How do I write the formula units of ionic compounds?

1 Answer
Apr 29, 2014

Answer:

You follow a systematic process to write formula units.

Explanation:

  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.

Example:

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

Solution

  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#.