posted by Kate on .
What is wrong with or ambiguous about the phrase "four molecules of NaCl"
I really don't understand my textbook says in the back that it is an ionic compound so I guess that its not really a molecule then I don't understand...
NaCl is an ionic compound. It is a solid made up of Na^+ and Cl^- interlaced into a three dimensional crystal lattice; thus there is no NaCl molecule as such. We write the formula as NaCl because that is the simplest empirical formula but it really is NaxClx because the Na^+ and Cl^- are repeating in all three dimensions. It is not possible to obtain four "molecules" of NaCl (or one or sixteen or any other number). It IS possible to obtain four formula weights of NaCl and there are many chemists in the world that don't like to use the terms molecular weight, molecular mass, or molar mass when talking about ionic crystals for the very reason that these don't exist as molecules. Those chemists prefer to use the term formula weight or formula mass. So the formula weight of NaCl is about 58.5 and that gets around the problem of NaCl not existing as a true molecule. CO2 is not an ionic compound and it does exist as a molecule. We can weigh a mole of molecules of CO2 (about 44 grams) or we can weigh a formula weight of NaCl (about 58.5 grams).