Multiple Choice
Which of the following represents the ground state electron configuration of boron (B)?
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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)?
A
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^5
B
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^1 4d^5
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^4
Verified step by step guidance1
Step 1: Identify the atomic number of molybdenum (Mo), which is 42. This means Mo has 42 electrons to be placed in orbitals according to the Aufbau principle.
Step 2: Recall the general order of orbital filling based on increasing energy levels: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, etc.
Step 3: Write the electron configuration by filling orbitals in order until you reach 42 electrons. Normally, you would expect 5s to fill before 4d, so the naive configuration would be $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^4$.
Step 4: Consider the special stability associated with half-filled or fully filled d subshells. For molybdenum, having a half-filled 4d subshell (4d^5) and one electron in 5s (5s^1) is more stable than 5s^2 4d^4 due to electron exchange energy and symmetry.
Step 5: Therefore, the correct ground-state electron configuration for molybdenum is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^1 4d^5$, reflecting the stability of the half-filled d subshell.
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