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QUESTION CORNER
Chemical symbol
Why is hydroxide represented as OH and not HO?
V. Eashwar
Puducherry
There is no set of hard and fast rules or regulations for this practice. Still, there are some broad guidelines.
The atom whose valence is unfulfilled in the group and hence is linked to another unspecified group or atom is placed on the left of the group along with the other atoms whose valence is fulfilled, in the group, to its right.
In the present case of hydroxyl group, the oxygen atom is in a bivalent state (or it needs two bonds) whereas the hydrogen is in its monovalent.
Since hydrogen is already bonded to oxygen, its valence is fulfilled whereas the oxygen atom with its linkage to hydrogen gets only one of its two valences satisfied.
In other words, oxygen in OH group has to be linked to some other atom or group to get its other valence fulfilled. Hence, oxygen atom is written to the left in OH group as OH but not to the right as HO.
The same logic holds in the cases of groups such as NH{-2}, CN, SO{-3}H, COOH, CH{-3}, C{-6}H{-5}, etc. An interesting example for this convention is in the isomeric pair of thiocyanate (SCN) and isothiocyante (NCS) wherein the unfulfilled valence is situated with sulphur and nitrogen atoms respectively and therefore placed leftmost in the respective groups.
There are some exceptions to this convention in cases such as HSO{-4}, H{-2}PO{-4}, CH{-3}CH{-2} etc, wherein it is not the leftmost atom with unfulfilled valence.
In molecules such as CO, HCl, AlCl{-3}, B{-2}H{-6}, HCN, SO{-2}, etc, where the valence of all the atoms is fulfilled, the atom with a formal positive oxidation state is written on the left. There are some exceptions to this rule also, e.g., NH{-3}, PH{-3}, CH{-4}, C{-2}H{-6}, etc.
Finally, we should understand that there is neither any natural cause nor any scientific basis for the way we write chemical formulae of groups and molecules.
Some amount of experience in chemistry enables users to fall in line with the tradition of writing the chemical formulae in their most acceptable formats.
PROF. A. RAMACHANDRAIAH
Department of Chemistry
National Institute of Technology
Warangal, Andhra Pradesh
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