How to determine a formula is molecule or formula unit?

Question

I read difference sources but still confuse on how to determine a formula is a "molecule" or "formula unit" as the unit when I write conversion factors.

For example,

C3H8 molecule?

NaCl formula unit?

CaCO3 formula unit?

Answer 1

A formula unit is reserved for when we talk about ionic salts, such as sodium chloride (NaCl) or to covalent network solids. Instead of referring to a molecule, we refer to a formula unit.

A formula unit is the empirical formula and is the lowest whole number ratio of ions represented in an ionic compound. If you look at NaCl for example, it is made up of a lattice network of Na and Cl ions, so it isn't a molecule in the true sense of the word. That's why they use another term, i.e. formula unit.

C3H8 molecule? This is a molecule because it isn't a salt or a network solid

NaCl formula unit? This is a formula unit because it is a salt

CaCO3 formula unit? This is a formula unit because it is a salt

Answer 2

Good question.

A molecule is whole ... A formula unit is part of the whole.

Water is H2O. It's molecule is H2O. It's formula unit is H2O.

Hydrogen peroxide's molecule is H2O2 but it's formula unit is HO.

Hexane's molecule is C6H14 and it's formula unit is C3H7.

Another example is cyclohexane. It's like a 6-bead bracelet ..... implied with "hex" and "cyclo".

It's molecule is C6H12 and it's formula unit is CH2. That formula unit represent one of the beads in the bracelet.