Polyatomic ions Knowledge-Based Study

Study notes

1. What is a Polyatomic Ion?

   A polyatomic ion is an electrically charged group of two or more covalently bonded atoms that behaves as a single unit in chemical reactions. Unlike monoatomic ions such as $\text{Na}^+$ or $\text{Cl}^-$, which derive from single atoms losing or gaining electrons, a polyatomic ion carries an overall net charge distributed across multiple atoms.

   Most polyatomic ions are anions (negatively charged), with the most common positively charged exception being the ammonium ion ($\text{NH}_4^+$).

   Common Polyatomic Ions to Know

   • Ammonium: $\text{NH}_4^+$
   • Hydroxide: $\text{OH}^-$
   • Cyanide: $\text{CN}^-$
   • Nitrate: $\text{NO}_3^-$
   • Sulfate: $\text{SO}_4^{2-}$
   • Carbonate: $\text{CO}_3^{2-}$
   • Phosphate: $\text{PO}_4^{3-}$

   Understanding Oxyanions

   An oxyanion is a polyatomic ion containing oxygen combined with another nonmetal element. The systematic naming of oxyanions relies on specific suffixes and prefixes that indicate relative numbers of oxygen atoms rather than absolute quantities.

   Suffix Rules (-ate and -ite)

   • The suffix -ate designates the most common or standard ion in a family (e.g., $\text{NO}_3^-$ is nitrate, $\text{SO}_4^{2-}$ is sulfate).
   • The suffix -ite designates an ion that has one fewer oxygen atom than the "-ate" counterpart, retaining the same ionic charge (e.g., $\text{NO}_2^-$ is nitrite, $\text{SO}_3^{2-}$ is sulfite).

   Prefix Rules (per- and hypo-)

   When an element forms four distinct oxyanions (notably chlorine, bromine, and iodine):

   • per- ... -ate: One more oxygen than the "-ate" form (e.g., $\text{ClO}_4^-$ is perchlorate).
   • -ate: Base standard form (e.g., $\text{ClO}_3^-$ is chlorate).
   • -ite: One fewer oxygen than "-ate" (e.g., $\text{ClO}_2^-$ is chlorite).
   • hypo- ... -ite: One fewer oxygen than "-ite" (e.g., $\text{ClO}^-$ is hypochlorite).

   Balancing Charges and Using Parentheses

   Ionic compounds containing polyatomic ions are electrically neutral. Therefore, the total positive charge from cations must equal the total negative charge from anions.

   When to Use Parentheses

   When a chemical formula requires two or more of the same polyatomic ion to balance charges, the entire polyatomic ion formula is placed inside parentheses, with a numerical subscript written outside to indicate the quantity.

   • If only one polyatomic ion is needed, do not use parentheses: $\text{Na}^+$ and $\text{NO}_3^-$ combine in a 1:1 ratio to yield $\text{NaNO}_3$.
   • If multiple polyatomic ions are needed, parentheses are mandatory: $\text{Ca}^{2+}$ and $\text{OH}^-$ combine in a 1:2 ratio to yield $\text{Ca(OH)}_2$, representing one calcium ion and two hydroxide units.
   • For $\text{Al}^{3+}$ and $\text{SO}_4^{2-}$, the lowest common multiple of charges is 6, requiring two aluminum ions and three sulfate ions: $\text{Al}_2(\text{SO}_4)_3$.

   Rules for Systematic Nomenclature

   Naming compounds that contain polyatomic ions is straightforward once the individual ions are identified.

   1. State the Cation Name First: Name the metal cation or $\text{NH}_4^+$ (ammonium). If the metal has variable oxidation states (such as iron or copper), indicate its charge using a Roman numeral in parentheses (e.g., Iron(III)).
   2. State the Polyatomic Anion Name Second: Do not modify the ending of the polyatomic ion name (e.g., keep nitrate, sulfate, or carbonate without changing to -ide).

   Worked Examples

   • $\text{K}_2\text{CO}_3$: Potassium cation ($\text{K}^+$) + carbonate anion ($\text{CO}_3^{2-}$) = Potassium carbonate
   • $\text{Fe(NO}_3)_3$: Iron(III) cation ($\text{Fe}^{3+}$) + nitrate anion ($\text{NO}_3^-$ $\times 3$) = Iron(III) nitrate
   • $(\text{NH}_4)_2\text{SO}_4$: Ammonium cation ($\text{NH}_4^+$) + sulfate anion ($\text{SO}_4^{2-}$) = Ammonium sulfate
   • $\text{CuSO}_4$: Copper(II) cation ($\text{Cu}^{2+}$) + sulfate anion ($\text{SO}_4^{2-}$) = Copper(II) sulfate