Ionic formulae

1. Jul 11, 2006

pivoxa15

When a cation and anion combine to form an ionic compound, the cation is usually written before the anion. I.e. NaCl

But with some ionic compounds like sodium acetate, it is written with the anion acetate written first. i.e. CH3COONa

Why like this?

2. Jul 11, 2006

bomba923

Where did you see sodium acetate written as CH3COONa ?
(with the cation expressed after the anion)

3. Jul 11, 2006

Gokul43201

Staff Emeritus
The requirement (of putting the cation first) is relaxed in the case of organic compounds (typically salts of carboxylic acids). The idea is that you want to minimize the modification to notation for the parent structure - in this case CH3COOH.

This clearly does not mean that the cation is always written at the end - for instance, in a dicarboxylic acid with 2 different cations, it would make more sense to write something like KOOC-(CH2)3-COONH4.

Because of the attendant loss of clarity over which is the cation and which the anion, it becomes customary to include the necessary information to elucidate this. So the above compound (Ammonium potassium pentanedioate - it is convention to list the cations in alphabetical order) would best be written as: K+ -OOC-(CH2)3-COO- NH4+

4. Jul 11, 2006

pivoxa15

In a senior high school textbook.

5. Jul 11, 2006

pivoxa15

So with organic compounds, preservation of the notion of the parent structure is more important than keeping witht the rigid rule of naming ionic compounds.

6. Jul 12, 2006

Gokul43201

Staff Emeritus
That's sort of the idea, but as Jack Sparrow would say, it's really more of a guideline. It's with the systematic nomenclature that the IUPAC has more elaborate "rules".