3-7

Ionic Compounds: Formula and Naming

Ionic vs. Molecular/Covalent Compounds

  • Propane (C3H8): Contains individual molecules.
  • Table salt (NaCl): Contains an array of Na+Na^+ and Cl−Cl^- ions.

Ionic Compounds

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

Polyatomic Ions

  • Oxyanions: Polyatomic anions containing oxygen (e.g., NO<em>3−NO<em>3^-, SO</em>42−SO</em>4^{2-}.)
  • Remember both the formula and charge of all the polyatomic ions in the handout list!

Patterns for Polyatomic Ions

  • If the polyatomic ion starts with H, add "hydrogen-" prefix and add 1+ to the charge (e.g., CO<em>32−CO<em>3^{2-} = carbonate, HCO</em>3−HCO</em>3^- = hydrogen carbonate/bicarbonate).
  • Most nonmetals have two forms of oxyanions: "-ate" (more oxygen), "-ite" (less oxygen).
    • C: CO<em>32−CO<em>3^{2-} carbonate, CO</em>22−CO</em>2^{2-} carbonite
    • N: NO<em>3−NO<em>3^- nitrate, NO</em>2−NO</em>2^- nitrite
    • S: SO<em>42−SO<em>4^{2-} sulfate, SO</em>32−SO</em>3^{2-} sulfite

Patterns for Halogen Oxyanions

  • Halogens form 4 types of similar oxyanions (except F).
  • -ate ion, 3 Os (e.g., chlorate = ClO3−ClO_3^-).
  • -ate ion + 1 O → per- prefix (e.g., perchlorate = ClO4−ClO_4^-).
  • -ite ion, 2 Os Þ same charge chlorite = ClO2−ClO_2^- .
  • -ite ion – 1 O → hypo- prefix, -ite suffix (e.g., hypochlorite = ClO−ClO^-).

Special Polyatomic Ions

  • CN−CN^-: cyanide.
  • OH−OH^-: hydroxide; ionic compounds formed by metal cations and OH−OH^- are called bases (e.g., KOH, Ca(OH)<em>2Ca(OH)<em>2, NH</em>4OHNH</em>4OH).
  • NH<em>4+NH<em>4^+: ammonium (NH</em>3+H+→NH4+NH</em>3 + H^+ \rightarrow NH_4^+).
  • C<em>2H</em>3O<em>2−C<em>2H</em>3O<em>2^-, also written as CH</em>3COO−CH</em>3COO^-: acetate (organic ions).

Compounds Containing Polyatomic Ions

  • Polyatomic ions are single ions containing more than one atom, often oxyanions.
  • Often identified by parentheses in formula; if only one polyatomic ion, parentheses are not needed (e.g., NaOH, Mg(OH)2Mg(OH)_2).
  • Naming: name of polyatomic ion does not change.

Zero Net Charge Rule

  • Ionic compounds must be neutral.
  • Total charge must be zero (e.g., Na2SNa_2S: 2 Na+Na^+ and 1 S2−S^{2-}, so 2*(+1) + (-2) = 0).
  • Cation name is the same as the element; anion name changes to -ide (except for polyatomic ions).

Criss Cross Method for Formulas

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

Remembering Polyatomic Ions

  • "Nick the Camel ate a Clam Supper in Pheonix”
    • Nick: N with 3 consonants and 1 vowel, therefore NO3−1NO_3^{-1}
    • Camel: C with 3 consonants and 2 vowels, therefore CO3−2CO_3^{-2}
    • Clam: Cl with 3 consonants and 1 vowel, therefore ClO3−1ClO_3^{-1}
    • Supper: S with 4 consonants and 2 vowels, therefore SO4−2SO_4^{-2}
    • Pheonix: P with 4 consonants and 3 vowels, therefore PO4−3PO_4^{-3}