Question

Differentiate between primitive and centred unit cells

Answer 1

The unit cell is defined as the smallest repeating unit having the full symmetry of a crystal structure. Crystal structure is described in terms of the geometry of arrangement of particles in the unit cell.

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A primitive cell is a crystal lattice of eight molecules bound in a geometric structure. When another molecule is added into this structure (base-centered, body-centered, or six molecules in face-centered) the structure is no longer considered primitive.

Answer 2

• Unit cells can be broadly divided into two categories, primitive and centred unit cells.

$\large{\underline{\textrm{Primitive Unit Cells:-}}}$

When constituent particles are present only on the corner positions of a unit cell , it is called as primitive unit cell.

$\large{\underline{\textrm{Centred Unit Cells:-}}}$

When a unit cell contains one or more constituent particle present at position other than corners in addition to those at corners it is called a Centred unit cell.

• Body Centred unit cells : Such a unit cell contains one constituent particle( atom, molecule or ion) at its body - centre besides the ones that are at its corner.
• Face - Centred unit cells: Such a unit cell contains one constituent particle present at the centre of each face, besides the ones that are its corners.
• End-Centred unit cells: In such a unit cell, one constituent particle is present at the center of any two opposite faces besides the ones present at its corners.