Molecular compounds or covalent compounds are those in which the elements share electrons via covalent bonds. The only type of molecular compound a chemistry student is expected to be able to name is a binary covalent compound. This is a covalent compound made up of only two different elements. Here is a look at the nomenclature rules for molecular compounds, plus some examples of how to name the compounds.
Identifying Molecular CompoundsMolecular compounds contain two or more nonmetals (not the ammonium ion). Usually you can recognize you are dealing with a molecular compound because the first element in the compound name is a nonmetal. Some molecular compounds contain hydrogen, but if you see a compound which starts with "H", you can assume it is an acid and not a molecular compound. Compounds consisting only of carbon with hydrogen are called hydrocarbons. Hydrocarbons have their own special nomenclature, so they are treated differently from other molecular compounds.
Writing Formulas for Covalent CompoundsCertain rules apply to the way names of covalent compounds are written:
- The more electropositive element (further left on the periodic table) is listed before the more electronegative element (further right on the periodic table).
- The second element is given an -ide ending.
- Prefixes are used to denote how many atoms of each element are present in the compound.
Prefixes and Molecular Compound NamesNonmetals may combine in a variety of ratios, so it is important that the name of a molecular compound indicates how many atoms of each type of element are present in the compound. This is accomplished using prefixes. If there is only one atom of the first element, no prefix is used. It is customary to prefix the name of one atom of the second element with mono-. For example, CO is named carbon monoxide rather than carbon oxide.
Examples of Covalent Compound NamesSO2 - sulfur dioxide
SF6 - sulfur hexafluoride
CCl4 - carbon tetrachloride
NI3 - nitrogen triiodide
Writing the Formula from the NameYou can write the formula for a covalent compound from its name by writing the symbols for the first and second element and translating the prefixes into subscripts. For example, xenon hexafluoride would be written XF6. It is common for students to confuse ionic compounds and covalent compounds and then have trouble trying to write formulae from the compounds names. You aren't balancing charges of covalent compounds; if the compound does not contain a metal, don't try to balance this!
Memorizing Common Names of Covalent CompoundsSome covalent compounds have common names that you may wish to memorize. For example, H2O is called 'water' and not 'dihydrogen monoxide'. Here is a list of important common names:
H2O - water
O3 - ozone
NH3 - ammonia
NO - nitric oxide
NO2 - nitrous oxide
Examples of CompoundsExamples of Ionic Bonds - Ionic Compounds
Examples of Covalent Bonds - Covalent Compounds
Examples of Compounds with Mixed Bonds
Molecular Compound Prefixes