Multiple Choice
What is the maximum number of electrons that the n = 4 shell can contain?
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10. Periodic Properties of the Elements
The Electron Configuration
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Which of the following is the correct ground-state electron configuration for molybdenum (Mo, z = 42)?
A
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^0 4d^6
B
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^6
C
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^4
D
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^1 4d^5
Verified step by step guidance1
Identify the atomic number of molybdenum (Mo), which is 42, meaning it has 42 electrons to place in orbitals according to the Aufbau principle.
Recall the order of orbital filling based on increasing energy levels: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, and so on.
Write the expected electron configuration by filling orbitals in order until you reach 42 electrons: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^4.
Consider exceptions to the Aufbau principle, especially for transition metals, where half-filled and fully filled d subshells provide extra stability. For molybdenum, this means one electron moves from 5s to 4d to achieve a half-filled 4d subshell.
Adjust the configuration to reflect this stability: 5s^1 4d^5, resulting in the ground-state electron configuration: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^1 4d^5.
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