Characteristics of a Unit Cell

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chemistry 12th class cbse notes

Parameters of Unit Cell

A unit cell is characterized by 6 parameters:

  • 3-edges a, b and c (refer the diagram given above). The edges may or may not be perpendicular to each other.
  • 3 angles α (between b and c), β(between a and c) and γ(between a and b).

Classification of unit cell:

Primitive Unit Cells:

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

Centred Unit Cells:

When a unit cell contains one or more constituent particles present at positions other than corners in addition to those at corners, it is called a centred unit cell.

Centred unit cells are of three types:

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 corners.

Face-Centred Unit Cells:

Such a unit cell contains one constituent particle present at the centre of each face, besides the ones that are at its corners.

End-Centred Unit Cells:

In such a unit cell, one constituent particle is present at the centre of any two opposite faces besides the ones present at its corners.

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