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How do you write formula units?
You follow a systematic process to write formula units.

Identify the symbol of the cation and the anion.

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

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

Place these numbers as subscripts after the symbols.
Example:
Write the formula for a formula unit of iron(III) carbonate.
Solution

The symbol for iron is Fe. The formula for carbonate is CO₃.

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⁻.

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⁻.

The formula is Fe₂(CO₃)₃.