Design matter with useful electrical, optical, and mechanical behavior.
Composition makes a structure. Atoms arrange into lattices, phases, grains, surfaces, and defects rather than one abstract material.
Answer from the model, then use the explanation to connect the particle picture to the chemical rule.
Disorder and defects add tail and mid-gap states at the band edges, narrowing the effective optical gap from 3.2 eV to 2.73 eV. Order lets carriers move coherently: relative mobility is 0.19 of a perfect crystal.
Illustrative trend for a wide-gap semiconductor with a 3.2 eV intrinsic gap (like TiO₂ or ZnO). The intrinsic gap is set by composition and crystal structure; disorder and defects add band-tail and mid-gap states that lower the effective (optical) gap. The coefficients are not fitted to one material, and real properties also depend on temperature, grain boundaries, and measurement direction.