a cation

A cation is an atom that has lost one or more electrons and so carries a positive charge. In ceramics these are almost always the metal atoms: sodium becomes Na+, magnesium Mg2+, aluminium Al3+, silicon Si4+, each written with a superscript plus and a number telling how many electrons it gave away. Because losing electrons shrinks an atom (the remaining electrons are pulled in tighter by the now-unbalanced nucleus), cations are the small partners in a ceramic, the little marbles that tuck into the gaps of a crystal.

In the ionic model the cation's two important properties are its charge and its size. The charge sets how strongly it pulls on surrounding anions and how much bond strength it contributes; the size, relative to the anion, decides how many anions can fit around it, its coordination number. A small, highly charged cation like Si4+ pulls hard but is so tiny that only four oxygens fit around it (a tetrahedron), while a bigger Ca2+ can gather eight. The same element can even be several cations: iron appears as Fe2+ or Fe3+, and switching between them is how ferrites and many colours and defects work.

Which cation you choose, and in which charge state, is the main lever a ceramist pulls. Swapping a bit of Y3+ for Zr4+ (aliovalent doping) forces oxygen vacancies that turn zirconia into an oxygen-ion conductor for fuel cells; the off-centre Ti4+ in BaTiO3 is what makes it ferroelectric. Cations are also usually the atoms that must diffuse for sintering and reactions to proceed, so their size and charge quietly govern processing too.