a substitutional impurity
A substitutional impurity is a foreign atom that moves in and takes a host atom's place — it sits on a normal lattice site, occupying it just as a host atom would, but it is the wrong element. Back to the wallpaper: it is as if one dot in the repeating pattern has been swapped for a dot of a different colour, still stamped in exactly the right spot, just not the right colour. The geometry of the site is respected; only the identity of the occupant has changed.
For a foreign atom to substitute comfortably, it needs to be about the same SIZE as the host atom it replaces — you cannot swap a marble for a beach ball and keep the pattern tidy. As a rough guide (part of the Hume-Rothery rules), the two atoms should differ in radius by no more than about 15 percent; they also help if they share the same crystal structure, similar electronegativity, and similar valence. Zinc dissolving in copper to make brass is the classic case: zinc (radius 0.133 nm) and copper (0.128 nm) differ by only a few percent, so zinc atoms slip onto copper sites easily.
Substitutional impurities are the atomic basis of most alloys and much of doping. Each foreign atom, slightly larger or smaller than the host it replaced, sets up a local strain field; those strain fields impede the dislocations that carry slip, which is why adding solute makes a metal stronger (solid-solution strengthening). The same substitution, done deliberately with a chosen element, is how metallurgists tune strength, corrosion resistance, and colour — from brass to stainless steel to sterling silver.
Sterling silver is copper substituting for silver: about 7.5 percent of the silver atoms are replaced by copper atoms sitting on silver sites. Silver (0.144 nm) and copper (0.128 nm) differ by roughly 11 percent — within the 15 percent guideline — so the two dissolve, and the copper substitutions harden the otherwise-too-soft pure silver.
A substitutional impurity swaps onto a host site; it fits best when its radius is within about 15 percent of the host's.
Size similarity permits substitution but does not guarantee full mixing; valence and electronegativity mismatches can still limit how much dissolves, or push the system to form a compound instead of a solid solution.