What is a vacancy defect in a crystal lattice?

• A. A missing atom at a lattice site.
• B. An extra atom occupying an interstitial site.
• C. A substitutional impurity that replaces a lattice atom.
• D. A dislocation line in the crystal.

Answer: (A)

A vacancy defect is a missing atom at a lattice site. In a crystal, atoms occupy regular positions in the lattice, and when one of these atoms is absent, the site becomes a vacant spot. This empty site can enable diffusion as neighboring atoms jump into it, contributing to mass transport in solids. The other possibilities describe different defects: an extra atom in an interstitial site is an interstitial defect; a substitutional impurity replaces a lattice atom; a dislocation line is a line defect that distorts the lattice along a line. So the scenario of a missing atom at a lattice position is exactly what a vacancy defect is.