Ask A Scientist Chemistry Archive

Question:
What is the chemical formula for ammonium dichromate?  When are parentheses 
used in writing chemical formulas?
 Lisa Malone

Answer:
Ammonium dichromate:  (NH4)2Cr2O7
Parentheses are used when there are more than one polyatomic 
"functionalities" in a molecule and the formula has to be written out.
The example above shows this.  The ammonium ion (four hydrogens connected
to one nitrogen which has an overall charge of +1) charge has to offset
the -2 charge of the dichromate ion.  Since two ammoniums are needed,
and there's already a 4 as a subscript, the parentheses are used and the
2 goes outside.  Otherwise it would look like NH42.  Sound confusing?
Maybe someone else can word this better...
-Joe Schultz