Electron Orbitals: Filling Order and 3d/4s Anomaly

Orbital Filling Order and Anomalies

• General Filling Procedure:

  • Orbitals fill in a specific sequence. For instance, after 2p orbitals are filled, the next electrons go into the 3s orbital.

• Filling the $n=3$ Level:

  • Sodium (Na): The first electron in the $n=3$ level goes into the 3s orbital, making it $3s^1$. This is the first element in the third period.

  • Magnesium (Mg): The second electron in the $n=3$ level fills the 3s orbital, resulting in $3s^2$.

  • The 3d Orbital: There are five 3d orbitals (a set of them). Theoretically, these belong to the $n=3$ energy level. However, electrons do not fill the 3d orbitals immediately after 3s.

  • Instead, electrons proceed to fill the 3p orbitals first, according to the periodic table's structure.

• The 3d vs. 4s Energy Anomaly:

  • Observation: For a significant portion of the periodic table, the 3d orbitals are at a higher energy level than the 4s orbitals.

  • Expected vs. Actual: This is considered