How do you write formula units?

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.

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 CO₃.

2. The charge on iron(III) is +3, so its symbol is Fe³⁺. The charge on a carbonate ion is -2, so its formula is CO₃²⁻.

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 Fe³⁺ and 3 CO₃²⁻.

4. The formula is Fe₂(CO₃)₃.