dsubshell

A dsubshell, or d subshell, refers to the set of atomic orbitals with orbital angular momentum quantum number l = 2. In a given principal level n, there are five d orbitals, corresponding to the magnetic quantum numbers m_l = -2, -1, 0, 1, 2. These orbitals are traditionally labeled by their shapes: dx2−y2, dz2, dxy, dxz, and dyz. Each orbital can hold up to two electrons with opposite spins, so a full d subshell can accommodate a total of 10 electrons.

In hydrogen-like atoms the five d orbitals are degenerate within a given n and l. In multi-electron atoms, electron–electron interactions and spin–orbit coupling lift this degeneracy, and the five d orbitals may have slightly different energies depending on the atomic environment. The d subshell becomes energetically relevant starting at n ≥ 3, and it plays a central role in the chemistry of transition metals.

In chemistry, the d subshell is crucial for understanding valence and bonding in transition metals and related elements. The arrangement of electrons in the d orbitals influences oxidation states, color, magnetism, and coordination chemistry. In crystal field theory and ligand-field theory, the five d orbitals split into sets with different energies in various geometries; in an octahedral field, for example, the dx2−y2 and dz2 (collectively called eg) typically lie higher in energy than the other three (the t2g set: dxy, dxz, dyz). This d-electron configuration underpins much of the observed chemical behavior of transition-metal complexes.