Formulas with polyatomic ions:

• Although compounds containing polyatomic ions will have more than two elements, write their formulas just like binary compounds.
   
• The polyatomic ion may be either the positive or the negative half of the formula - or both.
   
• If a subscript must be added to the polyatomic ion to balance oxidation numbers, put parenthesis around the ion and add the subscript outside the parenthesis.

  This is the only time parenthesis are used in a chemical formula.

  Example: combine calcium and cyanide
   
  • Calcium has an oxidation number of +2.
     
  • The cyanide ion is CN⁻.
     
  • Put parenthesis around CN before adding the subscript!!!
     
  • The formula for calcium cyanide is Ca(CN)₂.

NEVER change the subscripts in a polyatomic ion!  

Chemical Formulas

 
Steel Wool