Planar density is defined as

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

Answer: (D)

Planar density tells you how densely packed atoms are on a given crystal plane, measured as the number of atomic centers that lie on that plane divided by the area of the plane within the crystal reference region (usually the unit cell). This gives units of atoms per unit area (e.g., atoms per square angstrom). It’s a different quantity from linear density (atoms per unit length along a line) and from volume density (atoms per unit volume). It’s also not simply the total number of atoms in the unit cell, since that ignores the plane’s area. So the definition corresponds to atoms on the plane divided by the plane’s area.