A chemical base is fundamentally defined by its ability to increase the concentration of hydroxide ions (\(\text{OH}^-\)) when dissolved in water, following the Arrhenius definition. More broadly, a base is a substance that can accept a proton (\(\text{H}^+\)) from another molecule, which is the Brønsted-Lowry definition. Chemical nomenclature, governed by the International Union of Pure and Applied Chemistry (IUPAC), provides a systematic way to name these substances. For many common inorganic bases, the naming conventions closely follow the rules established for ionic compounds. This system ensures that a base’s name clearly communicates its chemical composition.
Naming Simple Metal Hydroxides
The most straightforward bases to name are the metal hydroxides, which are ionic compounds formed when a metal cation is bonded to one or more hydroxide anions (\(\text{OH}^-\)). Metals in Group 1 (alkali metals) and Group 2 (alkaline earth metals) form cations with fixed charges. The naming convention involves two parts: the full name of the metal cation, followed by the anion name, “hydroxide.” For instance, \(\text{NaOH}\) is named sodium hydroxide. Similarly, calcium hydroxide (\(\text{Ca}(\text{OH})_2\)) is formed by the calcium ion (\(\text{Ca}^{2+}\)) and two hydroxide ions. Since the charge of the metal is fixed, it is unnecessary to include any numbers or prefixes in the name.
Accounting for Transition Metal Charge
A more complex situation arises when bases are formed from transition metals or other metals that can exhibit multiple possible positive charges, known as variable charges. Because the metal’s charge can vary, the resulting compound requires a naming system to distinguish between them. Chemists use the Stock system, which employs Roman numerals in parentheses to specify the charge of the metal cation.
The Roman numeral directly corresponds to the positive charge of the metal ion in that specific compound. For example, iron can form both a \(+2\) ion (\(\text{Fe}^{2+}\)) and a \(+3\) ion (\(\text{Fe}^{3+}\)). The corresponding bases are iron(II) hydroxide (\(\text{Fe}(\text{OH})_2\)) and iron(III) hydroxide (\(\text{Fe}(\text{OH})_3\)). This system clearly communicates the specific oxidation state of the metal within the compound.
Naming Bases with Polyatomic Cations
While most bases are formed with metal cations, some ionic bases feature a polyatomic cation, which is a group of non-metal atoms bonded together that carries an overall positive charge. These compounds still adhere to the general rule of naming the cation first, followed by the anion. The most common example is ammonium hydroxide (\(\text{NH}_4\text{OH}\)), formed from the ammonium cation (\(\text{NH}_4^+\)) and the hydroxide anion (\(\text{OH}^-\)). The name of the polyatomic cation, ammonium, is substituted for the metal name. This maintains the familiar “cation name + anion name” structure. Although the formula \(\text{NH}_4\text{OH}\) is often used, the name ammonium hydroxide persists in common usage.
Common Names for Molecular Bases
Not all substances that act as bases contain the hydroxide ion in their formula. According to the Brønsted-Lowry definition, a base is any species that can accept a proton, and many molecular compounds fit this description. These compounds, which are typically composed of non-metals, are often referred to by established common names rather than strict, systematic IUPAC ionic nomenclature.
The primary example is ammonia (\(\text{NH}_3\)), a compound that accepts a proton from water to form the ammonium and hydroxide ions, making it a weak base. Instead of a systematic name, it is universally known simply as ammonia. Similarly, organic bases like methylamine (\(\text{CH}_3\text{NH}_2\)) and other amines use systematic organic nomenclature or common names. These molecular bases are an exception to the hydroxide-based naming rules, relying on long-standing names that identify them by their chemical structure rather than their ionic components.