1.12 Distinguish between

(i) Hexagonal and monoclinic unit cells

(ii) Face-centred and end-centred unit cells.

Answers (1)

(i) Hexagonal and monoclinic unit cells:-   

         Properties               Hexagonal               Monoclinic
    Possible Variation               Primitive

             Primitive,  End centered

     Axial distance           a=b\neq c             a\neq b\neq c
      Axial angles

        \alpha =\beta =90^{\circ}

         \gamma =120^{\circ}

       \alpha =\gamma =90^{\circ}

           \beta \neq 90^{\circ}

     Examples       Graphite, ZnO       Monoclinic sulphur

(ii) Face-centred and end-centred unit cells:-

  • A face-centred unit cell contains atoms at all the corners and at the centre of all the faces, whereas in end - centred unit cell one pair of opposite faces contains atoms apart from atoms at all the corners.
  • The total number of atoms in a unit cell differs by 1.