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Naming Ionic Compounds

Key Concepts

  • Positively charged ions are called cations

  • Negatively charged ions are called anions

  • The cation is always named first.


Cations can be metals or polyatomic ions

  • The ammonium ion (NH4+) is an example of a polyatomic cation

  • Hydrogen can also form a cation, H+, in which case the name hydrogen is used in naming.

  • For metals that have only one possible charge (valency) the name of the metal is used.
    Examples are Group 1 (IA) metals (charge 1+), Group 2 (IIA) metals (charge 2+), Aluminium (charge 3+), Zinc (charge 2+), Silver (charge 1+)

  • For metals that can have more than one charge (valency) the name of the metal is succeeded by the valency in capital Roman numerals in brackets
    by using the suffix -ous for the lowest valency and -ic for the highest valency and sometimes with the Latinised name for the metal
Element Cation Preferred Name Other Name
copper Cu+ copper(I) cuprous
Cu2+ copper(II) cupric

iron Fe2+ iron(II) ferrous
Fe3+ iron(III) ferric

lead Pb2+ lead(II) plumbous
Pb4+ lead(IV) plumbic

mercury Hg22+ mercury(I) mercurous
Hg2+ mercury(II) mercuric

tin Sn2+ tin(II) stannous
Sn4+ tin(IV) stannic


Anions can be a negatively charged element or a polyatomic ion

  • Negatively charged elements have the suffix -ide
    Examples are oxide (O2-), sulfide (S2-), fluoride (F-), chloride (Cl-), bromide (Br-), iodide (I-), nitride (N3-), hydride (H-)

  • Polyatomic ions which include oxygen in the anion have the suffixes -ate or -ite. "ate" means there is more oxygen in the anion than one ending in "ite"

    Examples: sulfate (SO42-) has more oxygen than sulfite (SO32-), nitrate (NO3-) has more oxygen in the anion than nitrite (NO2-)

    Other examples are carbonate (CO32-), phosphate (PO43-) and permanganate (MnO4-)

    Exception: OH- is named hydroxide


Ionic Compounds containing ions of elements

  • MgO
    CATION: Mg2+ is named magnesium as magnesium belongs to Group 2 (IIA) and can only have one charge (valency)
    ANION: O2- is named oxide
    Name of compound is magnesium oxide

  • FeS
    CATION: Fe2+ is named iron(II) or ferrous as iron can have a charge of either 2+ or 3+
    ANION: S2- is named as sulfide
    Name of compound is iron(II) sulfide or ferrous sulfide

  • LiH
    CATION: Li+ is named as lithium since lithium is a Group 1 (IA) metal and can have only one charge (valency)
    ANION: H- is named as hydride
    Name of compund is lithium hydride

  • H2S
    CATION: H+ is named as hydrogen
    ANION: S2- is named as sulfide
    Name of compound is hydrogen sulfide

Ionic compounds containing polyatomic ions

  • NaOH
    CATION: Na+ is named as sodium (Group 1 metal)
    ANION: OH- is named as hydroxide
    Name of compound is sodium hydroxide

  • CaCO3
    CATION: Ca2+ is named as calcium (Group 2 metal)
    ANION: CO32- is named as carbonate
    Name of compound is calcium carbonate

  • FeSO4
    CATION: Fe2+ named as iron(II) or ferrous
    ANION: SO42- named as sulfate
    Name of compound is iron(II) sulfate or ferrous sulfate

  • FeSO3
    CATION: Fe2+ named as iron(II) or ferrous
    ANION: SO32- named as sulfite
    Name of compound is iron(II) sulfite or ferrous sulfite

  • (NH4)3PO4
    CATION: NH4+ named as ammonium
    ANION: PO43- named as phosphate
    Name of compound is ammonium phosphate

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