Ionic Compounds: Formula and Naming

Ionic Compounds

  • Formed from metals and nonmetals.
  • Metals form cations, nonmetals form anions.
  • No individual molecule units, only empirical formulas.
  • Monatomic ions: Made of one type of element.
  • Polyatomic ions: Made of more than one type of element; atoms are held together by covalent bonds.

Polyatomic Ions

  • Several atoms attached together in one ion.
  • Oxyanion: One or more oxygen atoms in the polyatomic anion (e.g., NO<em>3−NO<em>3^-, SO</em>42−SO</em>4^{2-}.)
  • Remember both the formula and charge of common polyatomic ions.

Polyatomic Ion Patterns

  • If the polyatomic ion starts with H, add "hydrogen-" prefix (e.g., HCO3−HCO_3^- is hydrogen carbonate).
  • Most nonmetals have two forms of oxyanions:
    • "-ate": More oxygens (e.g., nitrate NO<em>3−NO<em>3^-, sulfate SO</em>42−SO</em>4^{2-}.)
    • "-ite": Less oxygen (e.g., nitrite NO<em>2−NO<em>2^-, sulfite SO</em>32−SO</em>3^{2-}.)

Halogen Oxyanions

  • Halogens (except F) form 4 types of oxyanions with 1-4 oxygen atoms and a 1- charge.
    • -ate ion + 1 O -> per- prefix (e.g. ClO<em>3−ClO<em>3^- = chlorate, ClO</em>4−ClO</em>4^- = perchlorate)
    • -ite ion -> (e.g. ClO2−ClO_2^- = chlorite)
    • -ite ion – 1 O -> hypo- prefix, -ite suffix (e.g. ClO−ClO^- = hypochlorite)

Common Polyatomic Ions

  • Acetate: C<em>2H</em>3O2−C<em>2H</em>3O_2^-
  • Carbonate: CO32−CO_3^{2-}
  • Hydrogen Carbonate (Bicarbonate): HCO3−HCO_3^-
  • Hydroxide: OH−OH^-
  • Nitrate: NO3−NO_3^-
  • Nitrite: NO2−NO_2^-
  • Chromate: CrO42−CrO_4^{2-}
  • Dichromate: Cr<em>2O</em>72−Cr<em>2O</em>7^{2-}
  • Ammonium: NH4+NH_4^+
  • Hypochlorite: ClO−ClO^-
  • Chlorite: ClO2−ClO_2^-
  • Chlorate: ClO3−ClO_3^-
  • Perchlorate: ClO4−ClO_4^-
  • Sulfate: SO42−SO_4^{2-}
  • Sulfite: SO32−SO_3^{2-}
  • Hydrogen Sulfate (Bisulfate): HSO4−HSO_4^-
  • Hydrogen Sulfite (Bisulfite): HSO3−HSO_3^-

Remembering Polyatomic Ions

  • Use the sentence "Nick the Camel ate a Clam Supper in Phoenix" to remember some polyatomic ions:
    • Nick: Nitrate (NO3−NO_3^-)
    • Camel: Carbonate (CO32−CO_3^{2-}
    • Clam: Chlorate (ClO3−ClO_3^-)
    • Supper: Sulfate (SO42−SO_4^{2-}
    • Phoenix: Phosphate (PO43−PO_4^{3-}
  • The number of consonants corresponds to the number of oxygen atoms, and the number of vowels corresponds to the charge.

Special Polyatomic Ions

  • Cyanide: CN−CN^-
  • Hydroxide: OH−OH^- (Ionic compounds with OH−OH^- are bases).
  • Ammonium: NH<em>4+NH<em>4^+ (formed by NH</em>3+H+→NH4+NH</em>3 + H^+ \rightarrow NH_4^+)
  • Acetate: C<em>2H</em>3O<em>2−C<em>2H</em>3O<em>2^-, also written as CH</em>3COO−CH</em>3COO^- (organic ion).

Compounds with Polyatomic Ions

  • Polyatomic ions are single ions containing multiple atoms.
  • Often identified by parentheses in the formula (not needed if only one polyatomic ion).
  • When naming ionic compounds with polyatomic ions, the name of the polyatomic ion does not change.

Zero Net Charge Rule

  • Ionic compounds must be neutral.
  • The total positive charge must equal the total negative charge.
  • Example: Sodium sulfide is Na2SNa_2S (2 Na+ and 1 S2-).
    • 2 Na+ has 2x(+1) = +2 charges
    • 1 S2- has -2 charges
    • +2 + (-2) = 0
  • Cation name is the same as the element; anion name changes to -ide (except for polyatomic ions).

Criss-Cross Method

  1. Write the metal/cation first, then the nonmetal/anion.
  2. Take the number of the charge on one ion and make it the other atom’s subscript (without the sign).
  3. Reduce subscripts to the smallest whole number ratio.
  4. Check that the sum of the charges of the cations cancels the sum of the anions.

Writing Formulas - Examples

  • Calcium ions and oxide ions: Ca2+Ca^{2+} and O2−O^{2-}\rightarrow CaO
  • Sodium oxide: Na+Na^+ and O2−O^{2-}\rightarrow Na_2O
  • Manganese(IV) sulfide: Mn4+Mn^{4+} and S2−S^{2-}\rightarrow MnS_2