How do I write the formula units of ionic compounds?

1 Answer
Apr 29, 2014

You follow a systematic process to write formula units.


  1. Identify the symbol of the cation and the anion.

  2. Identify the charge of each ion and place it as a superscript above the symbol.

  3. Calculate the numbers of each ion that make the total charge equal to zero.

  4. Place these numbers as subscripts after the symbols.


Write the formula for a formula unit of iron(III) carbonate.


  1. The symbol for iron is #"Fe"#. The formula for carbonate is #"CO"_3#.

  2. The charge on iron(III) is +3, so its symbol is #"Fe"^"3+"#. The charge on a carbonate ion is -2, so its formula is #"CO"_3^"2-"#.

  3. We must find the lowest common multiple of the charges. The lowest common multiple of 3 and 2 is 6. That means we need 6 + charges and 6 – charges. We need 2 #"Fe"^"3+"# and 3 #"CO"_3^"2-"#.

  4. The formula is #"Fe"_2("CO"_3)_3#.