An ionic compound forms when a metal transfers electrons to a non-metal, creating positively charged cations and negatively charged anions. These oppositely charged ions are held together by a strong electrostatic force. Chemists use a standardized system of chemical nomenclature to communicate the makeup of these substances. This system provides a clear language for identifying specific compounds but requires the inclusion of Roman numerals in some names.
Fixed Charge vs. Variable Charge Metals
Metals located in Groups 1 and 2 of the periodic table, along with a few others like aluminum, zinc, and silver, consistently form ions with only one possible positive charge. For example, sodium always forms a $+1$ ion, and magnesium always forms a $+2$ ion. Since the resulting charge for these fixed-charge metals is predictable based on their position on the periodic table, this information does not need to be explicitly stated in the compound’s name.
Most transition metals, which populate the central block of the periodic table, and certain post-transition metals exhibit a variable charge. This means they can form two or more stable cations with different positive charges. For instance, an iron atom might form an $\text{Fe}^{2+}$ ion or an $\text{Fe}^{3+}$ ion, depending on the chemical environment. Because the charge of the metal cation is not fixed, a system is required to distinguish between compounds that contain the same elements but possess different chemical properties.
The Role of Roman Numerals in Indicating Charge
When placed in parentheses immediately following the metal’s name, the Roman numeral specifies the exact magnitude of the positive charge, or oxidation state, of the metal in that particular compound. This addition ensures that two compounds containing the same metal but featuring different charges can be clearly differentiated. For example, Iron (II) chloride refers specifically to the compound containing the $\text{Fe}^{2+}$ ion, while Iron (III) chloride contains the $\text{Fe}^{3+}$ ion.
The Roman numeral is mathematically derived by balancing the known negative charge of the anion. All ionic compounds must be electrically neutral, meaning the total positive charge from the metal cations must cancel the total negative charge from the non-metal anions. If a compound contains a known anion, such as the chloride ion ($\text{Cl}^-$) which always has a $-1$ charge, chemists can work backward to determine the metal’s charge. In a compound like copper (I) oxide ($\text{Cu}_2\text{O}$), the single oxygen anion contributes a $-2$ charge, meaning the two copper cations must collectively contribute a $+2$ charge, thus making each copper ion $\text{Cu}^+$.
Writing the Name of Variable Charge Ionic Compounds
To name a variable-charge ionic compound, you must first determine the specific positive charge of the metal cation. This is accomplished by identifying the charge of the anion, which is usually fixed and known, and then calculating the necessary positive charge required to achieve overall electrical neutrality. Once the metal’s specific charge is established, it is converted into the corresponding Roman numeral; for example, a $+2$ charge becomes (II), and a $+3$ charge becomes (III).
The final name is constructed by writing the full name of the metal, followed immediately by the Roman numeral enclosed in parentheses. The name concludes with the root name of the anion, which is modified to end with the suffix “-ide,” such as oxide or sulfide. This rule also applies if the negative portion is a polyatomic ion, where its established name is used instead of the “-ide” root. For $\text{FeCl}_2$, the two $\text{Cl}^-$ ions give a total negative charge of $-2$, meaning the single iron ion must be $\text{Fe}^{2+}$, resulting in the name Iron (II) chloride.