Accurate structure factors for synthetic low-quartz permit the evaluation of residual charges of +1.0±0.1 e.u. on pseudoatom Si and −0.5±0.1 e.u. on pseudoatom O. A new definition of ionicity, based on experimental data exclusively, sets it equal to the ratio of residual charge to formal charge thus giving the Si–0 bond in quartz an ionicity of 25%. The value that is calculated from orbital electronegativity theory, taking the observed interbond angles into account, is 25.36%.