What, you say? Not a molecule?

If NaCl is not a molecule,what is it? Well, if a molecule is considered to be a discrete small unit made up of atoms joined together, then NaCl, and any other ionic solid is not really a molecule. In NaCl we have a repetitive crystal structure in which each Na⁺ ion is surrounded by Cl^- ions, and vice-versa, for as far as the crystal extends. There really isn't such a thing as an NaCl molecule. But the formula does tell us that there is 1 Na for every 1 Cl, which you could verify by counting the individual atoms in this crystal, so the formula of NaCl is correct. Instead of using the term molecular mass, we can legitimately speak of "calculating its formula mass." 

Of course there are some things that really are discrete individual molecules. H₂O, or CO₂ are simple examples, while glucose C₆H₁₂O₆ is somewhat more complex. 
 
These substances do have a molecular mass, though it is just as useful to call this a formula mass as well. Sometimes the general term molar mass is used for both molecular and formula masses. 

Copyright © 1998 - 2008 David Dice