percent ionic character

A chemical bond is two atoms sharing the glue of their electrons. In a perfectly ionic bond one atom hands its electrons over completely and the two become charged spheres that stick by simple electric attraction, like a magnet on a fridge; in a perfectly covalent bond the two atoms share electrons evenly, like two children each holding one end of the same rope. Real bonds in ceramics live somewhere between these extremes, and percent ionic character is a single number, from 0 to 100, that says how far along that spectrum a bond sits.

Pauling gave a simple recipe. Take the two atoms' electronegativities (a measure of how hard each pulls on shared electrons) and their difference, call it delta-X. Then the fraction ionic = 1 - exp(-0.25 times (delta-X)^2), and multiplying by 100 gives the percent. Work it for common salt, NaCl: sodium's electronegativity is about 0.9 and chlorine's about 3.0, so delta-X = 2.1, and 1 - exp(-0.25 times 2.1^2) = 1 - exp(-1.10) = 0.67, or about 67 percent ionic. For silicon carbide, SiC, silicon is 1.8 and carbon 2.5, delta-X = 0.7, which gives only about 12 percent ionic, so SiC is mostly covalent.

This one number is a first predictor of how a ceramic behaves. Highly ionic solids (MgO near 73 percent, NaCl near 67 percent) tend toward close-packed, high-coordination structures and cleave or dissolve in tidy ways; highly covalent solids (SiC, Si3N4, diamond) form open, directionally bonded frameworks that are extremely hard and stiff. Most ceramic oxides sit in the middle: silica SiO2 comes out near 51 percent and alumina Al2O3 near 63 percent, which is exactly why the purely ionic hard-sphere picture is only ever an approximation for them.