A student is analyzing the electron configuration of various elements. Which of the following elements has the greatest number of unpaired electrons in its ground state?
- ACarbon (C)
- BOxygen (O)
- CNitrogen (N)
- DFluorine (F)
Show answer & explanationAnswer & explanation
Correct answer: C. Nitrogen (N)
To determine the number of unpaired electrons, we write the electron configuration and apply Hund's Rule. Oxygen (Z=8): [He] 2s² 2p⁴. The 2p subshell has 4 electrons: ↑↓ ↑ ↑. This gives 2 unpaired electrons. Nitrogen (Z=7): [He] 2s² 2p³. The 2p subshell has 3 electrons: ↑ ↑ ↑. This gives 3 unpaired electrons. Carbon (Z=6): [He] 2s² 2p². The 2p subshell has 2 electrons: ↑ ↑ _. This gives 2 unpaired electrons. Fluorine (Z=9): [He] 2s² 2p⁵. The 2p subshell has 5 electrons: ↑↓ ↑↓ ↑. This gives 1 unpaired electron. Therefore, Nitrogen has the greatest number of unpaired electrons (3).
Why the other options are wrong
- A. Carbon has 2 unpaired electrons (2p²).
- B. Oxygen has 2 unpaired electrons (2p⁴).
- D. Fluorine has 1 unpaired electron (2p⁵).
Hund's Rule
States that every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin.
- Maximizes the number of unpaired electrons.
- Applies to degenerate orbitals (orbitals of the same energy).
- Used in conjunction with the Aufbau principle and Pauli exclusion principle.
Memory trick: Hund's Rule: Half-fill first, then pair up.