Polyatomic Ions – HSC Chemistry


This is part of preliminary HSC Chemistry course under the topic of Properties of Matter.

HSC Chemistry Syllabus

  • investigate the nomenclature of inorganic substances using International Union of Pure and Applied Chemistry (IUPAC) naming conventions

Nomenclature & Structure of Polyatomic Ions

This video discusses polyatomic ions that are commonly encountered in HSC Chemistry.

What Are Polyatomic Ions?

Polyatomic ions are groups of atoms that are covalently bonded yet carry an overall charge, making them capable of participating in ionic bonding with other ions. These ions can be either positively charged (cations) or negatively charged (anions), but anions are more common. The charge on a polyatomic ion arises because the total number of electrons in the molecule does not equal the total number of protons in the nuclei of the atoms making up the ion.

Common Polyatomic Ions

Here are some widely encountered polyatomic ions, along with their formulas and charges:

  • Ammonium (NH₄⁺): An example of a positively charged polyatomic ion.
  • Hydroxide (OH⁻): A basic ion consisting of oxygen and hydrogen.
  • Nitrate (NO₃⁻): A common ion in fertilisers and explosives.
  • Sulfate (SO₄²⁻): Found in many minerals and industrial chemicals.
  • Carbonate (CO₃²⁻): Present in limestone, marble, and baking soda.
  • Phosphate (PO₄³⁻): Crucial for biological molecules and fertilisers.


It is important to be familiar with the molecular formula and names of the common polyatomic ions listed above.

Less common poly atomic ions include:


  • Nitrite (NO₂⁻):

  • Phosphite (PO₃³⁻):

  • Hydrogen Carbonate (HCO₃⁻), also known as bicarbonate: 

  • Sulfite (SO₃²⁻):

Ionic Compounds with Polyatomic Ions

When polyatomic ions combine with suitable cations or anions, they form ionic compounds. These compounds can include a combination of a polyatomic ion with a simple monatomic ion or with another polyatomic ion. Knowing the name and charge of polyatomic ion is useful to determine the empirical formula of the ionic compound and its name.

Naming ionic compounds containing polyatomic ions follows a straightforward convention: the name of the cation comes first, followed by the name of the anion. If the compound contains a metal that can form ions with different charges, the charge is indicated in Roman numerals in parentheses immediately following the metal's name. For the polyatomic ion, its conventional name is used, which often ends in "-ate," "-ite," or "-ide," depending on the ion.


Here are examples of ionic compounds containing polyatomic ions:


  • Ammonium Chloride (NH₄Cl): the ratio between ammonium and chloride is 1:1 because the charge of ammonium ion is +1.

  • Sodium Hydroxide (NaOH): the ratio of between sodium and hydroxide ion is 1:1 between the charge of hydroxide ion is –1.

  • Calcium Carbonate (CaCO₃) the ratio of between calcium and carbonate ion is 1:1 between the charges of calcium and hydroxide ion are +2 and –2 respectively.

  • Potassium Nitrate (KNO₃): the ratio of between potassium and nitrate ion is 1:1 between the charge of nitrate ion is –1.

  • Ammonium Sulfate ((NH₄)₂SO₄): the ratio of between ammonium and sulfate ion is 2:1 between the charge of sulfate ion is –2.

  • Sodium Phosphate (Na₃PO₄)the ratio of between sodium and sulfate ion is 2:1 between the charge of sulfate ion is –2.