What it does
An ionic compound is made of positive ions (cations) and negative ions (anions) in just the right numbers for the charges to cancel. The quickest way to find those numbers is the criss-cross method: each ion’s valency becomes the other ion’s count. This page does it in front of you.
- Pick a cation (such as sodium, calcium, aluminium, ammonium or iron(III)) and an anion (such as chloride, oxide, hydroxide, sulfate or phosphate). Each list shows the valency.
- The picture shows the valencies crossing over. Below it, the formula appears with its name and four short steps: write the valencies, cross them over, divide out any common factor, add brackets round groups that repeat.
- Charges balance draws every positive and negative charge as a block, so you can see that, for example, two Al³⁺ (+6) exactly match three SO₄²⁻ (−6).
- Formula unit mass adds up the atoms. The Class 9 setting uses rounded masses (H 1, O 16, Na 23, Cl 35.5 …); the Class 11 setting uses the exact IUPAC atomic weights.
It follows Class 9 Science (atoms and molecules: ions, valency, writing chemical formulae, molecular mass and formula unit mass) and supports Class 10 and 11 Chemistry. See where the valencies come from in build an atom, then use your formulas in the equation balancer or turn them into moles with the mole bridge.
How to use it
- Choose the positive ion and the negative ion.
- Read the formula, the name and the steps.
- Check the charge picture: the two rows of blocks are always the same length.
- Switch between rounded and exact masses to see the formula unit mass both ways.
- Try the Check yourself questions. Type formulas with normal digits, such as Ca(OH)2 or Al2(SO4)3.
Worked example: aluminium sulfate
Pick aluminium (Al, valency 3) and sulfate (SO₄, valency 2).
- Write the valencies: Al 3, SO₄ 2.
- Cross them over: Al gets 2, SO₄ gets 3.
- 2 and 3 have no common factor, so the ratio stays 2 : 3.
- Sulfate is a group of atoms that appears three times, so it goes in brackets: Al₂(SO₄)₃.
Charges: 2 × (+3) = +6 and 3 × (−2) = −6.
Formula unit mass with rounded masses: 2 × 27 + 3 × 32 + 12 × 16 = 54 + 96 + 192 = 342 u. With exact IUPAC weights it is 342.132 u.
Now pick magnesium (2) and oxide (2). Crossing gives Mg₂O₂, and dividing by 2 gives MgO. That is why step 3 matters.
Tips
- Brackets only go round groups of atoms (polyatomic ions such as OH, NO₃, SO₄, CO₃, PO₄, NH₄), and only when there is more than one of them. Calcium chloride is CaCl₂, not Ca(Cl)₂.
- Iron, copper and lead can have more than one valency. The Roman numeral in the name, as in iron(III) oxide, tells you which.
- Formula unit mass is used for ionic compounds; molecular mass is the same sum for molecules such as H₂O or CO₂.
FAQ
Why does hydrogen with hydroxide give water? H⁺ and OH⁻ join to make H₂O, so the page shows water instead of “HOH”.
Is anything saved? No. Everything runs in your browser and nothing is stored or sent.
Is this an official textbook tool? No. It is an independent practice page that follows the same school topics.
Where the numbers come from
- Ion valencies and names: general chemistry facts (IUPAC naming with Roman numerals for iron, copper and lead), written fresh for fibub.
- Rounded atomic masses as used in school books; exact values are the IUPAC/CIAAW standard atomic weights from fibub's shared element data.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Read more on the Blog
- How to write chemical formulae with the criss-cross method(the full guide)
Last updated 2026-11-09.