Introduction to Polyatomic Ions
Polyatomic ions are charged chemical species (ions) that consist of two or more atoms covalently bonded together. Unlike monatomic ions, which are formed from single atoms, polyatomic ions behave as a single unit in chemical reactions. Understanding polyatomic ions is essential for chemistry students as they appear frequently in chemical formulas, naming compounds, and predicting reaction products. This cheatsheet provides a comprehensive reference for the most commonly encountered polyatomic ions, organized by charge and type for easy memorization and application.
Negative Polyatomic Ions (Anions)
-1 Charge Anions
| Formula | Name | Common Occurrence |
|---|---|---|
| OH⁻ | Hydroxide | Bases, pH adjustment |
| CN⁻ | Cyanide | Industrial processes, electroplating |
| NO₂⁻ | Nitrite | Food preservatives, chemical intermediates |
| NO₃⁻ | Nitrate | Fertilizers, explosives, food preservative |
| ClO⁻ | Hypochlorite | Bleaches, disinfectants |
| ClO₂⁻ | Chlorite | Water purification |
| ClO₃⁻ | Chlorate | Oxidizing agents, matches, explosives |
| ClO₄⁻ | Perchlorate | Rocket propellants, fireworks |
| MnO₄⁻ | Permanganate | Disinfectants, water treatment, oxidizing agent |
| CH₃COO⁻ | Acetate | Solvents, vinegar, esters |
| HCO₃⁻ | Hydrogen carbonate (Bicarbonate) | Baking soda, antacids, pH buffers |
| HSO₄⁻ | Hydrogen sulfate (Bisulfate) | pH adjustment, cleaning agents |
| HSO₃⁻ | Hydrogen sulfite (Bisulfite) | Food preservatives, wine making |
| H₂PO₄⁻ | Dihydrogen phosphate | Fertilizers, pH buffers |
| IO₃⁻ | Iodate | Disinfectants, analytical chemistry |
| BrO₃⁻ | Bromate | Water treatment, baking additives |
| SCN⁻ | Thiocyanate | Chemical analysis, photography |
| OCN⁻ | Cyanate | Industrial processes |
| N₃⁻ | Azide | Airbags, preservatives |
-2 Charge Anions
| Formula | Name | Common Occurrence |
|---|---|---|
| CO₃²⁻ | Carbonate | Limestone, chalk, antacids |
| SO₃²⁻ | Sulfite | Food preservatives, wine making |
| SO₄²⁻ | Sulfate | Gypsum, fertilizers, detergents |
| S₂O₃²⁻ | Thiosulfate | Photography, analytical chemistry |
| CrO₄²⁻ | Chromate | Pigments, wood preservatives |
| Cr₂O₇²⁻ | Dichromate | Oxidizing agents, leather tanning |
| HPO₄²⁻ | Hydrogen phosphate | Fertilizers, pH buffers |
| SiO₃²⁻ | Silicate | Glass, ceramics, detergents |
| C₂O₄²⁻ | Oxalate | Cleaning agents, rust removal |
| O₂²⁻ | Peroxide | Bleaching agents, disinfectants |
| S²⁻ | Sulfide | Metal ores, water treatment |
| MoO₄²⁻ | Molybdate | Pigments, catalysts |
| WO₄²⁻ | Tungstate | Pigments, flame retardants |
-3 Charge Anions
| Formula | Name | Common Occurrence |
|---|---|---|
| PO₄³⁻ | Phosphate | Fertilizers, detergents, biological systems |
| AsO₄³⁻ | Arsenate | Pesticides, wood preservatives |
| BO₃³⁻ | Borate | Detergents, glass production |
| PO₃³⁻ | Phosphite | Fertilizers, flame retardants |
| AsO₃³⁻ | Arsenite | Pesticides, wood preservatives |
| VO₄³⁻ | Vanadate | Catalysts, steel production |
| N³⁻ | Nitride | Ceramics, semiconductors |
Positive Polyatomic Ions (Cations)
+1 Charge Cations
| Formula | Name | Common Occurrence |
|---|---|---|
| NH₄⁺ | Ammonium | Fertilizers, household cleaning products |
| H₃O⁺ | Hydronium | Aqueous acid solutions |
| NO⁺ | Nitrosyl | Chemical intermediates |
| Hg₂²⁺ | Mercury(I) / Mercurous | Mercury compounds |
+2 Charge Cations
| Formula | Name | Common Occurrence |
|---|---|---|
| UO₂²⁺ | Uranyl | Uranium compounds, nuclear industry |
| VO²⁺ | Vanadyl | Catalysts |
| PO₂²⁺ | Phosphoryl | Phosphate chemistry |
Structural Patterns and Naming Rules
Common Structural Patterns
Oxyanions
Oxyanions consist of an element bonded to oxygen atoms. The naming follows these patterns:
| Element + Oxygen | Suffix | Example |
|---|---|---|
| Lowest oxygen number | -ite | NO₂⁻ = Nitrite |
| Higher oxygen number | -ate | NO₃⁻ = Nitrate |
| One less oxygen than -ite | Hypo-…ite | ClO⁻ = Hypochlorite |
| One more oxygen than -ate | Per-…ate | ClO₄⁻ = Perchlorate |
Halogen Oxyanions Series
| Formula | Name |
|---|---|
| ClO⁻ | Hypochlorite |
| ClO₂⁻ | Chlorite |
| ClO₃⁻ | Chlorate |
| ClO₄⁻ | Perchlorate |
Similar patterns exist for bromine, iodine, and fluorine oxyanions.
Hydrogen-Containing Anions
When hydrogen is added to an anion, it typically gets the prefix “hydrogen” or “bi-” (older nomenclature):
- CO₃²⁻ (Carbonate) → HCO₃⁻ (Hydrogen carbonate or Bicarbonate)
- PO₄³⁻ (Phosphate) → HPO₄²⁻ (Hydrogen phosphate) → H₂PO₄⁻ (Dihydrogen phosphate)
Memorization Strategies
Charge Grouping Method
Organize ions by charge, focusing on learning groups with the same charge at a time.
Suffix-Charge Association
- Most -ate and -ite ions have a -2 or -3 charge
- When hydrogens are added, the negative charge decreases by one for each H added
- -ide ions typically have a -1 or -2 charge
Visual Association
- Oxyanions with more oxygen atoms generally have more negative charge
- Ions with similar formulas often have similar charges (e.g., CrO₄²⁻ and SO₄²⁻)
Mnemonic Devices
Common -ate ions:
“Sam’s Crazy Purple N-ate”: Sulfate (SO₄²⁻), Carbonate (CO₃²⁻), Phosphate (PO₄³⁻), Nitrate (NO₃⁻)
-ite vs -ate oxygen count:
“The -ite is a little lighter” (fewer oxygen atoms than the corresponding -ate ion)
Polyatomic Ion Applications
Chemical Formula Writing
When writing formulas with polyatomic ions, enclose the polyatomic ion in parentheses if more than one is needed:
- Calcium nitrate: Ca(NO₃)₂
- Aluminum sulfate: Al₂(SO₄)₃
- Ammonium phosphate: (NH₄)₃PO₄
Compound Naming With Polyatomic Ions
- Name the metal (cation) first
- Name the polyatomic ion (anion) second
- No prefixes needed for binary ionic compounds
Examples:
- KNO₃: Potassium nitrate
- Ca(OH)₂: Calcium hydroxide
- Fe₂(SO₄)₃: Iron(III) sulfate or Ferric sulfate
Balancing Equations with Polyatomic Ions
When polyatomic ions remain unchanged throughout a reaction, treat them as a single unit:
Example: 2KMnO₄ + 10KI + 8H₂SO₄ → 2MnSO₄ + 5I₂ + 6K₂SO₄ + 8H₂O
Notice how MnO₄⁻, SO₄²⁻, and H₂O stay intact in the balanced equation.
Common Polyatomic Ions in Specific Chemical Contexts
Acids Containing Polyatomic Ions
| Polyatomic Ion | Corresponding Acid |
|---|---|
| NO₃⁻ (Nitrate) | HNO₃ (Nitric acid) |
| NO₂⁻ (Nitrite) | HNO₂ (Nitrous acid) |
| SO₄²⁻ (Sulfate) | H₂SO₄ (Sulfuric acid) |
| SO₃²⁻ (Sulfite) | H₂SO₃ (Sulfurous acid) |
| PO₄³⁻ (Phosphate) | H₃PO₄ (Phosphoric acid) |
| ClO⁻ (Hypochlorite) | HClO (Hypochlorous acid) |
| ClO₃⁻ (Chlorate) | HClO₃ (Chloric acid) |
| CO₃²⁻ (Carbonate) | H₂CO₃ (Carbonic acid) |
| C₂H₃O₂⁻ (Acetate) | HC₂H₃O₂ (Acetic acid) |
Polyatomic Ions in Common Household Substances
| Common Name | Chemical Name | Formula with Polyatomic Ion |
|---|---|---|
| Baking soda | Sodium bicarbonate | NaHCO₃ |
| Table salt | Sodium chloride | NaCl (no polyatomic ion) |
| Bleach | Sodium hypochlorite | NaClO |
| Vinegar | Acetic acid | HC₂H₃O₂ |
| Drain cleaner | Sodium hydroxide | NaOH |
| Chalk | Calcium carbonate | CaCO₃ |
| Epsom salt | Magnesium sulfate | MgSO₄ |
| Plaster of Paris | Calcium sulfate | CaSO₄ |
| Fertilizer | Ammonium nitrate | NH₄NO₃ |
| Antacid | Various carbonates | (e.g., CaCO₃, NaHCO₃) |
Biological Polyatomic Ions
| Polyatomic Ion | Biological Role |
|---|---|
| PO₄³⁻ (Phosphate) | DNA, ATP, energy transfer, buffer systems |
| HCO₃⁻ (Bicarbonate) | Blood pH buffer |
| NH₄⁺ (Ammonium) | Nitrogen excretion, protein metabolism |
| NO₃⁻ (Nitrate) | Nitrogen cycle, vasodilation |
| SO₄²⁻ (Sulfate) | Protein structure, detoxification |
| HPO₄²⁻/H₂PO₄⁻ | Phosphate buffer system |
Electronic Structure and Resonance
Many polyatomic ions exhibit resonance, where electrons are delocalized over multiple atoms, contributing to their stability.
Example Resonance Structures
Nitrate Ion (NO₃⁻)
The negative charge is distributed equally among all three oxygen atoms:
- N has a formal charge of +1
- One O has a formal charge of -1
- Two O atoms have formal charges of 0
Carbonate Ion (CO₃²⁻)
The negative charge is distributed among the oxygen atoms:
- C has a formal charge of 0
- Two O atoms have formal charges of -1 each
- One O atom has a formal charge of 0
Resources for Further Learning
Online Databases
- NIST Chemistry WebBook
- Royal Society of Chemistry Periodic Table
- PubChem Chemical Database
Reference Books
- CRC Handbook of Chemistry and Physics
- Lange’s Handbook of Chemistry
- Chemistry: The Central Science (Brown, LeMay, et al.)
Interactive Learning Tools
- Polyatomic Ion Flashcards
- Periodic Table Apps with Ion Information
- Chemical Formula Writing Practice Tools
This cheatsheet provides a comprehensive reference for common polyatomic ions in chemistry. The organization by charge and type facilitates quick reference and supports systematic learning of these important chemical components.