Formula Units and Molecules

[sonalivij]

Would someone be able to explain to me what the difference between molecules and formula units are?

[Duby3L]

Molecules and formula units essentially you could think about like the number of atoms. Formula units are used for ionic compounds whereas molecules are used for a compound with more than one element so if asked to solve number of formula units its the same process as solving for number of atoms.

[David S]

The difference is that a molecule consists of a fixed number (at least 2) atoms bound together by covalent bonds (e.g glucose C6H12O6). So when I say I have 2 glucose molecules, I mean two separate, discrete pieces of the same C6H12O6; I'm not saying I have C12H24O12, which would be an entirely different molecule.

With ionic compounds however, what we typically have are large, effectively continuous array of repeating ions and anions all together. Think of table salt. One piece of table salt might have 55 Na atoms, and 55 Cl atoms repeating in a pattern, while another chunk of table salt might have 20 Na atoms and 20 Cl atoms, and so on. So, to make things simple, rather than representing ionic compounds as the exact ratio of atoms (like we do for molecules), we simply write the chemical formula to represent the most basic, repeating unit of atoms (in the case of table salt, NaCl is the basic repeating unit). That is what we call a formula unit.