Ionic compounds are formed through the transfer of electrons between atoms, typically a metal and a nonmetal, resulting in oppositely charged ions (cations and anions). The strong electrostatic attraction between these ions creates the ionic bond. Chemists rely on systematic rules called chemical nomenclature to provide a unique name for every compound. This system allows chemists to unambiguously identify a compound or write its formula from its name.
Naming Simple Binary Compounds
The most straightforward ionic compounds are binary, meaning they are composed of only two elements, where the metal has a single, fixed charge. Metals from Group 1, Group 2, Aluminum (\(\text{Al}^{3+}\)), Zinc (\(\text{Zn}^{2+}\)), and Silver (\(\text{Ag}^{+}\)) consistently form cations with only one possible charge. Naming these compounds involves a simple two-part structure that identifies the metal and the nonmetal.
The naming process begins with the cation (the metal), using its full elemental name without modification. For example, the \(\text{Na}^{+}\) ion is named sodium. The anion (the nonmetal) is named by taking the root of the element’s name and changing the ending to the suffix “-ide.”
In the compound \(\text{NaCl}\), the sodium cation is named first, and the chlorine anion becomes chloride, resulting in Sodium Chloride. Similarly, \(\text{MgO}\) is named Magnesium Oxide. The ratio of ions is implied by the balance of charges and is not indicated in the name.
Naming Compounds with Transition Metals
Naming compounds involving metals capable of forming ions with multiple positive charges, such as most transition metals, requires an indicator of charge. This is accomplished using the Stock system, which employs Roman numerals.
The Roman numeral, placed in parentheses immediately after the metal’s name, specifies the cation’s positive charge. For example, iron can form both \(\text{Fe}^{2+}\) and \(\text{Fe}^{3+}\) ions. To determine the metal’s charge, one must identify the known charge of the anion and calculate the metal’s charge necessary to balance the neutral compound.
For \(\text{FeCl}_2\), the two chloride anions (\(\text{Cl}^{-}\)) have a total charge of \(2-\), meaning the iron cation must be \(2+\), making the name Iron(II) Chloride. In \(\text{FeCl}_3\), the three chloride anions contribute a \(3-\) charge, requiring the iron to be \(\text{Fe}^{3+}\), resulting in Iron(III) Chloride. The anion still uses the standard “-ide” ending.
Naming Compounds Containing Complex Ions
This category involves compounds that incorporate polyatomic ions, which are groups of covalently bonded atoms that carry a net electrical charge and act as a single unit. These ions have specific, established names that do not follow the simple “-ide” rule for monatomic anions. The core rule remains to name the cation first and the anion second.
If the polyatomic ion is the cation, such as Ammonium (\(\text{NH}_4^{+}\)), its name is used directly in the first position. For \(\text{(NH}_4\text{)}_2\text{SO}_4\), the name begins with Ammonium. When the polyatomic ion is the anion, its name is simply used as the second word, never changing its ending to “-ide.”
For example, \(\text{NO}_3^{-}\) is nitrate, \(\text{SO}_4^{2-}\) is sulfate, and \(\text{OH}^{-}\) is hydroxide. Therefore, \(\text{NaNO}_3\) is named Sodium Nitrate. Even when subscripts are necessary to balance the charges, such as in \(\text{(NH}_4\text{)}_2\text{SO}_4\), the name remains Ammonium Sulfate.