# Ionic Compound Naming Calculator - Formula to Name and Name to Formula

> Write ionic formulas from cation and anion by criss-cross, name compounds from formulas and find formulas from names, with acids, hydrates and a quiz.

- URL: https://qilingtools.com/en/math/ionic-compound
- Category: Math Tools (https://qilingtools.com/en/math)
- Tags: [Math](https://qilingtools.com/en/tags/math), [Chemistry](https://qilingtools.com/en/tags/chemistry), [Education](https://qilingtools.com/en/tags/education)
- Last updated: 2026-10-26
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

An ionic formula is written so that the total positive and negative charges cancel: cross the charge numbers over as subscripts, reduce to the lowest whole-number ratio, and put a polyatomic ion in parentheses when there is more than one of it. Fe³⁺ and SO₄²⁻ give Fe₂(SO₄)₃, iron(III) sulfate or ferric sulfate; NH₄⁺ and PO₄³⁻ give (NH₄)₃PO₄, ammonium phosphate; Ca²⁺ and HCO₃⁻ give Ca(HCO₃)₂, calcium hydrogen carbonate (calcium bicarbonate); and the mercury(I) ion Hg₂²⁺ stays together, so mercury(I) chloride is Hg₂Cl₂, not HgCl. This tool includes 27 common cations and 41 anions, polyatomic ions such as sulfate, permanganate, dichromate and acetate among them, with English and Chinese names. Pick a pair to get the formula, the criss-cross steps, Stock (Roman numeral) and classical -ous/-ic names, the molar mass and a solubility hint, plus warnings for pairs that do not really exist such as FeI₃ or Al₂S₃. In reverse, type FeCl2 and the tool deduces iron's +2 charge from the chloride ions to give iron(II) chloride; N2O4, P4O10 and SF6 are named with Greek prefixes (dinitrogen tetroxide); HCl, H2SO4 and HClO get acid names (hydrochloric, sulfuric, hypochlorous acid); and CuSO4·5H2O is copper(II) sulfate pentahydrate. Names work too: type “copper(II) nitrate”, “ferric sulfate”, “baking soda” or a Chinese name to get the formula. A practice mode asks random questions and keeps your score in your browser, and a printable ion list covers charges and naming patterns.

## How to use

1. In “Ions → formula”, pick a cation and an anion to see the formula, the criss-cross steps and the names
2. Or switch to “Formula → name” or “Name → formula” and type FeCl2, Fe2(SO4)3, “copper(II) nitrate” or “dinitrogen pentoxide”
3. Practise with random questions, or open the ion list to review and print common ions and charges

## Features

- 27 cations and 41 anions including common polyatomic ions, with English and Chinese names
- Criss-cross formula writing with reduction, parentheses and Hg₂²⁺ handled correctly
- Stock names with Roman numerals plus classical names (ferrous/ferric, cuprous/cupric)
- Formula → name deduces the charge of variable-charge metals and recognises acids, bases, salts, oxides and peroxides
- Greek-prefix names for binary molecular compounds, e.g. N₂O₅ dinitrogen pentoxide
- Hydrates, common names (baking soda, slaked lime, blue vitriol) and molar mass
- Warnings for pairs that do not exist or hydrolyse, plus a solubility table
- Scored practice quiz and a printable ion list

## FAQ

### How do you write the formula of an ionic compound?

Write the cation first and the anion second with their charges, then use the size of each charge as the subscript of the other ion, reduce to the lowest ratio, and bracket any polyatomic ion that appears more than once. Check that the charges add up to zero. Al³⁺ and SO₄²⁻ give Al₂(SO₄)₃; Mg²⁺ and O²⁻ give 2:2, which reduces to MgO.

### When do you use Roman numerals in a compound name?

Use a Roman numeral for metals that commonly form more than one cation, such as iron, copper, tin, lead, mercury, manganese, cobalt and chromium. The numeral is the charge of the cation: FeCl₂ is iron(II) chloride and FeCl₃ is iron(III) chloride. The older classical system uses -ous for the lower charge and -ic for the higher one: ferrous and ferric chloride. Metals with a single common charge, like sodium, calcium, zinc, silver and aluminum, take no numeral.

### How are binary molecular compounds like N₂O₅ named?

Compounds of two nonmetals are not ionic, so Greek prefixes give the number of each atom: mono-, di-, tri-, tetra-, penta-, hexa- and so on, with the second element ending in -ide. Mono- is dropped for the first element, and the final a or o of a prefix is usually dropped before oxide: CO is carbon monoxide, N₂O₅ dinitrogen pentoxide, P₄O₁₀ tetraphosphorus decoxide and SF₆ sulfur hexafluoride.

### How are acids and hydrates named?

Binary acids take hydro-…-ic acid: HCl is hydrochloric acid and H₂S hydrosulfuric acid. Oxyacids follow the anion: -ate becomes -ic acid (sulfate → sulfuric acid) and -ite becomes -ous acid (sulfite → sulfurous acid), so HClO from hypochlorite is hypochlorous acid. Hydrates add a Greek prefix and “hydrate”: CuSO₄·5H₂O is copper(II) sulfate pentahydrate and Na₂CO₃·10H₂O sodium carbonate decahydrate.

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