dsubshells

A d subshell is the set of five d-type atomic orbitals associated with the angular momentum quantum number l = 2. For any principal quantum number n ≥ 3, the d-sublevel can hold up to ten electrons, distributed among the five orbitals. The five d orbitals are typically named as dx2−y2, dz2, dxy, dxz, and dyz, and correspond to magnetic quantum numbers ml = −2, −1, 0, +1, +2. In a spherically symmetric atom, these five orbitals are degenerate in energy, meaning they have the same energy in the absence of external perturbations.

In electronic structure, d subshells are filled after the preceding s subshell within a given shell, though the relative energies of s and d subshells can vary with atomic number and ionization. The d subshell begins to appear in the third principal shell (3d), and it plays a central role in the chemistry of transition metals. Common configurations exhibit variations such as chromium and copper where half-filled or partly filled subshell stability leads to observed exceptions to simple filling rules.

In crystal field theory and ligand field theory, the degeneracy of the five d orbitals is lifted by the electrostatic field created by surrounding ligands. In an octahedral field, the d orbitals split into a lower-energy t2g set (dxy, dxz, dyz) and a higher-energy eg set (dx2−y2, dz2) with a splitting parameter Δo. In tetrahedral fields, the ordering is reversed and the splitting magnitude is smaller. This d-subshell splitting underpins many properties of coordination compounds, including color, magnetism, and reactivity.

Additional effects such as spin-orbit coupling further refine d-subshell energies in heavier elements, contributing to fine structure in spectroscopy and electronic structure.