Polyatomic Ions and Nomenclature in Introductory Chemistry

Polyatomic Ions

Introduction to Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms covalently bonded, that function as a single charged entity. They are commonly encountered in introductory chemistry, especially in the context of naming ionic compounds and understanding chemical formulas.

• Cations: Positively charged polyatomic ions.

• Anions: Negatively charged polyatomic ions.

Common Polyatomic Cations

• NH4+: ammonium

Common Polyatomic Anions

Polyatomic anions are grouped by their charge:

Charge	Ion	Name
-1	HSO3-	bisulfite
-1	HSO4-	bisulfate
-1	HCO3-	bicarbonate
-2	SO32-	sulfite
-2	SO42-	sulfate
-2	S2O32-	thiosulfate
-2	CO32-	carbonate
-3	PO33-	phosphite
-3	PO43-	phosphate
-2	HPO42-	hydrogen phosphate
-2	H2PO4-	dihydrogen phosphate

Other Common Polyatomic Anions

• CN-: cyanide

• SCN-: thiocyanate

• NO2-: nitrite

• NO3-: nitrate

• ClO-: hypochlorite

• ClO2-: chlorite

• ClO3-: chlorate

• ClO4-: perchlorate

• MnO4-: permanganate

• OH-: hydroxide

• C2H3O2-: acetate

• CrO42-: chromate

• Cr2O72-: dichromate

• C2O42-: oxalate

• O22-: peroxide

Additional Polyatomic Ions (Not Required to Memorize)

These ions may appear in some contexts but are less commonly required for memorization in introductory courses.

Ion	Name
HS-	hydrosulfide
OCN-	cyanate
BrO-	hypobromite
BrO2-	bromite
BrO3-	bromate
BrO4-	perbromate
IO-	hypoiodite
IO2-	iodite
IO3-	iodate
IO4-	periodate
AsO43-	arsenate
BO33-	borate

Prefixes and Suffixes in Polyatomic Ion Nomenclature

Understanding Prefixes and Suffixes

Prefixes and suffixes in the names of polyatomic ions indicate the number of oxygen atoms and the relative charge. Recognizing these patterns helps in identifying and naming ions correctly.

• -ate: Indicates the most common form of the ion (e.g., sulfate SO42-).

• -ite: One less oxygen than the -ate form, same charge (e.g., sulfite SO32-).

• hypo-: One less oxygen than the -ite form (e.g., hypochlorite ClO-).

• per-: One more oxygen than the -ate form (e.g., perchlorate ClO4-).

• -ide: Used for ions with only one kind of atom (e.g., chloride Cl-), or when oxygen atoms are replaced by other atoms.

Examples of Naming Patterns

• Chlorine Oxyanions:

  • ClO-: hypochlorite

  • ClO2-: chlorite

  • ClO3-: chlorate

  • ClO4-: perchlorate

• Sulfur Oxyanions:

  • SO32-: sulfite

  • SO42-: sulfate

Key Points for Students

• Memorize the most common polyatomic ions and their charges.

• Use prefixes and suffixes to deduce the composition of related ions.

• Recognize that hydrogen can be added to polyatomic ions, changing both the name and the charge (e.g., hydrogen phosphate HPO42-).

Example: Writing Formulas

• Given the name ammonium sulfate, write the formula:

  • Ammonium: NH4+

  • Sulfate: SO42-

  • Formula: $(NH_4)_2SO_4$