text: Iron sulfate and sodium borate are ionic. SiC2 is covalent.  (This is due to the proximity, or lack thereof, of each main element in the periodic table.)  The concept of polar/nonpolar is used only with covalent molecules to describe whether or not they have a sufficient dipole moment to interact with other polar/nonpolar molecules.  I don't know the arrangement of atoms in SiC2 but, if the molecule is linear, the moment of the two Si-C bonds cancel out and it is nonpolar.  If SiC2 is bent, it's polar.