chemistry #3

In terms of electrons, how do covalent bonds form? How do ionic bonds form? How can you identify whether a compound is a molecular (covalently bonded) or an ionic compound? Identify each of the following as being an ionic or a covalent bonded compound: CO2, NaCl, SO3, MgBr2, CaO, H2O, Na2O, BeF2, MgO, CO, KCl, SO2