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Nomenclature for Ionic Compounds

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Ionic compounds consist of cations (positive ions) and anions (negative ions). The nomenclature, or naming, of ionic compounds is based on the names of the component ions. Here are the principal naming conventions for ionic compounds, along with examples to show how they are used:

  • Roman Numerals
    A Roman numeral in parentheses, followed by the name of the element, is used for elements that can form more than one positive ion. This is usually seen with metals. You can use a chart to see the possible valences for the elements.

    Fe2+ Iron (II)
    Fe3+ Iron (III)
    Cu+ Copper (I)
    Cu2+ Copper (II)

  • -ous and -ic
    Although Roman numerals are used to denote the ionic charge of cations, it is still common to see and use the endings -ous or -ic. These endings are added to the Latin name of the element (e.g., stannous/stannic for tin) to represent the ions with lesser or greater charge, respectively. The Roman numeral naming convention has wider appeal because many ions have more than two valences.

    Fe2+ Ferrous
    Fe3+ Ferric
    Cu+ Cuprous
    Cu2+ Cupric

  • -ide
    The -ide ending is added to the name of a monoatomic ion of an element.

    H- Hydride
    F- Fluoride
    O2- Oxide
    S2- Sulfide
    N3- Nitride
    P3- Phosphide

  • -ite and -ate
    Some polyatomic anions contain oxygen. These anions are called oxyanions. When an element forms two oxyanions, the one with less oxygen is given a name ending in -ite and the one with more oxgyen is given a name that ends in -ate.

    NO2- Nitrite
    NO3- Nitrate
    SO32- Sulfite
    SO42- Sulfate

  • hypo- and per-
    In the case where there is a series of four oxyanions, the hypo- and per- prefixes are used in conjunction with the -ite and -ate suffixes. The hypo- and per- prefixes indicate less oxygen and more oxygen, respectively.

    ClO- Hypochlorite
    ClO2- Chlorite
    ClO3- Chlorate
    ClO4- Perchlorate

  • bi- and di- hydrogen
    Polyatomic anions sometimes gain one or more H+ ions to form anions of a lower charge. These ions are named by adding the word hydrogen or dihydrogen in front of the name of the anion. It is still common to see and use the older naming convention in which the prefix bi- is used to indicate the addition of a single hydrogen ion.

    HCO3- Hydrogen carbonate or bicarbonate
    HSO4- Hydrogen sulfate or bisulfate
    H2PO4- Dihydrogen phosphate

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