The tetrahedral voids are present in 0.5 mole of hcp crystal structure

Which of the following is correct order of packing efficiency?

Assertion : ZnO becomes yellow when it is heated.

Reason : The anionic sites occupied by unpaired electrons (due to F-centres).

Which of the following is an example of network solid?

Niobium crystallises in body centred cubic structure. If density is $$8.55 \mathrm{~g} \mathrm{~cm}^{-3}$$, then the atomic radius of niobium is (atomic mass of niobium $$=93 u$$ )

A solid has a structure in which W atoms are located at the corners of a cubic lattice. O atoms at the centre of edges and Na atom at centre of the cube. The formula for the compound is

Assertion (A) Graphite is an example of tetragonal crystal system.

Reason (R) For a tetragonal system, $$a=b \neq c$$ and $$\alpha=\beta=90^{\circ}, \gamma=120^{\circ}$$.