[plu]

Interesting inorganic question:

Silicon carbide and diamond are covalent solids which crystallize in a cubic structure.  In silicon carbide, carbon atoms occupy points of a face-centered cubic lattice (fcc) and silicon atoms occupy half of the terahedral holes available.  In diamond, the same proportion of tetrahedral holes is occupied by other carbon atoms.

Given that the density of silicon carbide is 3.21 g cm^-3 and that of diamond is 3.51 g cm^-3, calculate the covalent radius of a silicon atom in silicon carbide crystal.

 

[AWK]

It is a quite simple crystallographic problem. From these data you can calculate a volume, then a parameter of cubic unit cell.  1/4 of the diagonal of the cube is equal to 2 covalent radii of C in diamond or sum of covalent radii C and Si in SiC.