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