Polyatomic Ions and Oxyanions: Structure, Naming, and Patterns

Polyatomic Ions

Definition and General Properties

Polyatomic ions are tightly bound groups made of multiple atoms that possess an overall charge. These ions are commonly encountered in general, organic, and biological (GOB) chemistry due to their prevalence in biological molecules and inorganic compounds.

• Polyatomic ions can be either positively or negatively charged.

• They often consist of nonmetals bonded together, with the charge distributed over the entire ion.

Polyatomic Oxyanions

Structure and Classification

Oxyanions are a subset of polyatomic ions that contain oxygen atoms bonded to another element (usually a nonmetal). Their naming and structure depend on the number of oxygen atoms present.

• Trioxides: Oxyanions with three oxygen atoms.

• Tetraoxides: Oxyanions with four oxygen atoms.

Examples of Trioxides and Tetraoxides

Trioxides	Tetraoxides
BO33- (Borate)	PO43- (Phosphate)
CO32- (Carbonate)	SO42- (Sulfate)
NO3- (Nitrate)	SiO44- (Silicate)
SiO32- (Silicate)

Charge Distribution and Periodic Trends

The charge and number of oxygens in polyatomic ions often follow periodic trends. For example, as you move across a period, the number of oxygens and the charge can change predictably.

Deriving Oxyanions

Systematic Naming and Derivation

Oxyanions are named by changing the number of oxygen atoms and using specific suffixes:

• Decrease the number of oxygens by 1: change the ending to -ite.

• Keep the overall charge the same.

Example: The systematic name for PO33- is phosphite, while PO43- is phosphate.

Formulas

• $ext{Phosphate: } ext{PO}_4^{3-}$

• $ext{Phosphite: } ext{PO}_3^{3-}$

• $ext{Sulfate: } ext{SO}_4^{2-}$

• $ext{Sulfite: } ext{SO}_3^{2-}$

Halogen Oxyanions

Naming Patterns and Examples

Halogen-containing oxyanions are named using prefixes and suffixes that indicate the number of oxygen atoms:

• Per- prefix: Indicates one more oxygen than the base ion.

• -ate suffix: Indicates the base number of oxygens.

• -ite suffix: Indicates one fewer oxygen than the base ion.

• Hypo- prefix: Indicates two fewer oxygens than the base ion.

Halogen	Base Name	# of Oxygens	Base Name Example
Fluorine (F)	fluor	1	hypofluorite
Chlorine (Cl)	chlor	4	perchlorate
Bromine (Br)	brom	4	perbromate
Iodine (I)	iod	4	periodate

All halogen oxyanions possess a charge of $-1$.

Polyatomic Cations

Common Examples

• NH4+ (Ammonium): The only major polyatomic ion with a +1 charge.

• Hg22+ (Mercury(I)): Composed of two mercury ions bonded together.

The Other Polyatomic Ions

Other Tetraoxides and Miscellaneous Ions

Other Tetraoxides	Other Polyatomic Ions
MnO4- (Permanganate)	CN- (Cyanide)
CrO42- (Chromate)	OH- (Hydroxide)
Cr2O72- (Dichromate)	O22- (Peroxide)
CO42- (Additional info: inferred as a possible carbonate variant)	CH3COO- (Acetate)

Practice Problems and Systematic Naming

Examples and Applications

• NO2-: Nitrite ion

• CO22-: Carbonite ion (Additional info: not commonly encountered, but follows naming pattern)

• AsO42-: Arsenate ion

• SO42-: Sulfate ion

• FO-: Hypofluorite ion

• IO3-: Iodate ion

• SiO44-: Silicate ion

Special Naming Cases

• Thiosulfate ion: Replacement of an oxygen by a sulfur in sulfate. Formula: $S_2O_3^{2-}$

• Thiocyanate ion: Structure based on cyanate, with sulfur replacing oxygen. Formula: $SCN^-$

Summary Table: Common Polyatomic Ions

Ion	Formula	Systematic Name
Phosphate	PO43-	Phosphate
Phosphite	PO33-	Phosphite
Sulfate	SO42-	Sulfate
Sulfite	SO32-	Sulfite
Nitrate	NO3-	Nitrate
Nitrite	NO2-	Nitrite
Chlorate	ClO3-	Chlorate
Perchlorate	ClO4-	Perchlorate
Ammonium	NH4+	Ammonium
Acetate	CH3COO-	Acetate

Additional info: The notes provide a systematic approach to naming and recognizing polyatomic ions, including oxyanions and halogen-containing ions, which are essential for GOB Chemistry students.