Naming Ions and Ionic Compounds

Learning Objective

In this lesson we will learn how ions and ionic compounds are named.

Learning Outcomes

By the end of this lesson you will be able to:

  • Name positive and negative monatomic ions.

  • Give examples of metals that form more than one type of ion, and show how these ions are differentiated.

  • Look up the names and formulas of polyatomic ions on a valency table.

  • Determine the names of ionic compounds from their formulas.

 

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Introduction

  • Ionic compounds are named after the ions that form them.
    The first part of the compound name is the name of the positive ion.
    The second part of the compound name is the name of the negative ion.

 

Monatomic Ions

  • Monatomic ions are ions that contain one type of atom.

 

Monatomic Positive Ions

  • Monatomic positive ions have the same name as the metal atoms they are formed from.
    For example, sodium atoms (Na) form sodium ions (Na+).
  • Positive ions formed from metals among the first 20 elements are shown below.

 

    Element Name Element Symbol Ion Name Ion Charge Ion Symbol
    lithium Li lithium +1 Li+
    beryllium Be beryllium +2 Be2+
    sodium Na sodium +1 Na+
    magnesium Mg magnesium +2 Mg2+
    aluminium Al aluminium +3 Al3+
    potassium K potassium +1 K+
    calcium Ca calcium +2 Ca2+

 

Metals That Form More Than One Type of Ion

  • Many transition and post-transition metals, such as iron, copper and lead, form more than one type of ion.
    For example, copper atoms (Cu) can form copper ions with a charge of +1 (Cu+) or +2 (Cu2+).
  • Roman numerals corresponding to the size of the positive charge are written in brackets after the metal name to distinguish the different ions.
    For example, Cu+ ions are called copper (I) ions, and Cu2+ ions are called copper (II) ions.
  • Common metals that form more than one type of ion are shown below.

 

    Element Name Element Symbol Ion Name Ion Charge Ion Symbol
    chromium Cr chromium (II) +2 Cr2+
    chromium (III) +3 Cr3+
    copper Cu copper (I) +1 Cu+
    copper (II) +2 Cu2+
    iron Fe iron (II) +2 Fe2+
    iron (III) +3 Fe3+
    lead Pb lead (II) +2 Pb2+
    lead (IV) +4 Pb4+
    tin Sn tin (II) +2 Sn2+
    tin (IV) +4 Sn4+
  • Metals that form more than one type of ion can also be looked up in a valency table.

 

Monatomic Negative Ions

  • Monatomic negative ions have the same name as the non-metal atoms they are formed from, except the last part of their name is changed to “-ide”.
    For example, chlorine atoms (Cl) form chloride ions (Cl).
  • Negative ions formed from non-metal elements are shown below.

 

    Element Name Element Symbol Ion Name Ion Charge Ion Symbol
    nitrogen N nitride –3 N3–
    oxygen O oxide –2 O2–
    fluorine F fluoride –1 F
    phosphorus P phosphide –3 P3–
    sulfur S sulfide –2 S2–
    chlorine Cl chloride –1 Cl
    selenium Se selenide –2 Se2–
    bromine Br bromide –1 Br
    iodine I iodide –1 I

 

Polyatomic Ions

  • Polyatomic ions are ions that contain more than one type of atom.
    For example, sulfate ions (SO42–) consist of one sulfur atom and four oxygen atoms, and have an overall charge of –2.
  • The names of polyatomic ions cannot be predicted from their formulas.
    They can be looked up in a valency table, but you may wish to memorise the most commonly used ones.
  • The names of common polyatomic ions are shown below.

 

    Ion Name Ion Symbol
    ammonium NH4+
    hydroxide OH
    nitrate NO3
    hydrogen carbonate HCO3
    acetate CH3COO
    sulfate SO42–
    carbonate CO32–
    phosphate PO43–

Click here for a downloadable valency table..

 

Ionic Compounds

  • To form the name of an ionic compound, simply join the names of the positive and negative ions together.
    The positive ion forms the first part of the name and the negative ion forms the second part of the name.
    For example, potassium ions (K+) and bromide ions (Br) combine to form the ionic compound potassium bromide (KBr).
  • Examples of common ionic compounds are shown below.

 

    Positive Ion Name Positive Ion Symbol Negative Ion Name Negative Ion Symbol Compound Name Compound Formula*
    magnesium Mg2+ oxide O2– magnesium oxide MgO
    aluminium Al3+ nitride N3– aluminium nitride AlN
    iron (III) Fe3+ chloride Cl iron (III) chloride FeCl3
    sodium Na+ sulfate SO42– sodium sulfate Na2SO4
    calcium Ca2+ carbonate CO32– calcium carbonate CaCO3
    lead (IV) Pb4+ fluoride F lead (IV) fluoride PbF4
    ammonium NH4+ hydroxide OH ammonium hydroxide NH4OH
    * We will learn how the formulas for ionic compounds are determined in the next lesson.

 

Summary

  • Monatomic ions are ions that contain one type of atom.
    Polyatomic ions are ions that contain more than one type of atom.
  • Monatomic positive ions have the same name as the metal atoms they are formed from.
    For metals that form more than one type of ion, Roman numerals corresponding to the size of the positive charge are used to distinguish between the different ions.
  • Monatomic negative ions have the same name as the non-metal atoms they are formed from, except the last part of their name is changed to “-ide”.
  • The names of polyatomic ions cannot be predicted from their formulas, but can be looked up in a valency table.
  • The names of ionic compounds are derived by joining the names of the positive ion (first) and the negative ion (second).

 
 
sodium chloride ions lattice

Sodium chloride lattice, showing sodium and chloride ions.

(Image: Eyal Bairey, Wikimedia Commons)

 

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