Nomenclature for Covalent or Molecular Compounds

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.

Molecular compounds contain two or more nonmetals (not the ammonium ion). Usually, you can recognize a molecular compound because the first element in the compound name is a nonmetal. Some molecular compounds contain hydrogen, however, 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.

Certain 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.

Nonmetals 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.

SO₂ - sulfur dioxide
SF₆ - sulfur hexafluoride
CCl₄ - carbon tetrachloride
NI₃ - nitrogen triiodide

You can write the formula for a covalent compound from its name by writing the symbols for the first and second elements and translating the prefixes into subscripts. For example, xenon hexafluoride would be written XF₆. It is common for students to have trouble writing formulae from compounds names as ionic compounds and covalent compounds are often confused. You aren't balancing charges of covalent compounds; if the compound does not contain a metal, don't try to balance this!

Molecular Compound Prefixes