Course Outline

General Chemistry

04/25/09

Molecular Compounds

Description:

Molecular compounds are composed of molecules in which atoms share electrons via bonds. The chemistry of molecular compounds is varied and complicated, far more so than ionic compounds. For this reason (plus some tradition and a pinch of inertia), most of the content of a general chemistry course involves ionic compounds. See the bond theory section of the Chemistry WebBook for more information about molecular compounds.

Identification:

Most molecular compounds contain two or more non-metals (but not the ammonium ion). While there are exceptions, the absence of metals signals the beginning chemistry student that the compound is molecular. While many molecular compounds contain hydrogen, if the formula of a compound has hydrogen written first, assume it is an acid rather than a molecular compound.

Nomenclature:

The only type of molecular compound a beginning student is expected to name is the binary molecular compound. These contain two non-metallic elements (but not carbon with hydrogen; hydrocarbons have their own special nomenclature). Many binary molecular compounds have common names. For example, H₂O is water and NH₃ is ammonia. You just have to memorize these. There is also an older system of naming binary compounds that contain oxygen. For example, N₂O is nitrous oxide and NO is nitric oxide. You may need to memorize a list of these compounds early in the course, but that depends on your instructor.

You will be expected to know how to binary molecular compounds. Unlike ionic compounds, the elements in a molecular can combine with a variety of ratios. We use prefixes to indicate the number of atoms of each element.

To name the compound using the formula, name the first element then the second element giving it an –ide ending. For example, the compound PCl₃ is phosphorus trichloride, and the compound SO₂ is sulfur dioxide.

Formulae:

A student can write the formula of a binary molecular compound from its name by translating the prefixes into subscripts. Given the name, write the symbol for the first element followed by the symbol for the second element. Translate the prefixes into subscripts. For example, SeCl₂ is selenium dichloride and PF₅ is phosphorus pentafluoride.

!Warning! It is very common for beginning chemistry students to mix up the system for naming ionic compounds with the system for naming binary molecular compounds. Some students write the formula for binary molecular compound then attempt to make the charges balance. They then get frustrated because both elements seem to have negative charges and they can’t get the compound neutral! Another common mistake is to become fond of the prefixes used for binary molecular compounds and use that system with ionic compounds. Some of the polyatomic ions have names that seem similar to these prefixes. Chromate ion is CrO₄^-2 and dichromate ion is Cr₂O₇^2-. Sn(CrO₄)₂ is tin(IV) chromate NOT tin dichromate. There are two tin dichromates: tin(II) dichromate is SnCr₂O₇, and tin(IV) dichromate is Sn(Cr₂O₇)₂.

Concept Check: What is the name of the compound that has the formula XeF6? What is the formula for sulfur dioxide?

Answer: XeF6 is xenon hexafluoride. Sulfur dioxide has the formula SO₂.

© Copyright 2006, Kelley Whitley, ChemProfessor. All rights reserved.

This site was last updated 08/02/06