In a solid solution, a foreign atom occupying a lattice site normally occupied by a host atom describes what?

• A. Vacancy defect
• B. Substitutional defect
• C. Interstitial defect
• D. Dislocation

Answer: (B)

In crystal lattices, a foreign atom that takes the place of a host atom on a lattice site is a substitutional defect. This is the hallmark of a substitutional solid solution, where solute atoms replace host atoms within the regular lattice. It differs from interstitial defects, where small atoms fit into the gaps between lattice points, and from vacancy defects, where a lattice site is simply empty. Dislocations are line defects related to misalignment of planes, not occupancy of lattice sites. So the described scenario matches a substitutional defect.