A covalent bond is a bond in which two atoms share electrons. The shared electrons have the effect of gluing two magnets together. The glue turns the two magnets into one molecule. Substances that consist of discrete molecules, on the other hand, do not have covalent bonds. However, bonding still does occur between these molecules. Several types of intermolecular forces allow discrete molecules to bond with each other as many small magnets would, with no glue needed.
Intermolecular hydrogen bonding is the attraction between two separate molecules. Each molecule must have a hydrogen atom that is covalently bonded to another atom that is more electronegative. The atom that is more electronegative than hydrogen will tend to pull the shared electrons in their covalent bond toward itself, away from hydrogen. Electrons have negative charges. This results in a momentary slightly positive charge on the hydrogen atom and a momentary slightly negative charge on the more electronegative atom. These two slight charges turn each discrete molecule into a weak “mini-magnet.” Many mini-magnets, like the water molecules (H2O) in a cup of water, give a substance a slightly sticky property.
London Dispersion Forces
London dispersion forces fall within the category of what are called Van der Waals forces. Nonpolar molecules are molecules that do not have an actual electrical charge or do not have highly electronegative atoms. However, nonpolar molecules can have momentary slightly negative charges. The reason is that the electrons surrounding the atoms that make up each molecule do not stay in one place, but can move around. So if many of the electrons, which have negative charges, happen to be near one end of the molecule, then the molecule now has a slightly -- but momentarily -- negative end. At the same time, the other end will be momentarily slightly positive. This behavior of electrons can give a nonpolar substance, such as long hydrocarbon chains, a stickiness that makes them harder to boil. Indeed, the larger the hydrocarbon chain, the more heat is required to boil it.
Dipole-dipole interactions are another type of Van der Waals force. In this case, a molecule has a highly electronegative atom attached at one end and nonpolar molecules on the other end. Chloroethane is an example (CH3CH2Cl). The chlorine atom (Cl) is covalently bound to a carbon atom, meaning they share electrons. Since chlorine is more electronegative than carbon, chlorine attracts the shared electrons better and has a slightly negative charge. The slightly negative chlorine atom is referred to as one pole and the slightly positive carbon atom is another pole -- like the north and south poles of a magnet. In this way, two more discrete molecules of chloroethane can bond with each other.
Organic salts such as calcium phosphate (Ca3(PO4)2) are insoluble, meaning they form a solid precipitate. The calcium (Ca++) ions and the phosphate ions (PO4---) are not covalently linked, meaning they do not share electrons. However, the two ions form a solid network because they have full, not partial, electrical charges. The calcium ion is positively charged and the phosphate ion is negatively charged. Though the calcium ion is an atom, the phosphate ion is a molecule. Thus, ionic bonding is a type of bonding that happens in a substance that consists of discrete molecules.
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