Writing ionic formulas

5 minAS Level2.3.1

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can write the formula of any ionic compound from the charges of its ions or from its name (Roman numerals included), using brackets correctly.

The one rule

An ionic compound has no overall charge. Total positive charge = total negative charge. That's it; everything else is bookkeeping.

The method:

Write each ion with its charge (from the group, the Roman numeral, or your list of nine).
Find the lowest common multiple of the charge sizes.
Use just enough of each ion to reach it. Those numbers become subscripts.
Put brackets round a compound ion if you need more than one of it. Check: charges add to zero.

Make the positive row and the negative row the same length. Then count the tiles.

Worked example: Aluminium sulfate

Write the formula of aluminium sulfate.

Have a go first?

Quick check

One-line answerQuestion 1 · Apply

Write the formula of magnesium nitride. (Type subscripts as normal numbers, e.g. Al2O3.)

Multiple choiceQuestion 2 · Recall

What is the formula of calcium hydroxide?

\text{Ca(OH)}_2

\text{CaOH}_2

\text{CaOH}

\text{Ca}_2\text{OH}

Answer the 2 questions above to continue.

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