Electron Configuration: Quiz

Electron Configuration: Quiz

7 questions

1. Question 1EasyWhat is the electron configuration of fluorine (Z = 9)?

   A$1s^2\, 2s^2\, 2p^5$B$1s^2\, 2s^2\, 2p^6$C$1s^2\, 2s^1\, 2p^6$D$1s^2\, 2p^7$
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   Answer: $1s^2\, 2s^2\, 2p^5$

   9 electrons: 1s holds 2, 2s holds 2, and the remaining 5 go into 2p.

2. Question 2EasyWhat is the noble-gas configuration of potassium (Z = 19)?

   A[Ar] 3d¹B[Ar] 4s¹C[Ne] 3s¹D[Ar] 4p¹
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   Answer: [Ar] 4s¹

   After argon (18 electrons), the next subshell to fill is 4s, not 3d.

3. Question 3MediumHow many unpaired electrons does an oxygen atom have?

   A0B1C2D4
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   Answer: 2

   Oxygen 2p⁴: by Hund's rule, [↑↓] [↑] [↑], so two unpaired electrons.

4. Question 4MediumWhat is the configuration of $\ce{Fe^2+}$ (Fe: Z = 26)?

   A[Ar] 3d⁶B[Ar] 3d⁴ 4s²C[Ar] 3d⁵ 4s¹D[Ar] 3d⁸
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   Answer: [Ar] 3d⁶

   Fe is [Ar] 3d⁶ 4s². Transition metals lose 4s electrons first, so Fe²⁺ is [Ar] 3d⁶.

5. Question 5MediumWhich element has the configuration [Ne] 3s² 3p³?

   ANitrogenBPhosphorusCSulfurDArsenic
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   Answer: Phosphorus

   10 (Ne) + 2 + 3 = 15 electrons, so Z = 15: phosphorus.

6. Question 6HardWhich is the correct ground-state configuration of copper (Z = 29)?

   A[Ar] 3d⁹ 4s²B[Ar] 3d¹⁰ 4s¹C[Ar] 3d¹¹D[Ar] 4s² 4p⁹
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   Answer: [Ar] 3d¹⁰ 4s¹

   Copper is an exception: a completely filled 3d subshell is especially stable, so one 4s electron moves to 3d.

7. Question 7EasyWhat does the Pauli exclusion principle state?

   AElectrons fill the lowest energy subshells firstBAn orbital holds at most two electrons, with opposite spinsCElectrons spread out before pairingDElectrons cannot be found in the nucleus
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   Answer: An orbital holds at most two electrons, with opposite spins

   That is Pauli. Filling lowest energy first is the Aufbau principle; spreading out before pairing is Hund's rule.