Chapter 3 | Polyatomic Ions and Molecular Compounds

Molecular Compounds Identification and Naming

  • Molecular compounds consist of molecules formed by nonmetal atoms. The common naming conventions involve prefixes to indicate the number of atoms of each element in the compound.

    • Carbon Monoxide (CO)

    • Carbon Dioxide (CO₂)

    • Nitrogen Trioxide (N₂O₃)

    • Silicon Tetrachloride (SiCl₄)

Prefixes for Molecular Compounds

  • The prefixes used for naming molecular compounds include:

    1. Mono- (1)

    2. Di- (2)

    3. Tri- (3)

    4. Tetra- (4)

    5. Penta- (5)

    6. Hexa- (6)

    7. Hepta- (7)

    8. Octa- (8)

Important Polyatomic Ions

  • The following are common polyatomic ions with their respective formulas and notable attributes:

    • Per iodate (IO₄⁻)

    • Chlorate (ClO₃⁻)

    • Bromide (Br⁻)

    • Iodite (IO₂⁻)

    • Hypobromite (BrO⁻)

    • Superoxide (O₂⁻)

    • Hydrogen Phosphide (PH₃)

    • Borate (BO₃³⁻)

Overview of Key Ions and Their Charges

  • Carbonate (CO₃²⁻)

  • Nitrate (NO₃⁻)

  • Bicarbonate / Hydrogen Carbonate (HCO₃⁻)

  • Sulfate (SO₄²⁻)

  • Sulfite (SO₃²⁻)

  • Hydroxide (OH⁻)

  • Acetate (C₂H₃O₂⁻)

    • Corresponds to a common organic ion.

Naming Conventions for Polyatomic Ions

  • The naming of polyatomic ions often follows certain patterns:

    • Oxyanions that have a higher number of oxygen atoms end in "-ate" while those with fewer end in "-ite".

    • The prefix "per-" indicates more oxygens than the "-ate" form, while "hypo-" indicates fewer than the "-ite" form.

    • Example: Chlorate (ClO₃⁻), Chlorite (ClO₂⁻), Hypochlorite (ClO⁻), and Perchlorate (ClO₄⁻).

Charge and Symbol Information for Select Ions

  • Permanganate ⟶ MnO₄⁻

  • Thiosulfate ⟶ S₂O₃²⁻

  • Oxide ⟶ O²⁻

  • Nitride ⟶ N³⁻

  • Ammonium ⟶ NH₄⁺ (notable as a positively charged ion)

  • Cyanide ⟶ CN⁻

  • Hydronium ⟶ H₃O⁺

Polyatomic Ion Characteristics

  • Most polyatomic ions contain oxygen atoms and can be recognized by their systematic naming based on the aforementioned rules.

  • Phosphate Ions:

    • Phosphate (PO₄³⁻)

    • Hydrogen Phosphate (HPO₄²⁻)

    • Dihydrogen Phosphate (H₂PO₄⁻)

  • Oxalate (C₂O₄²⁻)

  • Bromate (BrO₃⁻)

  • Sulfide (S²⁻)

Common Naming Tips for Polyatomic Ions

  • To identify the charge and the appropriate name of a polyatomic ion, consider:

    • The presence of oxygen usually indicates a polyatomic ion, and its ending in "-ate" or "-ite" hints the number of oxygen atoms it contains in relation to its base element.

    • Memorization strategies for polyatomic ions can be helpful for chemistry classes, often relying on prefix indicators to track variations in ions (e.g., nitrite NO₂⁻, nitrate NO₃⁻, etc.).

Summary of Select Common Ions

  • Chloride (Cl⁻)

  • Nitrite (NO₂⁻)

  • Sulfate (SO₄²⁻) and Sulfite (SO₃²⁻)

  • Phosphate (PO₄³⁻), Phosphite (PO₃³⁻), Phosphide (P³⁻)