Linear density is defined as

• A. # atoms on plane / area of plane
• B. # atoms per unit length along the direction vector
• C. # atoms in the unit cell
• D. Density of atoms per volume

Answer: (C)

Linear density is about how many atoms you encounter along a line in the crystal per unit length. Put simply, it’s the number of atoms per unit length along the chosen direction. This is different from densities per area or per volume, which define how many atoms sit in a given plane or in a given volume, respectively. Counting the total atoms in a unit cell isn’t a density at all—it’s just a discrete count of atoms in that cell. So the description that matches linear density is the one that specifies atoms per unit length along the direction vector.