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4.5: Nomenclature of Molecular Compounds

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  • The nomenclature of simple binary molecular compounds (covalently bonded compounds consisting of only two elements) is slightly more complicated than the nomenclature of ionic compounds because multipliers must be used to indicate the ratio of the elements in the molecule; the multiplier mono is only used for the second element in a compound.

    Further, when you are naming a molecular compound, you must also decide which element should be listed first. In general, elements appearing to the left or lower in the periodic table are listed first in the name. Once you have decided on the order, the second element is named using the element root and ide, just like in ionic compounds. Thus, for CCl4, carbon is to the left of chlorine (Group 4A vs. Group 5A), so it is listed first. There are four chlorines, so the multiplier tetra is used, and the name is carbon tetrachloride. Compounds containing hydrogen are generally an exception, and the hydrogen is listed as the first element in the name. Thus, H2S would be named using the multiplier di to indicate that there are two hydrogens and mono to indicate that there is only one sulfur, or, dihydrogen monosulfide.

    For the molecule SO2; they are both Group 6A elements, but sulfur is lower in the periodic table (Row 3 vs. Row 2) so it is first in the name. There are two oxygens, so the multiplier is di and the name is sulfur dioxide.

    For the molecule NO; nitrogen is to the left of oxygen (Group 6A vs. Group 5A) so it is first in the name. There is one oxygen, so the multiplier is mono and, following the rules, the name would be “nitrogen monooxide”. In this case, however, the second “o” in the name is dropped (to allow for easier pronunciation ) and the name is shortened to nitrogen monoxide. Distinguish this from another oxide of nitrogen, N2O4. Again nitrogen is first and needs the multiplier di. There are four oxygens, so the multiplier is tetra, but once again the multiplier is shortened (again, the “a” is dropped) and the name is dinitrogen tetroxide.

    Example \(\PageIndex{1}\):

    Write a correct chemical formula for each of the following molecular compounds:

    1. Chlorine monofluoride
    2. Dihydrogen monosulfile
    3. Carbon tetrabromide
    4. Bromine


    1. ClF
    2. H2S
    3. CBr4
    4. Br2

    Example \(\PageIndex{1}\):

    Write a proper chemical name for each of the following molecular compounds:

    1. IF
    2. PCl3
    3. I2
    4. N2F2


    1. Iodine monofluoride
    2. Phosphorus trichloride
    3. Iodine
    4. Dinitrogen difluoride

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