How this instrument works
Ionic compounds form when a positively charged cation and a negatively charged anion combine in whatever ratio makes the overall compound electrically neutral. Sodium (Na⁺) and chloride (Cl⁻) both carry a single charge, so they pair one-to-one: NaCl. Calcium (Ca²⁺) and phosphate (PO₄³⁻) don't share a charge, so it takes three calciums and two phosphates to balance out — 3×(+2) = +6 and 2×(−3) = −6 — giving the formula Ca₃(PO₄)₂. That charge-balancing arithmetic, reduced to its simplest whole-number ratio using the greatest common divisor of the two charges, is exactly what this instrument runs.
Naming follows the formula: state the cation's name first, then the anion's, dropping the word 'ion' from both. Simple nonmetal anions get an -ide ending (chlorine becomes chloride), while polyatomic ions like sulfate, nitrate, and phosphate keep their own established names. When a polyatomic ion appears more than once in a formula, it gets wrapped in parentheses before the subscript is applied — (PO₄)₂, not PO₄₂ — so the formula still reads unambiguously as 'two phosphate ions,' not some entirely different four-oxygen-and-two-more grouping.
Metals like iron and copper form more than one stable cation charge, and for those the Stock system adds a Roman numeral in parentheses right after the metal's name to say which charge is in play — Iron(III) Oxide (Fe₂O₃, using Fe³⁺) is chemically distinct from Iron(II) Oxide (FeO, using Fe²⁺), and the Roman numeral is the only thing in the name that tells them apart.
- Choose a cation from the first dropdown — the list covers common metal and ammonium cations, with their charge shown alongside the name.
- Choose an anion from the second dropdown — common monatomic and polyatomic anions, charge included.
- Read the resulting formula, already reduced to its simplest whole-number ratio with parentheses added where a polyatomic ion appears more than once.
- Read the compound name alongside it, including a Stock-system Roman numeral for any cation that can carry more than one charge.
Worked example — calcium and phosphate build a real fertilizer compound
Calcium carries a 2+ charge and phosphate carries a 3− charge. Their greatest common divisor is 1, so the charge-balancing ratio is the full 3:2 — three calcium ions for every two phosphate ions, giving 3×(+2) + 2×(−3) = +6 − 6 = 0, a neutral compound. Because the phosphate polyatomic ion appears twice, it's wrapped in parentheses in the formula: Ca₃(PO₄)₂. The name follows directly from the ions involved, with no Roman numeral needed since calcium only ever forms a 2+ ion: Calcium Phosphate.
This isn't just a textbook exercise — Ca₃(PO₄)₂ is the same tricalcium phosphate used as a common calcium supplement and food additive, and it's chemically related to the calcium phosphate minerals that make up the mineral portion of bone and tooth enamel. Getting the 3:2 ratio right isn't optional trivia; it's the difference between a real, stable, charge-neutral compound and a formula that doesn't actually exist.
Questions
How do you name an ionic compound from its formula?
State the cation's name first, unchanged, then the anion's name second. For a simple nonmetal anion, change its ending to -ide (chlorine becomes chloride, oxygen becomes oxide); for a polyatomic ion like sulfate or nitrate, use its established name as-is. If the cation is a metal capable of more than one charge, add a Roman numeral in parentheses after its name showing which charge is present in this particular compound.
Why do some names have a Roman numeral and others don't?
The Roman numeral (the Stock system) only shows up for metals that can form more than one stable cation charge — iron, copper, and tin, among others, can each show up as more than one charge, so 'Iron Oxide' alone would be ambiguous between FeO and Fe₂O₃. Metals with only one common charge, like sodium (always 1+) or calcium (always 2+), never need a Roman numeral, since there's nothing to disambiguate.
Why do some formulas have parentheses and others don't?
Parentheses appear specifically around a polyatomic ion when its subscript in the final formula is greater than one — it keeps the ion's own internal atom count from getting confused with the compound's overall subscript. Ca₃(PO₄)₂ means three calcium ions and two whole phosphate ions (eight oxygens total across those two ions); without the parentheses, 'CaPO₄₂' would misleadingly suggest something different. A single polyatomic ion (subscript of one) doesn't need parentheses at all.
Can this calculator name any chemical compound?
It covers a curated set of common cations and anions combined as binary or polyatomic ionic compounds — the kind of straightforward salt formulas taught in an introductory chemistry nomenclature unit. It does not handle covalent (molecular) compound naming, acids, organic compounds, or free-text chemical formulas typed in directly; those follow different naming rules entirely and are outside this tool's scope.
Why is rust named Iron(III) Oxide and not just Iron Oxide?
Because iron commonly forms two different cations — Fe²⁺ (iron(II)) and Fe³⁺ (iron(III)) — and each pairs with oxide to make a genuinely different compound: FeO (iron(II) oxide) versus Fe₂O₃ (iron(III) oxide, the reddish compound found in rust and the mineral hematite). Without the Roman numeral, 'iron oxide' wouldn't tell you which of two real, distinct substances you mean, so the Stock-system numeral is doing real chemical work, not just decoration.