Ionic compounds form when positive ions and negative ions attract to make a neutral substance. Naming them correctly helps chemists identify what ions are present and in what charge balance. The name usually lists the cation first and the anion second, matching the order in the chemical formula.
A clear naming system prevents confusion between compounds such as iron(II) chloride and iron(III) chloride.
For simple binary ionic compounds, the metal keeps its element name and the nonmetal ending changes to -ide. Polyatomic ions keep their standard ion names, such as nitrate, sulfate, or ammonium. Transition metals often need Roman numerals because the same metal can form more than one positive charge.
To name a compound from its formula, identify each ion, determine any variable metal charge, then write the correct name without using prefixes.
Key Facts
- Ionic compound names list the cation first and the anion second.
- Binary ionic compound pattern: metal name + nonmetal root + ide, such as NaCl = sodium chloride.
- The total charge of an ionic compound is always zero.
- Roman numerals show the charge of a variable-charge metal, such as FeCl2 = iron(II) chloride.
- Polyatomic ions keep their names in compounds, such as CaCO3 = calcium carbonate.
- Do not use Greek prefixes for ionic compounds, so MgCl2 is magnesium chloride, not magnesium dichloride.
Vocabulary
- Cation
- A cation is a positively charged ion, usually a metal ion or ammonium in ionic compounds.
- Anion
- An anion is a negatively charged ion, usually a nonmetal ion or a polyatomic ion.
- Polyatomic ion
- A polyatomic ion is a group of bonded atoms that carries an overall charge and acts as one ion.
- Roman numeral
- A Roman numeral in an ionic compound name shows the positive charge of a metal ion that can have more than one charge.
- Binary ionic compound
- A binary ionic compound is made from only two elements, usually one metal and one nonmetal.
Common Mistakes to Avoid
- Using prefixes such as mono-, di-, or tri- for ionic compounds. Prefixes are used for many covalent compounds, but ionic compound names are based on ion charges.
- Forgetting Roman numerals for transition metals with variable charges. CuCl and CuCl2 contain copper ions with different charges, so their names must show copper(I) and copper(II).
- Changing the name of a polyatomic ion incorrectly. In NaNO3, nitrate stays nitrate, so the name is sodium nitrate, not sodium nitrogen oxide.
- Assuming subscripts become Roman numerals. In Fe2O3, the oxygen charge is used to calculate Fe as 3+, so the name is iron(III) oxide, not iron(II) oxide.
Practice Questions
- 1 Name the ionic compound CaBr2.
- 2 Name the ionic compound Fe2(SO4)3. Show how you determine the iron charge.
- 3 Explain why MgCl2 is named magnesium chloride instead of magnesium dichloride.