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Name:  Lisa Malone
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Age: N/A
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Date: 1999 

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

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

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