Why do chromium and molybdenum move an electron down in their configurations?

Why do chromium and molybdenum move an electron down in their configurations?

• A. To get their electrons unpaired
• B. To have it in a lower energy state and be more stable
• C. To maximize pairing
• D. To fill a half-filled subshell

Answer: (A)

The key idea is that certain electron arrangements are unusually stable, especially when a subshell is half-filled. For chromium and molybdenum, promoting one electron from the 4s orbital into the 3d subshell creates a 3d5 configuration with the 4s still holding one electron. A half-filled d subshell (five electrons with one in each of the five d orbitals and all spins aligned) gains extra stability from exchange interactions and the symmetry of the arrangement. That extra stabilization lowers the overall energy enough to compensate for the energy cost of moving the electron, so the configuration with a half-filled d subshell becomes favorable. So the move isn’t simply about making electrons unpaired or about maximizing pairing. It’s about taking advantage of the extra stability that comes with a half-filled d subshell. The other ideas don’t fit because maximizing pairing would reduce unpaired electrons, and simply lowering energy isn’t the specific stabilizing mechanism at play here; the notable factor is the stability associated with a half-filled subshell.

The key idea is that certain electron arrangements are unusually stable, especially when a subshell is half-filled. For chromium and molybdenum, promoting one electron from the 4s orbital into the 3d subshell creates a 3d5 configuration with the 4s still holding one electron. A half-filled d subshell (five electrons with one in each of the five d orbitals and all spins aligned) gains extra stability from exchange interactions and the symmetry of the arrangement. That extra stabilization lowers the overall energy enough to compensate for the energy cost of moving the electron, so the configuration with a half-filled d subshell becomes favorable.

So the move isn’t simply about making electrons unpaired or about maximizing pairing. It’s about taking advantage of the extra stability that comes with a half-filled d subshell. The other ideas don’t fit because maximizing pairing would reduce unpaired electrons, and simply lowering energy isn’t the specific stabilizing mechanism at play here; the notable factor is the stability associated with a half-filled subshell.