Polyatomics and Acid Naming

Overview of Polyatomic Ions

• Date & Time: Monday, September 21, 2026, 10:14 AM
• Definition: A polyatomic ion is a covalent compound that carries an overall electric charge.
  • Example: Sulphate ($SO_4^{2-}$)

Rules and Characteristics of Polyatomic Ions

• Naming Suffixes:
  • The vast majority of polyatomic ions end in -ate or -ite.
  • Exceptions:
  • Cyanide ($(CN)^{-1}$
  • Hydroxide ($(OH)^{-1}$
• Charge Types:
  • Most polyatomic ions are negative ions (anions).
  • Positive Polyatomic Ion Exceptions (Cations):
  • Ammonium ($(NH_4)^{+1}$
  • Hydronium ($(H_3O)^{+1}$
• Subscript and Grouping Rules:
  • The internal subscripts within a polyatomic group cannot be changed.
  • To represent more than one polyatomic unit in a chemical formula, bracket the whole polyatomic group and place the subscript outside.

Formula-to-Name Conversions

• $Na_2SO_4$: Sodium sulphate
• $Mg(OH)_2$: Magnesium hydroxide
• $FeSO_4$: Iron ( II ) sulphate
• $Ru(NO_3)_3$: Ruthenium (III ) nitrate
• $Au_2Cr_2O_7$: Gold (I) dichromate

Nomenclature Rules for Acids

• Aqueous Notation:

  • The symbol $(aq)$ stands for aqueous, which means dissolved in water.

• Acid Classification Systems:

  • Halogen-Based Acids ($H$ + Halogen):

  • Structure: Hydrogen bonded to a halogen element in aqueous solution.

  • Naming Rule: hydro + "halogen root" + ic acid

  • Example Formula: $HCl(aq)$

  • Example Name: Hydrochloric acid

  • Polyatomic Oxyacids with "-ate" Ions ($H$ + Polyatomic with Oxygen):

  • Structure: Hydrogen bonded to an oxygen-containing polyatomic anion that originally ends in "-ate".

  • Naming Rule: polyatomic/element root + "ic acid"

  • Example Formula: $H_2SO_4(aq)$

  • Example Name: Sulphuric acid

  • Polyatomic Oxyacids with "-ite" Ions ($H$ + Polyatomic with Oxygen):

  • Structure: Hydrogen bonded to an oxygen-containing polyatomic anion that originally ends in "-ite".

  • Naming Rule: poly/element root + "ous acid"

  • Example Formula: $H_2SO_3(aq)$

  • Example Name: Sulphurous acid

Name-to-Formula Conversions

• Sodium Carbonate:

  • Formula: $Na_2CO_3$

• Lead (IV) Cyanide:

  • Formula: $Pb(CN)_4$

• Ammonium Phosphate:

  • Formula: $(NH_4)_3PO_4$