Ionic Compounds, Polyatomic Ions, and Nomenclature

Polyatomic Ions and Ionic Formulas

• Carbonate ion: a carbon atom with three surrounding oxygens, existing as a single polyatomic ion with charge
  $ext{CO}_3^{2-}.$ It’s not just one atom; it’s one unit (the carbonate ion).

• Sulfate ion: sulfur with four oxygens; charge is
  $ext{SO}_4^{2-}.$ Also a polyatomic ion.

• Sodium ion: sodium forms the cation
  $ext{Na}^+,$ which has a +1 charge.

• To make a neutral compound with a polyatomic ion, balance charges by combining cations and anions.

• Example: Forming a compound with carbonate using sodium.

  • Carbonate has charge $-2$, sodium has charge $+1$.

  • We need two $ext{Na}^+$ ions to balance one $ext{CO}_3^{2-}$ ion.

  • Resulting formula: $ext{Na}<em>2 ext{CO}</em>3.$

• If you need more than one polyatomic ion in a compound, place the polyatomic ion in parentheses and specify the number outside the parentheses.

  • Example: Sodium sulfate involves one sulfate ion, but when needed in larger formulas you might see $ext{Na}<em>2( ext{SO}</em>4).$

• Criss-cross (charge balancing) shortcut:

  • Swap the charges of the ions to determine subscripts that balance the overall charge to zero.

  • Example: Sodium ion (charge +1) with sulfate (charge −2): cross the charges to get $ext{Na}<em>2 ext{SO}</em>4.$

• General rule for criss-cross and balancing: use the smallest whole-number ratio of ions that balances charges.

  • If you have $ext{A}^{a+}$ and $ext{B}^{b-}$, the formula uses:

  • number of A’s = frac{b}{ ext{gcd}(a,b)}

  • number of B’s = frac{a}{ ext{gcd}(a,b)}

  • Examples:

  • Na^+ (a = 1) and SO4^{2-} (b = 2): gcd(1,2) = 1 → Na2SO4
    $ext{A formula unit: } ext{Na}<em>2 ext{SO}</em>4.$

  • Al^{3+} (a = 3) and SO4^{2-} (b = 2): gcd(3,2) = 1 → Al2(SO4)3 $ext{A formula unit: } ext{Al}</em>2( ext{SO}<em>4)</em>3.$

• Reducing to simplest whole-number ratios:

  • If the criss-cross yields coefficients that can be simplified, reduce them.

  • Example: If you somehow obtained $ext{K}_4 ext{O}$ (hypothetical for illustration), you’d divide by the greatest common divisor to obtain the simplest ratio.

  • Realistic example:

  • If you had $ext{Fe}<em>2 ext{O}</em>3$ and mixed with another ion that yields a common factor, you’d reduce the coefficients accordingly to simplest integers.

• Polyatomic ions mentioned:

  • Carbonate: $ext{CO}_3^{2-}$

  • Sulfate: $ext{SO}_4^{2-}$

  • Ammonium: $ext{NH}_4^{+}$

• Naming ionic compounds (cation name followed by anion name):

  • Ammonium sulfate = $ext{(NH}<em>4 ext{)}</em>2 ext{SO}_4$ when balancing requires two ammonium ions for one sulfate ion, giving the neutral compound name "ammonium sulfate."

  • If there is a metal with multiple possible oxidation states, include the oxidation state in roman numerals after the metal name:

  • Iron(II) sulfate vs iron(III) sulfate (FeSO$4$ vs Fe$2$(SO$4$)$3$).

  • Common polyatomic ions with -ate/-ite endings: sulfate ($ext{SO}<em>4^{2-}$) vs sulfite ($ext{SO}</em>3^{2-}$); carbonate ($ext{CO}_3^{2-}$).

• Covalent (molecular) naming (nonmetals): use prefixes to indicate the number of atoms; first element typically does not take the prefix monoprefix for the word "one";

  • Examples:

  • Carbon dioxide: $ext{CO}_2 <br>ightarrow ext{carbon dioxide}.$

  • Carbon monoxide: $ext{CO} <br>ightarrow ext{carbon monoxide}.$

  • Prefix usage for other molecules:

  • Dinitrogen tetroxide: $ext{N}<em>2 ext{O}</em>4 <br>ightarrow ext{dinitrogen tetroxide}.$

  • Sulfur hexafluoride: $ext{SF}_6 <br>ightarrow ext{sulfur hexafluoride}.$

  • Note on “mono-” for the first element: often omitted (e.g., carbon monoxide is not named as “carbon monoxide” with a prefixed mono-; the term is simply monoxide for the second element).

• Quick practice prompts (to reinforce the rules):

  • Write the formula for sodium sulfate using criss-cross.

  • Name the compound formed from Fe^{2+} and NO_3^{-} (nitrate): iron(II) nitrate.

  • Write the formula that balances Al^{3+} with carbonate CO_3^{2-}.

  • Provide the name for N2O5 (dinitrogen pentoxide) and for CO_2 (carbon dioxide).

• Real-world relevance and connections:

  • Ionic bonding and charge balance underpin formation of salts used in fertilizers (e.g., ammonium sulfate) and in industry (e.g., sodium sulfate).

  • Understanding polyatomic ions simplifies naming and formulation of commonly encountered compounds.

  • Distinguishing ionic (metal/nonmetal ions with charges) vs covalent (nonmetals with prefixes) compounds is foundational for chemical nomenclature and stoichiometry.

• Summary tips:

  • Always check the total charge to zero before finalizing a formula.

  • Use parentheses when you have more than one polyatomic ion in the formula.

  • Use criss-cross to balance charges, then reduce to the smallest whole-number ratio.

  • Ionic compound naming: cation name + anion name; use roman numerals for metals with multiple oxidation states when required.

  • Covalent compound naming: use prefixes to indicate numbers of atoms; omit the "mono-" for the first element in common practice; end the second element with "-ide".