Binary compounds

Binary ionic compounds

The nomenclature for binary ionic compounds simply entails naming the ions according to the following rules:

1. The positive ion (called a cation) is named first and the negative ion (anion) second.
2. A simple cation (obtained from a single atom) takes its name from its parent element. For example, Li⁺ is called lithium in the names of compounds containing this ion. Similarly, Na⁺ is called sodium, Mg²⁺ is called magnesium, and so on.
3. A simple anion (obtained from a single atom) is named by taking the root of the parent element’s name and adding the suffix -ide. Thus, the F⁻ ion is called fluoride, Br⁻ is called bromide, S²⁻ is called sulfide, and so on.

The following examples illustrate the nomenclature rules for binary ionic compounds:

sodium chloride(opens in a new tab)

In the formulas of ionic compounds, simple ions are represented by the chemical symbol for the element: Cl means Cl⁻, Na means Na⁺, and so on. When individual ions are shown, however, the charge is always included. Thus, the formula of potassium bromide is given as KBr, but, when the potassium and bromide ions are shown individually, they are written K⁺ and Br⁻.

When a given metal atom can form more than one type of cation, the charge on the particular cation present must be specified in the name of the compound. For example, lead (Pb) can exist as Pb²⁺ or Pb⁴⁺ ions in ionic compounds. Also, iron (Fe) can form Fe²⁺ or Fe³⁺ ions, tin (Sn) can form Sn²⁺ or Sn⁴⁺ ions, gold (Au) can form Au⁺ or Au³⁺ ions, and so on. Therefore, the names of binary compounds containing metals such as these must include a Roman numeral to specify the charge on the ion. For example, the compound FeCl₃, which contains Fe³⁺, is named iron(III) chloride. On the other hand, the compound FeCl₂, which contains Fe²⁺, is designated as iron(II) chloride. In each case, the Roman numeral in the name specifies the charge of the metal ion present.

An alternative system for naming compounds containing metals that form only two ions is sometimes seen, especially in older literature. The ion with the higher charge has a name ending in -ic, and the one with the lower charge has the suffix -ous. For example, Fe³⁺ is called the ferric ion, and Fe²⁺ is called the ferrous ion. The names for FeCl₃ and FeCl₂ are then ferric chloride and ferrous chloride, respectively.