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Ionic Compound Formula Builder

Chemistry

Build the formula of an ionic compound from a cation and an anion, name it, turn names into formulas and formulas into names, or fill a whole ion grid with formulas and solubility tags. It covers common ions, hydrates and the general solubility rules, but not complex ions such as [Fe(CN)₆]³⁻, double salts and alums, carbides, or acid names.

Name parsing accepts both spellings either way.
Search by symbol or name: Fe, ferric, ammonium
Search by formula or name: SO4, sulfate, bicarbonate

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Quick examples

Fe₂(SO₄)₃

iron(III) sulfate

Classical name: ferric sulfate

Ions

2 × Fe³⁺ iron(III)

3 × SO₄²⁻ sulfate

Molar mass

399.86 g/mol, 17 atoms per formula unit

Full molar mass breakdown →

Solubility in water

soluble

Most sulfates are soluble.

Classroom rules at about 25 °C. Real solubility is a continuum, so check the Ksp for borderline salts. Solubility Product Calculator →

Steps

  1. Fe³⁺ and SO₄²⁻: charges 3 and 2
  2. Criss-cross: Fe₂(SO₄)₃
  3. gcd(2, 3) = 1, already in lowest terms
  4. Check: 2 × (+3) + 3 × (−2) = 0 ✓
  5. SO₄²⁻ is polyatomic and appears 3 times, so it goes in parentheses

Charge balance

Fe³⁺+3Fe³⁺+3SO₄²⁻−2SO₄²⁻−2SO₄²⁻−2+6 / −6→ neutral2 Fe³⁺ + 3 SO₄²⁻

Criss-cross

Fe3+SO42−Fe2(SO4)3already in lowest terms

Criss-cross is a shortcut for the lowest common multiple of the charges: L = lcm(3, 2) = 6, so 6 ÷ 3 = 2 Fe and 6 ÷ 2 = 3 SO₄.

See every atom's oxidation number →

About This Tool

Ionic Compound Formula Builder – Write, Name and Check Ionic Formulas

An ionic compound is built from cations (positive ions, usually metals) and anions (negative ions, usually nonmetals or groups of atoms). The compound as a whole is always electrically neutral, so the total positive charge must cancel the total negative charge. This tool does that bookkeeping for you: it writes the formula, names it, converts names to formulas and back, and tags each salt as soluble or not.

The criss-cross method

To write a formula, take the charge of each ion and swap the numbers: the cation's charge becomes the anion's subscript and the anion's charge becomes the cation's subscript. Then divide by any common factor. Criss-cross is really a shortcut for the lowest common multiple of the two charges.

Al³⁺ + O²⁻   swap → Al₂O₃      (2 : 3, nothing to reduce)
Pb⁴⁺ + O²⁻   swap → Pb₂O₄ → ÷ 2 → PbO₂   (2 : 4 reduces to 1 : 2)

Polyatomic ions and parentheses

Groups such as sulfate SO₄²⁻ or ammonium NH₄⁺ behave as one unit. When a polyatomic ion appears more than once, put it in parentheses so the subscript applies to the whole group: Fe₂(SO₄)₃ and (NH₄)₂SO₄. With a count of one there are no parentheses, as in NH₄NO₃.

Naming: fixed and variable charges

Metals such as sodium, calcium and aluminum form one common ion, so their names carry no numeral: sodium chloride, calcium oxide. Metals that form several ions, such as iron, copper, tin and lead, need a Roman numeral in the Stock name, written with no space: iron(III) sulfate, copper(I) oxide. Older classical names use the endings -ous (lower charge) and -ic (higher charge): ferrous and ferric, cuprous and cupric.

Tip: the charge-number form
IUPAC also allows the charge itself in brackets, as in iron(3+) oxide. The converter accepts it, along with lowercase numerals and extra spaces.

Hydrates

Many salts crystallise with water. The water is written after a dot and named with a Greek prefix: CuSO₄·5H₂O is copper(II) sulfate pentahydrate, and CaSO₄·½H₂O is calcium sulfate hemihydrate. The water carries no charge, so it never changes the ion ratio, but it does change the molar mass.

Traps to watch for

Common mistakes
Mercury(I) is the dimer Hg₂²⁺, so its chloride is Hg₂Cl₂, not HgCl. Peroxides and superoxides contain O₂ units, so sodium peroxide is Na₂O₂ and not NaO. A reduced formula can mislead you: SnO₂ would un-cross to Sn²⁺, but tin has to be +4 because oxide is 2−. And Fe₃O₄ has no whole-number iron charge because it is a mixed-valence oxide.

Solubility rules

Every result carries a verdict from the general rules: all group 1 and ammonium salts, nitrates and acetates are soluble; most chlorides, bromides and sulfates are soluble with a few well-known exceptions; carbonates, phosphates, hydroxides and most sulfides are not. These are classroom rules at about 25 °C, so for borderline salts use the Solubility Product Calculator and the real Ksp.

Frequently Asked Questions

Is the Ionic Compound Formula Builder free?

Yes, Ionic Compound Formula Builder is totally free :)

Can I use the Ionic Compound Formula Builder offline?

Yes, you can install the webapp as PWA.

Is it safe to use Ionic Compound Formula Builder?

Yes, any data related to Ionic Compound Formula Builder only stored in your browser (if storage required). You can simply clear browser cache to clear all the stored data. We do not store any data on server.

How do you write the formula of an ionic compound?

Find the charge of the cation and the anion, then balance them so the total charge is zero. The criss-cross method swaps the charges into subscripts (Al³⁺ and O²⁻ give Al₂O₃), and you then reduce to lowest terms. It is a shortcut for the lowest common multiple of the two charges.

When do you put parentheses in a formula?

Only when a polyatomic ion appears more than once. Fe₂(SO₄)₃ and (NH₄)₂SO₄ need them so the subscript applies to the whole group, while NH₄NO₃ and KMnO₄ do not because each group appears once.

When does an ionic name need a Roman numeral?

Only for metals that form more than one common ion, such as iron, copper, tin, lead and mercury. The numeral gives the charge: iron(III) sulfate has Fe³⁺. Metals with a single ion, like sodium or calcium, take no numeral.

Why is mercury(I) chloride Hg₂Cl₂ and not HgCl?

Mercury(I) exists as the dimer Hg₂²⁺, two bonded Hg atoms sharing a 2+ charge, so one unit balances two chloride ions. The same idea makes sodium peroxide Na₂O₂, because the peroxide ion is O₂²⁻ and not a single oxide.

What is the difference between ferric and iron(III)?

They name the same ion, Fe³⁺. Iron(III) is the Stock name, which uses a Roman numeral and works for every metal. Ferric is the older classical name, where -ic marks the higher charge and -ous the lower, so ferrous is Fe²⁺.

Why don't ionic compound names use prefixes like di or tri?

The charges fix the ratio, so a prefix adds nothing: aluminum chloride can only be AlCl₃. Greek prefixes belong to molecular compounds such as dinitrogen pentoxide. Dichromate and dihydrogen phosphate are ion names, not count prefixes.