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.
Understanding Chemistry: Naming Ionic Compounds
The charge on an ion comes from an imbalance between protons and electrons. Metals usually lose electrons, so they become positive ions. Nonmetals usually gain electrons, so they become negative ions.
The periodic table gives useful clues for many common ions. Group one metals usually form charge positive one ions. Group two metals usually form charge positive two ions.
Aluminum commonly forms charge positive three. Halogens commonly form charge negative one ions, while oxygen and sulfur commonly form charge negative two ions. These patterns help students predict charges before they begin naming or writing a formula.
Roman numerals do not tell how many atoms of a metal are present. They state the charge on each metal ion. This distinction is important with metals such as iron, copper, tin, lead, and chromium.
For example, imagine a compound with two iron ions and three oxide ions. Each oxide ion has charge negative two, giving a total negative charge of six. The two iron ions must provide total positive six, so each iron ion has charge positive three.
Its name uses iron(III). The numeral is found by balancing charge, not by guessing from the metal's position on the periodic table.
Polyatomic ions act as grouped units with their own overall charge. Nitrate has charge negative one. Sulfate has charge negative two.
Carbonate has charge negative two. Ammonium has charge positive one, even though it contains only nonmetals. When more than one copy of a polyatomic ion is needed in a formula, parentheses show that the whole group is repeated.
In calcium nitrate, one calcium ion balances two nitrate ions. Students should learn the most common polyatomic ions as complete names and charges. Changing the ending of nitrate to nitride would name a completely different ion.
Careful naming matters outside class because chemical labels must distinguish substances with different properties. Iron(II) compounds and iron(III) compounds can behave differently in water, soil, batteries, medicines, and industrial processes. A missing Roman numeral can make a label incomplete.
When checking a name, first identify whether the negative ion is a single element or a polyatomic ion. Next check whether the positive metal has more than one possible charge. Finally, confirm that the charges could balance to zero.
A useful habit is to write each ion with its charge in words on scratch paper before deciding on the final name. This slows the process slightly, but it prevents most naming mistakes.
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.