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3.17: Covalent Bonding: Writing Chemical Names of Covalent Molecules

  • Page ID
    213174
  • Learning Objectives
    • Write the chemical names of covalent molecules.

    In the previous section, the process for using a Lewis structure to determine the chemical formula of a covalent molecule was presented and applied.  The chemical name of a molecule, in turn, is derived based on the information included in its chemical formula.  As stated previously, a chemical name should uniquely correspond to a single chemical formula.  In other words, no two chemical formulas should share a common chemical name. 

    Recall that covalent bonds are produced when unpaired electrons found within two atoms interact to form a shared pair of electrons.  As a Lewis structure is the result of correctly executing this sharing process, the chemical formula and the chemical name of the corresponding covalent molecule must accurately reflect the information shown in that Lewis structure.  The procedure for writing the chemical name of a covalent molecule is explained in the following paragraphs.

    Naming Covalent Molecules

    For a covalent molecule, the information represented in its chemical name must be a direct reflection of its Lewis structure.  Therefore, the chemical name of a covalent molecule must not only contain information that indicates the identities of its constituent elements, but also must reflect how many of each of those elements are present within the molecule.  Note that, if written properly, the chemical formula for a covalent molecule also contains this information and, therefore, can be used as the basis for developing a chemical name.  Elemental names are incorporated into a covalent molecule's chemical name in the order in which their corresponding elemental symbols appear in the chemical formula.  The suffix on the second elemental term is replaced with "-ide," in order to indicate its secondary placement within the chemical formula.  Finally, since the subscripts in a covalent chemical formula are used to indicate how many times each elemental symbol appears in the molecule's Lewis structure, corresponding numerical prefixes, which are listed below in Table \(\PageIndex{1}\), must be incorporated into the molecule's chemical name.

    Table \(\PageIndex{1}\):  Numerical Prefixes Used When Naming Covalent Molecules
    Number of Atoms in the Molecule
    (Indicated by a Subscript)
    Numerical Prefix 
    1 mono-*
    2 di-
    3 tri-
    4 tetra-
    5 penta-
    6 hexa-
    7 hepta-
    8 octa-
    9 nona-
    10 deca-

    Note that the prefix "mono-" is never used to change the first elemental term in a covalent chemical name and should only be used as a modifier on the remaining term if the secondary element is oxygen.  Finally, an "a" or "o" at the end of a prefix is usually dropped if the name of the element that is being altered begins with a vowel.

    These prefix rules are best exemplified when writing the chemical name of "CO," which is the chemical formula of a commonly-known molecule. 

    This molecule is formed using covalent bonds, as its constituent elements, C (carbon) and (oxygen), are both non-metals.

    Since the elemental symbol "C" appears first in the given chemical formula, "carbon" is the basis of the first word in the molecule's chemical name.  The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-."  However, this prefix is not used to alter the first elemental term in a covalent chemical name.  Therefore, the first word in the chemical name of this molecule is "carbon."

    Because the elemental symbol "O" is written second in the given chemical formula, "oxide" becomes the basis of the second word in the molecule's chemical name.  The suffix on this elemental term is "-ide," as an indicator of its secondary placement within the chemical formula.  The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-."  Because the secondary element is oxygen, this prefix is used as a modifier on the term "oxide."  Finally, since "oxide" begins with a vowel, the "o" of "mono-" is dropped.  Therefore, the second word in the chemical name of this covalent molecule is "monoxide."

    The result of combining these words, "carbon monoxide," is the chemically-correct name for CO.

    Example \(\PageIndex{1}\)

    Write the chemical name of SF2, a covalent molecule that is formed when fluorine and sulfur bond with one another.

    Solution

    Since the elemental symbol "S" appears first in the given chemical formula, "sulfur" is the basis of the first word in the molecule's chemical name.  The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-."  However, this prefix is not used to alter the first elemental term in a covalent chemical name.  Therefore, the first word in the chemical name of this molecule is "sulfur."

    Because the elemental symbol "F" is written second in the given chemical formula, "fluoride" becomes the basis of the second word in the molecule's chemical name.  The suffix on this elemental term is "-ide," as an indicator of its secondary placement within the chemical formula.  The subscript on this elemental symbol, a "2," corresponds to prefix of "di-."  Therefore, the second word in the chemical name of this covalent molecule is "difluoride."

    The result of combining these words, "sulfur difluoride," is the chemically-correct name for SF2.

    Exercise \(\PageIndex{1}\)

    Write the chemical name that corresponds to each of the following chemical formulas. 

    1. CCl4, the covalent molecule that is formed when carbon and chlorine bond with one another.
    2. PBr3, a covalent molecule that is formed when bromine and phosphorus bond with one another.
    3. N3P6, a covalent molecule that is formed when nitrogen and phosphorus bond with one another.
    4. I2O5, a covalent molecule that is formed when oxygen and iodine bond with one another.
    5. HF, the covalent molecule that is formed when hydrogen and fluorine bond with one another.
    Answer a
    Since the elemental symbol "C" appears first in the given chemical formula, "carbon" is the basis of the first word in the molecule's chemical name.  The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-."  However, this prefix is not used to alter the first elemental term in a covalent chemical name.  Therefore, the first word in the chemical name of this molecule is "carbon."

    Because the elemental symbol "Cl" is written second in the given chemical formula, "chloride" becomes the basis of the second word in the molecule's chemical name.  The suffix on this elemental term is "-ide," as an indicator of its secondary placement within the chemical formula.  The subscript on this elemental symbol, a "4," corresponds to prefix of "tetra-."  Therefore, the second word in the chemical name of this covalent molecule is "tetrachloride."

    The result of combining these words, "carbon tetrachloride," is the chemically-correct name for CCl4.
    Answer b
    Since the elemental symbol "P" appears first in the given chemical formula, "phosphorus" is the basis of the first word in the molecule's chemical name.  The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-."  However, this prefix is not used to alter the first elemental term in a covalent chemical name.  Therefore, the first word in the chemical name of this molecule is "phosphorus."

    Because the elemental symbol "Br" is written second in the given chemical formula, "bromide" becomes the basis of the second word in the molecule's chemical name.  The suffix on this elemental term is "-ide," as an indicator of its secondary placement within the chemical formula.  The subscript on this elemental symbol, a "3," corresponds to prefix of "tri-."  Therefore, the second word in the chemical name of this covalent molecule is "tribromide."

    The result of combining these words, "phosphorus tribromide," is the chemically-correct name for PBr3.
    Answer c
    Since the elemental symbol "N" appears first in the given chemical formula, "nitrogen" is the basis of the first word in the molecule's chemical name.  The subscript on this elemental symbol, a "3," corresponds to prefix of "tri-."  Therefore, the first word in the chemical name of this molecule is "trinitrogen."

    Because the elemental symbol "P" is written second in the given chemical formula, "phosphide" becomes the basis of the second word in the molecule's chemical name.  The suffix on this elemental term is "-ide," as an indicator of its secondary placement within the chemical formula.  The subscript on this elemental symbol, a "6," corresponds to prefix of "hexa-."  Therefore, the second word in the chemical name of this covalent molecule is "hexaphosphide."

    The result of combining these words, "trinitrogen hexaphosphide," is the chemically-correct name for N3P6.
    Answer d
    Since the elemental symbol "I" appears first in the given chemical formula, "iodine" is the basis of the first word in the molecule's chemical name.  The subscript on this elemental symbol, a "2," corresponds to prefix of "di-."  Therefore, the first word in the chemical name of this molecule is "diiodine."

    Because the elemental symbol "O" is written second in the given chemical formula, "oxide" becomes the basis of the second word in the molecule's chemical name.  The subscript on this elemental symbol, a "5," corresponds to prefix of "penta-."  Since "oxide" begins with a vowel, the "a" of "penta-" is dropped.  Therefore, the second word in the chemical name of this covalent molecule is "pentoxide."

    The result of combining these words, "diiodine pentoxide," is the chemically-correct name for I2O5.
    Answer e
    Since the elemental symbol "H" appears first in the given chemical formula, "hydrogen" is the basis of the first word in the molecule's chemical name.  The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-."  However, this prefix is not used to alter the first elemental term in a covalent chemical name.  Therefore, the first word in the chemical name of this molecule is "hydrogen."

    Because the elemental symbol "F" is written second in the given chemical formula, "fluoride" becomes the basis of the second word in the molecule's chemical name.  The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-."  However, this prefix is only used as a modifier on the second term in a covalent chemical name if the secondary element is oxygen.  Because the secondary element is fluorine, the "mono-" prefix is not applied.  Therefore, the second word in the chemical name of this molecule is "fluoride."

    The result of combining these words, "hydrogen fluoride," is the chemically-correct name for HF.