How do I write the formula units of ionic compounds?
1 Answer
You follow a systematic process to write formula units.
Explanation:

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