Ionic compounds – aka, salts – (you may want to read my post on them here<\/a>) don’t generally exist in separate, self contained units. They more often exist as matrices of alternating metal and nonmetal participants that spread out in all directions. Here’s a quick example of NaCl:<\/p>\n In such a structure, it’s hard to tell which Na atom belongs to which Cl atom, if they were to be separated into partnerships. The lines between the atoms are not strictly “bonds”. (A Cl atom, for instance, can only form one bond; it’s the same for Na.) Rather, they can be thought of as associations or else rotating bonds that move to each adjacent atom in turn.<\/p>\n By the diagram, we can see that for every Na atom there is one Cl atom. For this case, then, a formula unit is the ratio between the Na and the Cl atoms: NaCl. In the case of CaCl2<\/sub>, a formula unit is CaCl2<\/sub>. The math works the same as if it were a molecule. It’s just best not to call CaCl2<\/sub> a molecule, because it likely isn’t usually found as separate units. Hence, for salts, such as CaCl2<\/sub> or NaCl, we refer to formula units, rather than molecules.<\/p>\n HTH:)<\/p>\n Sources<\/em>:<\/p>\n dictionary.reference.com<\/a><\/p>\n Turner, A. Mason and Curtis T. Sears, Jr: Inquiries in Chemistry<\/em>. Toronto: Allyn and Bacon Canada Ltd., 1977.<\/p>\n HTH:)<\/p>\n![](\"blogfiles\/formun2.png\")
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