling order and matches the shell electron counts. For example, choice B has 3s¹ 3p², which is still 3 electrons in shell 3 but violates filling order by not completing 3s first—always fill s before p in the same shell! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell!
- Using the Aufbau filling order (1s, 2s, 2p, 3s, 3p, 4s), what is the electron configuration of the neutral magnesium atom (Mg, atomic number 12)?
Answer: 1s² 2s² 2p⁶ 3s²
This question tests your ability to construct electron configurations showing how electrons are distributed in shells and subshells around the nucleus, following the Aufbau principle (filling order), Pauli exclusion principle (max 2 per orbital), and recognizing valence electrons. Electron configuration describes where electrons are located using notation like 1s² 2s² 2p⁶ where the number indicates the shell (1, 2, 3...), the letter indicates the subshell type (s, p, d), and the superscript shows how many electrons are in that subshell. Electrons fill in a specific order from lowest to highest energy: 1s (holds 2), then 2s (holds 2), then 2p (holds 6), then 3s (holds 2), then 3p (holds 6), then 4s, and you keep adding electrons until you've placed all of them (total electrons = atomic number for neutral atoms). Valence electrons are the electrons in the outermost shell—these are the ones involved in bonding and chemical reactions! For magnesium (Mg, atomic number 12), we fill: 1s² (2 electrons), 2s² (4 total), 2p⁶ (10 total), 3s² (12 total), so the configuration is 1s² 2s² 2p⁶ 3s², with 2 valence electrons in the 3s subshell. Choice B correctly constructs the electron configuration following Aufbau filling order and properly accounts for total electrons (atomic number 12). For example, choice A incorrectly places electrons in 3p before fully filling 3s, which violates the filling order—keep practicing to spot these! The electron configuration recipe for elements 1-20: (1) Determine total electrons: atomic number for neutral atoms, atomic number minus charge for ions (Na⁺ has 11 - 1 = 10 electrons). (2) Fill in order: 1s (add 2 electrons), 2s (add 2 more), 2p (add 6 more), 3s (add 2 more), 3p (add 6 more), 4s (add 2 more). Stop when you've placed all electrons. (3) Write configuration: 1s² 2s² 2p⁶ 3s¹ for sodium (11 total: 2+2+6+1=11). Check your total matches atomic number! (4) Identify valence: the outermost shell (highest n) electrons. For sodium 1s² 2s² 2p⁶ 3s¹, the outermost shell is shell 3 with 1 electron, so 1 valence electron. For oxygen 1s² 2s² 2p⁴, outermost is shell 2 with 2+4=6 electrons, so 6 valence electrons. Quick valence shortcut for main group elements: group number often equals valence electrons! Group 1 = 1 valence, group 2 = 2 valence, group 13 = 3 valence, group 14 = 4 valence, etc. For ions, remember: cations (positive) LOSE electrons from outermost shell first. Na (1s² 2s² 2p⁶ 3s¹) loses that 3s¹ to become Na⁺ (1s² 2s² 2p⁶). Anions (negative) GAIN electrons into valence shell. F (1s² 2s² 2p⁵) gains 1 in 2p to become F⁻ (1s² 2s² 2p⁶). Check: does your ion configuration make sense? Cations should look like previous noble gas, anions should complete the outer shell!
