3
Bonds and Interaction Forces
Atoms, groups of atoms, ions, molecules, macromolecules, and particles
always are subject to forces between them. These interaction forces may
cause chemical reactions to occur, i.e., cause the formation of other
molecular species, but they are also responsible for the existence of
condensed phases (solids and liquids), for adherence of a liquid to a solid
surface, or for aggregation of particles in a liquid. In short, all structures
form because of interaction forces. Generally, formation of a structure
causes a decrease in entropy, and this may counteract the tendency of
formation, depending on its magnitude compared to that of the energy
involved.
There are several, rather different, types of interaction forces, although
all of them are ultimately due to the electromagnetic force. This force can
thus become manifest in various ways. The interactions greatly differ in
specificity: what group or molecule will interact with what other group etc.?
For instance, they all decrease in magnitude with interparticle distance, but
the relation between energy and distance may vary widely. One generally
considers the energy needed to bring two particles (or molecules, etc.) from
infinite distance to close proximity. Since there is always more than one type
of force acting, this energyðUÞmay be negative or positive, depending on
the interparticle distanceðhÞ, for instance as depicted in Figure 3.1. The