How to write chemical formulae with the criss-cross method

Guide · Updated 2026-11-10

Learn what valency is, how to cross valencies over to write ionic formulae, and how to find formula unit mass, with a worked example.

Open the Formula writer Free, in your browser, nothing uploaded.

The chemical formula writer helps you turn two ions into one correct formula, with the name and the mass. It is made for Class 9 and 10 students learning to write formulae, and for older students who want a quick check. To see where valencies come from, use build an atom. To use your finished formulae in a reaction, go to the equation balancer.

What it is

Many compounds are made of ions: atoms or groups of atoms with an electric charge. A metal usually loses electrons and becomes a cation (positive). A non-metal usually gains them and becomes an anion (negative).

The valency of an ion is the size of its charge, ignoring the sign. Sodium is Na⁺, valency 1. Oxide is O²⁻, valency 2.

A compound has no overall charge, so the plus and minus must cancel:

(number of cations × cation charge) = (number of anions × anion charge)

The criss-cross method is a shortcut that makes this true. Write the valencies, swap them so each becomes the other ion’s subscript (the small number after the symbol), and divide by any common factor. Groups of atoms that stay together, such as sulfate SO₄ or nitrate NO₃, are put in brackets if you need more than one of them.

Formula unit mass is the total of the atomic masses in the formula, in atomic mass units (u). For an ionic compound we say “formula unit” because it is a repeating pattern, not a separate molecule.

How to use it

1. Choose the positive ion (cation) from the list. Each entry shows its valency.
2. Choose the negative ion (anion).
3. Read the picture of the valencies crossing over, then the formula, name and four steps below it.
4. Check the charge blocks: positive and negative rows must be the same length.
5. Choose Class 9: rounded masses or the exact masses to see the formula unit mass.
6. Try the Check yourself questions. Type digits normally, such as Ca(NO3)2.

A worked example

Let us build calcium nitrate. Pick calcium (Ca, valency 2) and nitrate (NO₃, valency 1).

1. Write the valencies: Ca 2, NO₃ 1.
2. Cross them over: Ca gets 1, NO₃ gets 2.
3. The numbers 1 and 2 have no common factor, so the ratio stays 1 : 2.
4. Nitrate is a group and we need two of it, so it goes in brackets. A subscript of 1 is not written. The formula is Ca(NO₃)₂.

Check the charges: 1 × (+2) = +2 and 2 × (−1) = −2. They cancel.

Formula unit mass with rounded masses: 40 + 2 × 14 + 6 × 16 = 40 + 28 + 96 = 164 u. (There are six oxygen atoms, three in each of the two nitrate groups.)

With exact atomic weights: 40.078 + 2 × 14.007 + 6 × 15.999 = 164.086 u.

A second case with a metal that has two valencies: iron(III) hydroxide. Fe has valency 3 and OH has valency 1, so the formula is Fe(OH)₃. With rounded masses (Fe 56, O 16, H 1): 56 + 3 × (16 + 1) = 107 u.

Where it is used

• Labs. Before you weigh a salt, you need its formula and mass to work out how much to use.
• Industry. Fertilisers, cement and cleaning products are written and costed by formula. Calcium nitrate, for example, is a plant fertiliser.
• Medicine and food. Labels list mineral salts such as calcium carbonate. The formula tells you how much of the useful metal is in each gram.
• Water and the environment. Reports on hardness or pollution name ions such as nitrate and sulfate. Knowing their formulae helps you read them.

Common mistakes

• Brackets around a single atom. Write MgBr₂, not Mg(Br)₂. Brackets are for groups of atoms only.
• Forgetting to simplify. Al³⁺ with N³⁻ crosses to Al₃N₃. Divide by 3 to get AlN.
• Changing the group itself. Never change the number inside the ion’s formula to balance charge. Only change how many of the whole ion you take.
• Ignoring the Roman numeral. Iron(II) and iron(III) give different formulae. The numeral is the valency.
• Counting atoms inside brackets once. In Ca(NO₃)₂ the subscript 2 multiplies everything inside, so there are two N and six O.

Questions people ask

Does the criss-cross method always give the right answer? It works for simple compounds whose valencies you know, and the tool always checks that the charges cancel. It cannot predict compounds that are not in the simplest ratio, such as hydrogen peroxide, H₂O₂, or elements whose valency you have not been told.

Why are there two sets of masses? Schools round masses to whole numbers or halves to make sums easy. Exact values come from the standard atomic weights. They differ slightly, so use the set your teacher or exam asks for.

Can I use the formula unit mass for moles? Yes. The same number in grams per mole is the molar mass, which the mole bridge uses.