Naming ionic compounds Knowledge-Based Study

Study notes

1. Foundations of Ionic Nomenclature

   An ionic compound is formed by the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). In systematic chemical nomenclature, the cation is always named first, followed by the anion.

   Binary Compounds with Fixed-Charge Metals

   Binary ionic compounds consist of exactly two elements: a metal and a nonmetal. Metals from Group 1 (alkali metals), Group 2 (alkaline earth metals), as well as silver, zinc, and aluminum, consistently form ions with fixed charges:

   • Group 1 metals: Form $+1$ cations (for example, sodium ion, $\text{Na}^+$).
   • Group 2 metals: Form $+2$ cations (for example, magnesium ion, $\text{Mg}^{2+}$).
   • Silver and zinc: Form $\text{Ag}^+$ and $\text{Zn}^{2+}$, respectively.
   • Aluminum: Consistently forms $\text{Al}^{3+}$.

   For these fixed-charge metals, the elemental name of the metal is used directly without Roman numerals. The monoatomic nonmetal anion takes the root of the element name and adds the suffix -ide.

   Examples:

   • $\text{NaCl}$: Sodium chloride
   • $\text{MgO}$: Magnesium oxide
   • $\text{Ca}_3\text{N}_2$: Calcium nitride
   • $\text{Al}_2\text{O}_3$: Aluminum oxide
   • $\text{K}_2\text{S}$: Potassium sulfide

   Variable-Charge Cations

   Many transition metals and post-transition metals (such as lead and tin) can form ions with more than one oxidation state. Because the same pair of elements can combine in different ratios, the charge on the metal cation must be specified explicitly in the compound's name.

   The IUPAC Stock System

   Under the IUPAC Stock naming system, the magnitude of the cation's positive charge is indicated by a Roman numeral placed in parentheses immediately after the metal's name, with no intervening space.

   • Iron can form $\text{Fe}^{2+}$ or $\text{Fe}^{3+}$.
   • Copper can form $\text{Cu}^+$ or $\text{Cu}^{2+}$.
   • Tin can form $\text{Sn}^{2+}$ or $\text{Sn}^{4+}$.
   • Lead can form $\text{Pb}^{2+}$ or $\text{Pb}^{4+}$.

   Determining the Cation Charge

   To determine the Roman numeral from a chemical formula, balance the total negative charge contributed by the known anions, because an ionic compound is electrically neutral overall:

   1. Identify the anion charge: In $\text{FeCl}_3$, each chloride ion carries a $-1$ charge, giving a total negative charge of $-3$.
   2. Calculate the required cation charge: For neutrality, the single iron cation must carry a $+3$ charge.
   3. State the full name: Iron(III) chloride.

   Contrasting Examples:

   • $\text{FeO}$: The oxide ion is $\text{O}^{2-}$, so iron is $\text{Fe}^{2+}$. Name: Iron(II) oxide.
   • $\text{Fe}_2\text{O}_3$: Three oxide ions contribute $-6$; therefore, two iron ions must total $+6$, meaning each is $\text{Fe}^{3+}$. Name: Iron(III) oxide.
   • $\text{Cu}_2\text{O}$: Copper(I) oxide.
   • $\text{CuO}$: Copper(II) oxide.

   Naming Polyatomic Ionic Compounds

   A polyatomic ion is a tightly bound group of atoms that behaves as a single charged unit. When naming compounds containing polyatomic ions, the name of the polyatomic ion is preserved without modifying its ending to -ide (unless its standard name already ends in -ide, such as hydroxide or cyanide).

   Common Cation:

   • Ammonium: $\text{NH}_4^+$

   Common Oxyanions:

   • Nitrate: $\text{NO}_3^-$; Nitrite: $\text{NO}_2^-$
   • Sulfate: $\text{SO}_4^{2-}$; Sulfite: $\text{SO}_3^{2-}$
   • Phosphate: $\text{PO}_4^{3-}$
   • Carbonate: $\text{CO}_3^{2-}$; Hydrogen carbonate (bicarbonate): $\text{HCO}_3^-$
   • Hydroxide: $\text{OH}^-$

   Patterns in Oxyanion Nomenclature

   Many polyatomic anions contain oxygen and belong to related series:

   • The ion with more oxygen atoms receives the suffix -ate (e.g., $\text{NO}_3^-$, nitrate).
   • The ion with fewer oxygen atoms receives the suffix -ite (e.g., $\text{NO}_2^-$, nitrite).

   Naming Rules in Practice

   • $\text{NaNO}_3$: Sodium nitrate
   • $\text{MgSO}_4$: Magnesium sulfate
   • $\text{NH}_4\text{Cl}$: Ammonium chloride
   • $\text{Fe(OH)}_3$: Iron(III) hydroxide
   • $\text{CuSO}_4$: Copper(II) sulfate

   Summary and Decision Framework

   Follow this systematic decision tree whenever converting an ionic chemical formula into its systematic name:

   1. Identify the cation: Is the cation a metal or a polyatomic ion (such as ammonium)? Write the metal's elemental name or write 'ammonium'.
   2. Check for variable charge: Does the metal form ions of variable charge (most transition metals, $\text{Sn}$, $\text{Pb}$)? If yes, deduce the charge using the known anion charges and express it as a Roman numeral in parentheses. If the metal has a fixed charge (Group 1, Group 2, $\text{Al}$, $\text{Zn}$, $\text{Ag}$), do not include a Roman numeral.
   3. Identify the anion:
      • If the anion is a monoatomic nonmetal, take its root and add the suffix -ide.
      • If the anion is a polyatomic ion, use its standard accepted name directly without changing its suffix.
   4. Combine the names: State the cation first, followed by the anion.

   Common Pitfalls to Avoid

   • Do not use Greek prefixes: Never use prefixes like mono-, di-, or tri- for ionic compounds (for example, $\text{MgCl}_2$ is magnesium chloride, never magnesium dichloride).
   • Do not include Roman numerals for fixed-charge metals: Writing 'sodium(I) chloride' or 'calcium(II) oxide' is incorrect because their charges are invariant.