Writing formulas from names Knowledge-Based Study

Study notes

1. Classification of Compounds

   To write the correct chemical formula from a systematic chemical name, the first essential step is to classify the substance as either an ionic compound or a molecular (covalent) compound.

   • Ionic Compounds: Formed between cations (typically metals or the ammonium ion) and anions (nonmetals or polyatomic ions). Their formulas represent the simplest whole-number ratio of ions (empirical formula) that results in an electrically neutral unit.
   • Molecular Compounds: Composed of two or more nonmetals bonded together via shared valence electrons. Their names typically use Greek numerical prefixes to directly indicate the number of atoms of each element present in a single molecule.

   Balancing Charges in Binary Ionic Compounds

   Binary ionic compounds consist of a monatomic metal cation and a monatomic nonmetal anion. Metals in Group 1, Group 2, as well as aluminum ($\\text{Al}^{3+}$), zinc ($\\text{Zn}^{2+}$), and silver ($\\text{Ag}^{+}$), always form ions of a fixed, predictable charge.

   Step-by-Step Procedure:

   1. Identify the symbol and positive charge of the metal cation.
   2. Identify the symbol and negative charge of the nonmetal anion (derived from the root name ending in -ide).
   3. Determine the lowest common multiple of the charges so that the sum of positive and negative charges equals zero.
   4. Write the cation first, followed by the anion, using subscripts to denote the ratio of ions. A subscript of 1 is omitted.

   Example: For magnesium chloride, magnesium forms $\\text{Mg}^{2+}$ and chlorine forms $\\text{Cl}^{-}$. Two chloride ions are required to balance one magnesium ion, yielding the neutral formula $\\text{MgCl}_2$.

   Transition Metals and Complex Ions

   Many transition metals and post-transition metals can form cations with multiple stable oxidation states. The Stock system indicates the specific ionic charge using a Roman numeral in parentheses immediately following the metal's name.

   Example: Iron(III) oxide indicates an iron cation with a charge of $3+$ ($\\text{Fe}^{3+}$), paired with the oxide ion ($\\text{O}^{2-}$). Balancing the total positive charge ($+6$) with the total negative charge ($-6$) gives $\\text{Fe}_2\\text{O}_3$.

   Polyatomic Ions:

   Polyatomic ions are charged groups of covalently bonded atoms that act as a single unit (such as $\\text{NH}_4^{+}$, $\\text{NO}_3^{-}$, $\\text{SO}_4^{2-}$, and $\\text{PO}_4^{3-}$).

   • If more than one polyatomic ion is required to achieve electrical neutrality, enclose the entire polyatomic group in parentheses before adding the subscript outside.
   • Example: In calcium nitrate, calcium forms $\\text{Ca}^{2+}$ and nitrate is $\\text{NO}_3^{-}$. Balancing charges requires two nitrate ions for every calcium ion, resulting in $\\text{Ca(NO}_3)_2$.

   Direct Interpretation Using Prefixes

   Binary molecular compounds are named using prefixes that directly specify the subscript for each nonmetal element in the formula.

   Common Greek Prefixes:

   • mono- = 1 (often omitted for the first element)
   • di- = 2
   • tri- = 3
   • tetra- = 4
   • penta- = 5
   • hexa- = 6
   • hepta- = 7
   • octa- = 8

   Because these compounds consist of individual molecules rather than an ionic lattice, subscripts reflect the actual number of atoms in the molecule rather than a simplified empirical ratio.

   Example: Dinitrogen pentoxide directly translates to two nitrogen atoms ($\\text{N}_2$) and five oxygen atoms ($\\text{O}_5$), giving the formula $\\text{N}_2\\text{O}_5$.

   Systematic Strategy for Formula Writing

   When given any chemical name, follow this logical sequence:

   1. Inspect the first element: Is it a nonmetal with a prefix (e.g., carbon dioxide, dinitrogen monoxide)? If so, write the element symbols and assign subscripts directly matching the prefixes.
   2. Check for metal or ammonium cation: If the first component is a metal or ammonium ($\\text{NH}_4^{+}$), the compound is ionic.
   3. Determine cation charge: Use the periodic table group for fixed-charge metals, or read the Roman numeral for variable-charge metals.
   4. Determine anion charge: Monatomic anions end in -ide; polyatomic anions generally end in -ate or -ite (with exceptions such as hydroxide, $\\text{OH}^{-}$, and cyanide, $\\text{CN}^{-}$).
   5. Cross-balance to zero net charge: Simplify the subscript ratio for ionic compounds to the lowest whole numbers, and apply parentheses to polyatomic ions when their count is two or greater.