Paul Arveson (bridges@his.com)
Wed, 11 Feb 1998 18:58:36 -0400

>At 10:48 PM 2/8/98 -0800, David B. Fenske wrote:
>>>>First, calcium carbonate is a salt, not a molecule.
>>>OH you really didn't say that did you? Have you never heard the definition
>>>of a molecule?
>>Glenn, I hate to quibble, but I think they're right, and this is what I
>>learned in high school and university chemistry. A molecule is a
>>collection of atoms covalently bonded together. In a salt, you don't have
>>individual molecules, you have a collection of positive and negative ions
>>forming a crystal lattice structure, held together by ionic interactions.
>>When you dissolve a salt in water, you get positive and negative ions in
>>solution, individually hydrated by water, you don't get a CaCO3 molecule as
>>an individual entity.
>>That's why when you buy a bottle of an organic molecule from Sigma, it has
>>the MW (moleculear weight) on the label; when you buy a bottle of any salt,
>>it gives you the FW (formula weight), which may or may not include water of
>Well If that is what they are meaning then I stand corrected. I must have
>missed class that day in chemistry. :-(

You needn't have apologized. CaCO3 is a salt but a very weak one, and it
is only barely soluble in water; it doesn't ionize and dissociate anywhere
near as much as a salt like sodium chloride, because the alkaline earth
metals like Ca form stronger double bonds than the single bonds of the
alkali metals. And most of the CaCO3-related minerals (aragonite, calcite,
dolomite etc.) are not hydrates.

Perhaps it's most correct to say that in general the distinction between
ionic and covalent bonds is a matter of degree. The physics is not as
simple as what was presented in high school chemistry class.