Previous Page: Covalent Substances
Covalent compounds are groups of atoms held together by covalent (shared electron) bonds. They come, very roughly speaking, in two sizes; smallish-to-largish and very, very large.
The very, very large ones include the diamond and graphite forms of carbon, and silicon dioxide as well as most polymers. In all these cases any atom in the compound is joined to several other atoms in a network. The main feature of this network is that it goes on and on and on. A crystal of sugar or salt dissolves in water because it is made up of many very small particles which the water molecules are able to separate from each other. A crystal of sand or diamond does not dissolve because it is essentially ONE particle. On an atomic scale it is HUGE. One 'particle' is composed of billions upon billions of atoms. Such compounds are called covalent networks.
The smallish-to-largish compounds are what we properly call molecules and are the only things we 'properly' call molecules.
While the networks have billions upon billions of atoms, molecules may have as few as two or as many as a few hundred. The main difference is that the atoms in a molecule can be precisely numbered. Examples of covalent networks are graphite, diamond and silicon carbide.
Methane has a formula of CH4. One methane molecule (one 'piece' of methane) has EXACTLY one carbon atom and four hydrogen atoms. Silicon dioxide (quartz) has the formula SiO2, but this only tells us that the ratio of silicon to oxygen is 1:2. It says absolutely nothing about the actual number of atoms present in the grain (one 'piece' of silicon dioxide). To find that out you'd have to weigh it.
Cholesterol, a steroid derivative, has the formula C27H46O. That's 74 atoms per molecule. Cholesterol is a larger than average molecule, but it's still a molecule for two reasons. First, you can designate the actual number of atoms in the smallest particle - 74. Second, the atoms are non-metals, and therefore the bonding is covalent. The first reason sets molecules apart from covalent networks. The second reason sets them apart from metallic and ionic substances.