# Does it change the compound if the element letters in its molecular formula are switched?

Does it change the compound if the element letters are switched? ex. $\ce{HO}$ or $\ce{OH}$. Are those two different compounds or are they one in the same?

• Actually neither HO or OH is a compound. – MaxW Jul 25 '18 at 14:39

In some cases it can matter. $\ce{COH}$ usually refers to an alcohol. $\ce{CHO}$ refers to an aldehyde. But since acetic acid is a carboxylic acid, it is written as $\ce{CH3COOH}$

• How does the edited version answer the question. It doesn't. He was asking if there was a difference between OH and HO. $\ce{CH3COOH}$ doesn't show any difference in the use of OH and HO. Too bad you have never seen acetic acid written as AcOH. – LDC3 Jul 27 '18 at 3:40

Apart from affecting organic compounds, it also affects coordination compounds.

1. $\ce{Hg[Co(SCN)4]}$ is a significantly different compound from $\ce{Co[Hg(SCN)4]}$. The central metal atom and the counter-ions in both cases differ, as does the complex anion.
2. In fact, in the above compound itself, $\ce{Hg[Co(SCN)4]}$ differs from $\ce{Hg[Co(CNS)4]}$ or $\ce{Hg[Co(NCS)4]}$: this is called linkage isomerism. The order in which ligand atoms are written denotes the atom of attachment to the central metal atom, which happens to differ in this case.
3. The complex salts $\ce{[Co(NH4)6]^3+[Cr(CN)6]^3-}$ and $\ce{[Co(CN)6]^3+[Cr(NH4)6]^3-}$ - formed by swapping the two radicals - are coordination isomers, and not identical.

Around year 1830, two different compounds (with different chemical and physical properties) having almost the same formulae except from the order of the elements (in the molecule/ion structure) were found. This is when the concept of chemical isomerism has originated.

The compounds are

It may or may not make a difference depending on the intention to explain the structure of the compound.

The trouble is that chemical formulae (like $\ce{CH3COOH}$) are not a very good way to explain the structure of a compound. In some cases all the formula does is to tell us the total composition if the compound in terms of the rations of the elements making it up. But there are plenty of examples where several distinct compounds consist of different arrangements of the same atoms ($\ce{C2H6O}$ for example could be dimethyl ether or ethanol, two very different compounds).

Chemists don't like this ambiguity so often write such formulae with some hints as to the real connectivity of the molecules. In this case the two isomers are usually written as $\ce{CH3OCH3}$ and $\ce{CH3CH2OH}$ which, in this simple case is quite a good hint as to the actual connectivity in the molecules.

The trouble is that, for more complex molecules, writing the structure as a line of text rapidly becomes unwieldy or ambiguous and the structure needs to be explained with a picture that unambiguously shows the connectivity of the atoms in the molecule.

So to know whether switching the position of atoms on a text formula matters, you have to know the intention of the writer. For simple molecules this is usually clear but for complex ones it may rapidly become ambiguous. Chemists have many conventions that are widely understood even when they are not completely unambiguous to a novice. The best approach is to learn the conventions but, when learning, demand clear structural pictures as well. Also, read the formula in context and ask whether it is trying to convey structure as well as composition.