What properties distinguish molecular compounds from other materials?

How a given chemical can be shown to be a covalent molecular compound, given either a sufficient description of its properties or its elemental composition?
Jackie Smith

A covalent molecular compound consists of individual molecules that contain only covalent bonds. The covalent bonds within the molecules are very strong and highly directional, so the molecules usually have definite shapes and retain their identities during physical changes. The forces between the molecules are by comparison very weak. It's these weak intermolecular forces that determine many of the properties of covalent molecular compounds.

Elemental Composition of Covalent Compounds. Compounds that contain both metals and nonmetals are usually ionic, not covalent. So KBr and Na₂SO₄ are easily recognized as ionic compounds; CO₂ and CH₄ are covalent. You have to watch for a few exceptions when using this rule of thumb. NH₄Cl is an ionic compound, not a covalent one, because the NH₄⁺ (ammonium) ion is combined with a chloride ion.

Properties of Covalent Molecular Compounds.

• Low melting points and boiling points. A relatively small amount of energy is required to overcome the weak attractions between covalent molecules, so these compounds melt and boil at much lower temperatures than metallic and ionic compounds do. In fact, many compounds in this class are liquids or gases at room temperature.
• Low enthalpies of fusion and vaporization These properties are usually one or two orders of magnitude smaller than they are for ionic compounds.
• Soft or brittle solid forms. The weak intermolecular forces makes the solid form of covalent molecular compounds easy to distort or break.
• Poor electrical and thermal conductivity. Ionic compounds conduct electricty well when melted; metallic solids do as well. Covalent molecular compounds do not.

Author: Fred Senese senese@antoine.frostburg.edu

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/solids/faq/properties-of-molecular-compounds.shtml