ionicity

Ionicity, in chemistry and materials science, refers to the degree to which a bond or a compound exhibits ionic character as opposed to covalent character. It describes how strongly electrons are transferred or polarized between atoms, and it is best thought of as a continuum rather than a binary property. In solids, high ionicity is associated with ions arranged in a lattice and strong electrostatic interactions, while low ionicity corresponds to more shared, covalent bonding.

Ionicity is commonly estimated from differences in electronegativity between bonded atoms. A widely cited empirical relation links the percent ionic character to the electronegativity difference (Δχ) between elements, often written in the form percent ionic character ≈ [1 − exp(−0.25(Δχ)²)] × 100%. This provides a qualitative trend: larger Δχ generally yields greater ionic character. Other methods use quantum-chemical analyses or lattice-energy models, such as Born effective charges, charge-transfer calculations, or Bader charge analysis, to quantify ionic versus covalent contributions in a given system.

In practice, ionicity helps explain material properties. Ionic compounds like sodium chloride and lithium fluoride typically have high ionicity, translating to high melting points, low electrical conductivity in the solid state, and large dielectric responses that depend on the lattice. Covalent substances, such as elemental carbon or hydrogen, exhibit low ionicity, characterized by directional bonding and different thermal and electronic properties. Many materials show intermediate or mixed character, where the balance of ionic and covalent interactions influences conductivity, optical behavior, and mechanical properties.

Because ionicity is context-dependent and not uniquely defined, it is best used as a qualitative descriptor supported by quantitative models or computations for the system in question.