Ionic Compound Formula Writer

Writing a formula from a name

The tool above is a free ionic compound formula calculator. Type a compound name, get the formula, and see the charges crossed and reduced one step at a time rather than just an answer. It works the other way too, so you can use it to check homework or to practise against it.

The method itself is short. Find the charge on each ion, cross the charges over to become the other ion’s subscript, reduce to the lowest whole numbers, and bracket any polyatomic ion that ended up with a subscript greater than one.

Crossing the charges

Aluminium is Al³⁺ and oxide is O^2-. The 3 becomes the subscript on oxygen and the 2 becomes the subscript on aluminium, giving Al₂O₃.

This is not a trick. It works because it produces the smallest whole-number ratio with no net charge. Two aluminiums carry 6+, three oxides carry 6-, and the compound is neutral. If you ever forget the shortcut you can get the same answer by asking what numbers make the charges cancel.

Reducing, and when it matters

Crossing charges sometimes gives numbers that share a factor. Tin(IV) is Sn⁴⁺ and oxide is O^2-, which crosses to Sn₂O₄. Both subscripts divide by two, so the formula is SnO₂.

A formula gives a ratio, not a count. Sn₂O₄ and SnO₂ describe the same substance, and only the reduced form is correct.

The same thing happens whenever the two charges are equal. Calcium 2+ with carbonate 2- crosses to Ca₂(CO₃)₂, which reduces to CaCO₃. Magnesium oxide is MgO, not Mg₂O₂.

The one place reducing is wrong

Mercury(I) is Hg₂²⁺. That subscript is part of the ion: two mercury atoms sharing a single 2+ charge, not two separate mercury ions.

So mercury(I) chloride is Hg₂Cl₂, and it does not reduce to HgCl. HgCl is not a compound. You never divide inside an ion, only across ions, and mercury(I) is the one common case where the distinction is visible.

Brackets

A polyatomic ion that takes a subscript greater than one goes in brackets, so the number multiplies the whole group.

Iron(III) is Fe³⁺ and sulfate is SO₄^2-, which crosses to two irons and three sulfates. Written correctly that is Fe₂(SO₄)₃. Written without brackets it becomes Fe₂SO₄₃, which reads as forty three oxygen atoms.

The rule is about the subscript, not about the ion. Ammonium nitrate is NH₄NO₃ with no brackets at all, because each ion appears once. Ammonium phosphate needs three ammoniums, so it is (NH₄)₃PO₄.

Reading the numeral

The Roman numeral in a name is the charge on the metal ion, and it is the only thing in the name that tells you. Iron(III) means Fe³⁺. Copper(I) means Cu⁺. Lead(IV) means Pb⁴⁺.

Mercury(I) is the exception worth knowing: the numeral is I because each mercury carries 1+, but the ion as a whole is 2+ because there are two of them.

Metals that form only one ion carry no numeral. Sodium is always 1+, magnesium always 2+, aluminium always 3+, zinc always 2+ and silver always 1+.

Four formulas worth knowing by heart

They come up constantly and each one demonstrates a different part of the method:

• Al₂(SO₄)₃, aluminium sulfate: a straight cross with brackets
• Ca₃(PO₄)₂, calcium phosphate: a 2 against a 3, the bone mineral
• Fe₂O₃, iron(III) oxide: a cross with no brackets, because oxide is one atom
• NaHCO₃, sodium hydrogen carbonate: baking soda, and a reminder that the H belongs to the ion

Checking your own answer

Multiply each charge by its subscript and add. If the total is not zero, the formula is wrong regardless of how it looks. In Ca₃(PO₄)₂ that is three calciums at 2+ giving 6+, and two phosphates at 3- giving 6-, which cancels.

This check catches almost every arithmetic slip in under five seconds, and it is the last step in every worked solution the tool prints.

Where to go next

The reverse direction, reading a formula and naming it, is naming ionic compounds practice. If the ion charges are what slow you down, the polyatomic ions quiz drills those on their own.

For reference, see the complete list of polyatomic ions and the list of binary compounds. How ionic bonds work covers why charges cancel in the first place.

Common questions

Is there a calculator that writes ionic formulas?

The tool above. Type a name such as iron(III) sulfate and it returns Fe₂(SO₄)₃ with the criss-cross shown step by step, rather than only the answer.

When do I need brackets?

When a polyatomic ion takes a subscript greater than one. A single polyatomic ion needs none, which is why ammonium nitrate is NH₄NO₃.

Why is calcium carbonate CaCO₃ and not Ca₂(CO₃)₂?

Because the charges are equal and opposite, so they cancel one to one. Crossing them gives 2 and 2, and both divide by two. A formula is always the reduced ratio.