The Building of Atoms and Molecules

Molecular vibration frequencies, force constants, bond lengths, and dissociation energies provide experimental methods for studying interatomic forces in detail. 

Van der Waals forces are also electrostatic, and, generally speaking, dipolar they exist in virtue of (o) permanent dipoles in molecules, (6) dipoles induced by other permanent dipoles and, most characteristically, (c) the coupling of the zero-point oscillations of positive and negative charge which must occur even in molecules without an observable moment. Van der Waals forces normally fall off as the inverse seventh power of the distance. 

When one atom has received electrons from another so that a pair of ions is formed, the Coulomb forces between the two obey the inverse square law. Such forces act therefore over long ranges and profoundly affect the properties of solutions of ionized substances. 

The science of mechanics provides objective criteria by which forces may be compared. Newton s laws relate force, mass, and acceleration. In a strict sense the mere formulation of the rule force = mass X acceleration is not helpful, since by it force cannot be defined except in terms of mass, nor mass except in terms of force. But the value of mechanics lies in the coherence of the system to which it leads. If masses are compared in terms of the accelerations imparted by a constant force, such as gravity at a given place (about which nothing need be postulated save its constancy), then the order found will in fact determine their relative behaviour under the infiuence of quite different forces. A very vast system of observed phenomena can be correlated in terms of the auxiliary quantities force and mass, even though a logician uninterested in calculations about the actual behaviour of matter may see little point in their introduction. 

Force interests the less sophisticated investigator because it suggests familiar sensations, but for most purposes the quantity known as the potential energy is more convenient to work with. For a particle of mass m, if a is a space coordinate and the component of force in that direction, J dx = work. 

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