## Molecules, and Formula Units

Peter DePaul 1E
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### Molecules, and Formula Units

I am doing homework exercise E.21
I'm confused on how to determine whether or not it is molecules or formula units. I found a different post on the forum https://lavelle.chem.ucla.edu/forum/viewtopic.php?t=32906#p105869 but I'm still unclear. For example the first asks about $Al_2{}O_3{}$ which I know is an ionic bond, so it's not covalently bonded. On this post https://lavelle.chem.ucla.edu/forum/viewtopic.php?t=15367 that a molecule is when the bond is covalent (nonmetal-nonmetal) and formula units when it is ionic (metal-nonmetal) but then goes on to explain that $Al_2{}O_3{}$ would be listed with molecules even though it is an ionic bond. If anyone could distinguish these two any better it would be appreciated because I'm very confused on the difference.

Ryan Agcaoili 2E
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### Re: Molecules, and Formula Units

To clarify, a formula unit is the amount of units used when converting an empirical formula to a molecular formula. Thus, it represents the lowest whole number ratio in an ionic compound. On the other hand, a molecule is created when 2 or more elements are bonded together covalently.

Chem_Mod
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### Re: Molecules, and Formula Units

From the textbook, a formula unit is the smallest repeating unit showing the smallest repeating unit of an ionic compound. In this case, the formula unit of sodium chloride would be Na Cl and for calcium chloride would be Ca Cl2. So a formula unit is often used to describe ionic compounds that aren't bonded covalently.

A molecule describes a group of covalently bonded atoms.

Xavier Herrera 3H
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### Re: Molecules, and Formula Units

So, based on this textbook question, the number of atoms would apply to a single element, the number of molecules would apply to a covalently-bonded molecule, and formula units would apply to ionic compounds?

Connie Liang 3L
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### Re: Molecules, and Formula Units

Yes, that should be how you approach it based on the textbook question. I suppose the textbook could also ask for the number of atoms of element in a certain covalently-bonded molecule (which is why the questions says "or atoms, if indicated"). And in that case, it would be appropriate to find the number of atoms in a covalently-bonded molecule.