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How do I write the formula units of ionic compounds?

Answer 1

Avery Adams

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 #"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#.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.