Writing Formulas

Binary Compounds
Compounds made from only two elements

** Note: Some browsers are not capable of showing superscripts and subscripts; realize that oxidation numbers are written as superscripts and subscripts in formulas are subscripted . Be sure to use the proper format when completing work

Before you can write the formula for a compound, you must know the oxidation numbers for the elements making up the compound

  1. Write symbol for the element with the positive oxidation number
  2. Write symbol for element with the negative oxidation number (on same line)
  3. Write in subscripts to make the sum of the oxidation numbers zero

      Try these

      1. compound of calcium and oxygen

      2. compound of Copper(II) and sulfur

      3. compound of magnesium and chlorine
      Check your answers

      Compounds with Polyatomic Ions

      The rules for writing formulas for compounds containing polyatomic ions(a group of covalently bonded atoms that act as an ion) are basically the same as for binary compounds; just treat the polyatomic ion as you would a single atom

      1. Write the symbol(formula) of the element(polyatomic ion) that has a positive oxidation number
      2. Write the name of the negative ion (element or polyatomic) directly beside it (on same line)
      3. Use subscripts to make the sum of the oxidation numbers zero.(use same hints as listed above)
      4. If the polyatomic ion is used once, just write it as is
        Do NOT delete its original subscripts! That is not allowed!!!!
      5. If the polyatomic ion is used more than once, you must enclose it in parentheses. Then write the balancing subscripts outside of the partentheses
        Be sure to include these parentheses, if necessary!!
      Note A partial listing of polyatomic ions is in your book on page 286 (chemistry book- p. 123) and on the back of the Navy's periodic table. It would be in your best interest to memorize these (add cyanide, CN-1)