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How to Name Ionic Compounds

When atoms share electrons, they form covalent bonds. When ions with opposite charges attract, they create ionic compounds that arrange themselves into a crystalline lattice known as a salt. Naming these compounds is systematic: first identify the cation (positive ion), then the anion (negative ion). For transition‑metal cations, the oxidation state is indicated with Roman numerals.

TL;DR

1. Cation first. 2. Anion second. 3. Add “‑ide” to elemental anions. 4. Keep polyatomic anions unchanged. 5. For transition metals, specify charge in Roman numerals.

The Cation Comes First

The cation is the positively charged ion—usually a metal. Elements in Groups 1 and 2 (alkali and alkaline‑earth metals) form ions with fixed charges (1+ and 2+ respectively). Thus, a compound containing sodium will always begin with “sodium” and a calcium compound will begin with “calcium.”

Transition metals (Groups 3–12) can form ions with multiple oxidation states. The ion’s charge is shown in parentheses after the element’s name. For example:

• Fe(III) – ferric iron (Fe³⁺)
• Fe(II) – ferrous iron (Fe²⁺)

The Anion Comes Next

The anion is the negatively charged species. It can be a single element from Groups 15–17 or a polyatomic ion. For elemental anions, change the ending to “‑ide”: chlorine → chloride, bromine → bromide, oxygen → oxide.

For polyatomic anions, use the ion’s established name unchanged. Examples: sulfate (SO₄^2‑), nitrate (NO₃^‑), carbonate (CO₃^2‑).

Determining the Cation’s Charge from the Formula

To name a compound containing a Group 1 or 2 cation, simply write the cation name followed by the anion name (with the “‑ide” suffix if needed). Examples: sodium chloride, magnesium sulfate, calcium oxide.

For transition‑metal cations, the oxidation state is inferred from the overall neutrality of the compound. The anion’s subscript and valency guide the calculation. For instance:

• FeO: Oxide has a 2‑ charge; iron must be 2+ → iron(II) oxide.
• Fe₂O₃: Two oxides (2 × 2‑ = 4‑). To balance, iron must be 3+ → iron(III) oxide.

Following these rules ensures accurate, universally accepted names for all ionic compounds.