Polyatomic Ions & Hydrates — Quick Reference

Polyatomic Ions

• Monatomic ions are formed from a single atom; polyatomic ions are charged groups of two or more atoms that act as a unit.

• Oxyanions are polyatomic ions that contain one or more oxygen atoms.

• You should memorize the names, formulas, and charges of common polyatomic ions.

• Naming system for some polyatomic ions:

  • suffixes -ate and -ite indicate more or fewer oxygen atoms

  • prefixes per- (hyper) and hypo- (under) indicate more or fewer oxygens than -ate/-ite

  • examples: perchlorate $\mathrm{ClO<em>4^-}$, chlorate $\mathrm{ClO</em>3^-}$, chlorite $\mathrm{ClO_2^-}$, hypochlorite $\mathrm{ClO^-}$

• The exact number of oxygens corresponding to a suffix/prefix is not entirely consistent (e.g., nitrate $\mathrm{NO<em>3^-}$ vs sulfate $\mathrm{SO</em>4^{2-}}$).

• Polyatomic ions can be cations, anions, or both; ionic compounds must be electrically neutral, so formulas reflect discrete polyatomic units.

• In formulas, parentheses indicate a polyatomic group treated as a unit (e.g., $\mathrm{Ca<em>3(PO</em>4)<em>2}$): three calcium ions (Ca^{2+}) for every two phosphate groups (PO4^{3-}).

• PO_4^{3-} groups consist of one phosphorus atom and four oxygen atoms with an overall 3− charge; the compound is neutral overall with calcium.

Writing Chemical Formulas for Ionic Compounds

• Steps:

  1. Identify the cation and anion and their charges.

  2. Combine in the smallest whole-number ratio so the total positive charge equals the total negative charge.

  3. Write the chemical formula reflecting this ratio.

• Example: Ca^{2+} and PO4^{3-} combine in a 3:2 ratio → $\mathrm{Ca</em>3(PO<em>4)</em>2}$.

Naming Ionic Compounds with Polyatomic Ions

• Steps:

  1. Name the cation first.

  • If the cation is monatomic, its name is the element name.

  • Use a roman numeral for cations with multiple possible charges.

  1. Name the anion second.

  • If the anion is monatomic, change its ending to -ide.

  • If the anion is polyatomic, use its name (no -ide ending).

• Ammonium is the only common positive polyatomic ion listed: $\mathrm{NH_4^+}$.

• In most compounds (except those containing ammonium), the first element is the cation and the rest of the formula is the anion.

• Examples:

  • Potassium acetate: cation = potassium; anion = acetate ($\mathrm{C<em>2H</em>3O<em>2^-}$) → $\mathrm{KC</em>2H<em>3O</em>2}$.

  • Barium dichromate: cation = barium; anion = dichromate ($\mathrm{Cr<em>2O</em>7^{2-}}$) → $\mathrm{BaCr<em>2O</em>7}$.

Ionic Hydrates: Naming and Formulas

• Hydrates are ionic compounds with water molecules integrated into the crystal.

• Name = base anhydrous compound name + a prefix (indicating number of H_2O) + "hydrate".

• Examples:

  • Copper(II) sulfate pentahydrate: $\mathrm{CuSO<em>4 \cdot 5H</em>2O}$.

  • Washing soda (sodium carbonate decahydrate): $\mathrm{Na<em>2CO</em>3 \cdot 10H_2O}$.

• Hydrate formula notation uses a dot: base formula, a dot, then the number of water molecules, then H_2O.

• Greek prefixes (number of water molecules):

  • mono- (1, sometimes omitted), di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-

  • used with "hydrate" as in pentahydrate, decahydrate, etc.