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Name: George N.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 2001 - 2002

A fellow teacher asked me a question a student inquired of him. I hope you can help us out. The question was "Why is the formula for water H2O and not OH2?"

It is conventional to write the more electropositive element in a compound first and the more electronegative element last -- so NaCl rather than ClNa. Having said that sometimes it is not all that clear, for example iodine monochloride is written ICl rather than ClI. I doubt that any chemist would be confused if you wrote OH2 rather than H2O.

Vince Calder

Hi, George !! During the development of the chemistry there were a number of symbols used by the chemists. Nowadays, the modern way to write the water is H2O. The positive charged ion remains on the left side and the negative charged ion remains on the right side of the molecule. So, instead of SO4H2, you set down H2SO4. Water is represented by H2O.

Best regards
Alcir Grohmann

It is all a matter of convention. For simple molecules such as water, the convention is to list the more electropositive elements first and the more electronegative elements later - H2O, NaCl. This basic principle is also used for ionic compounds in which the anion and/or cation are composed of many atoms - LiClO4, NH4Cl. Other conventions are used, however. Organic compounds are usually listed as carbon, then hydrogen, followed by all the other elements in alphabetical order of element symbol.

Any chemist will, however, recognize OH2 as water. In fact, any chemist will also be able to give examples of situations in which it would make sense to write the formula for water as H2O.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

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