d-block

The region of the periodic table in which the differentiating electron enters a d subshell. Its elements occupy groups 3–12, with some boundary conventions varying.

Atomic orbital: A region described by a wavefunction in which an electron has a particular distribution of position and energy. D subshells are orbitals that receive the differentiating electron in this block.
Transition metal: An element whose atom or common ion has an incomplete d subshell. Many d-block elements meet this stricter definition, though the terms are not identical.
Iron: A chemical element with symbol Fe and atomic number 26, widely used in alloys and biological systems. Iron anchors steel production and illustrates the industrial reach of the block.
f-block: The periodic-table region in which the differentiating electron enters an f subshell. It is the neighboring block whose lanthanides and actinides fill f orbitals instead.
Electron configuration: A description of how electrons are distributed among an atom’s shells, subshells, and orbitals. The block’s defining pattern is the placement of the differentiating electron in a d subshell.
Oxidation state: A formal charge assigned to an atom by treating its bonds as ionic. Variable oxidation states arise as d and s electrons participate in bonding.
Copper: A reddish chemical element with symbol Cu and atomic number 29, valued for electrical and thermal conductivity. Its conductivity makes copper a major example of d-block use in electrical systems.
s-block: The periodic-table region in which the differentiating electron enters an s subshell. Its elements occupy the table’s left side and fill s orbitals rather than d orbitals.
Aufbau principle: A model for constructing electron configurations by filling orbitals in an approximate order of increasing energy. Its filling sequence helps explain the d-block’s position and recurring configurations.
Crystal field theory: A model that explains how surrounding ligands split the energies of a metal ion’s d orbitals. It connects d-orbital splitting to color, magnetism, and coordination geometry.

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