Q.(a) Which of the following is a molecular solid?

(a) Ice is a molecular solid; (b) primitive and centred unit cells differ only in whether extra lattice points sit at the body/face/edge centres; (c) NaCl coloration is a metal-excess defect — F-centres trap electrons that absorb visible light.

(a) Which is a molecular solid?

Diamond, graphite and quartz (SiO2) are all covalent (network/giant-molecule) solids held together by a continuous lattice of covalent bonds — very hard, very high melting point. Ice, in contrast, is built of discrete H2O molecules held to each other only by hydrogen bonds and weaker van der Waals forces — this is the hallmark of a molecular solid (soft, low melting point).

Answer: (c) Ice.

(b) Primitive vs centred unit cells

• A primitive (simple) unit cell has constituent particles located ONLY at the eight corners of the cell.
• A centred unit cell has particles at the corners AND at one or more additional positions:
  • Body-centred (I): one extra particle at the centre of the cell body.
  • Face-centred (F): one extra particle at the centre of every face.
  • End- (base-) centred (C): extra particles at the centre of just one pair of opposite faces. So centring simply adds extra lattice points beyond the corner points that a primitive cell is limited to.

(c) Why excess-Na NaCl is coloured …