[noiseordinance]

Ok, I've done searches and have looked in two different chem books and I can't find anything that seems clear to me. Is a formula unit the same thing as an atom or molecule? For instance... a single formula unit of KI is just a single molecule of KI right?

[Astrokel]

Strictly speaking, the appropriate term for KI should be formula unit and not molecular formula because KI is an ionic compound.

Check this out: http://wiki.answers.com/Q/What_is_the_difference_between_a_formula_unit_and_a_molecular_formula

[noiseordinance]

LOL, it's funny you linked that. That's actually one of the things I tried to read but I still didn't and don't get it...

[Astrokel]

a single formula unit of KI is just a single molecule of KI right?

As mentioned, KI is not a covalent compound, thus it can't be considered as a molecule. So this is where the term formula unit comes in useful for ionic compound. Is that what you are confused on?

[noiseordinance]

OOOoooooooohhh, ok, I get it. Covalents are considered molecules but ionics are not. That makes sense. I never knew that. So any time we have polyatomic ions, they are referred to as FU instead of molecules, is that right?

[Astrokel]

Yep, but don't forget most polyatomic ion itself is covalently bonded.