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What are polyatomic ions?

  • A polyatomic ion is a charged group of atoms covalently bonded together.
  • Majority of the polyatomic ions are anions, meaning they are negatively charged.
  • Ammonium is the only positively charged polyatomic ion that does not have an oxygen atom.
  • And many polyatomic ions are combined with one or more oxygen atoms, and their names end in either “ite” or “ate”.

A polyatomic ion compound

polyatomic ions

Polyatomic ions are usually anions (negatively charged), so they would be attracted to cations (positively charged).

A polyatomic ion can be attracted to an oppositely charged element, or an oppositely charged polyatomic ion.

  • NH₄⁺ attracted to F⁻
  • NH₄⁺ attracted to SO₄²⁻

In all cases, the charges must always be balanced in a compound.


Balance the charges

Cation (Na⁺) Anion (SO₄²⁻)
+ 2-
+

Formula: Na₂SO₄

Cation (NH₄⁺) Anion (SO₄²⁻)
+ 2-
+

Formula: (NH₄)₂SO₄

Make sure you always put brackets around the polyatomic ion if there are more that one of them in the formula


Naming compounds containing polyatomic ions

The convention for naming polyatomic ion compounds is the same as for the ionic compounds.

Name the cation first before the anion.

Name Cation Anion Formula
Magnesium carbonate Mg²⁺ CO₃²⁻ MgCO₃
Ammonium nitrate NH₄⁺ NO₃⁻ NH₄NO₃
Calcium carbonate Ca²⁺ CO₃²⁻ CaCO₃

 


Transition elements

If transition elements are present, use Roman numerals to indicate the charges of the cation.

Name Cation Anion Formula
Chromium(II) phosphate Cr²⁺ PO₄³⁻ Cr₃(PO₄)₂
Chromium(III) phosphate Cr³⁺ PO₄³⁻ CrPO₄

 

The diagrams below show how the charges are balanced for the compounds.

Balancing charges in a polyatomic ion compound

chromium(II) phosphate

chromium(III) phosphate - balancing charges

chromium(III) phosphate


Acids

Acids are compounds in which the “cation” is H+ .

Acids are named based on their anion.

A polyatomic ion with suffix ending in “-ate”  is changed to the suffix “ic” as an acid.

A polyatomic ion with suffix ending in “-ite”  is changed to the suffix “ous” as an acid.

acids


Memorizing polyatomic ions names

How to memorize the names and chemical formulas of polyatomic ions such as carbonate (CO3), phosphate (PO43), nitrate (NO3) and sulfate (SO42)? If you notice the pattern, these names end in either “ite” or “ate”. These species contain oxygen atoms, and their names are based on the number of oxygen atoms in the ion. You can find the charges on the periodic table, as we will discuss on the next page.

How to memorize the names of polyatomic ions and their charges >>


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Concept check

 

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