Q

# Distinguish between (i) Hexagonal and monoclinic unit cells (ii) Face-centred and end-centred unit cells.

1.12 Distinguish between

(i) Hexagonal and monoclinic unit cells

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

Views

(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.

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