Classify the bonds in the following as ionic or covalent. for bonds that are covalent, indicate whether they are polar or nonpolar.
a. na2o b. cacl2
c. nbr3 d. cs2

Ionic bonding involves the transfer of electrons from two oppositely charged ions. This reaction usually involves a metal (electron donor) and a non-metal (acceptor). 

Covalent bonding occurs between elements close to each other or of the same nature on the periodic table. Instead of transferring electrons, they share electrons.

Therefore, both a. (Na2O) and b. (CaCl2) are ionic bonds and polar, as they are from two different spectra of the periodic table, with one ion being the donor and the other being the acceptor respectively.
Both c. (NBr3) and d. (CS2) are covalent, non-polar bonds, as they are sharing electrons due to the fact that they are situated close to each other on the periodic table and do not need to transfer electrons from their valence shells, as they have similar valence shell structures.