Condensed Matter: General Characteristics, the Chemical Bond, and Particle Interactions

Abstract

It is classic to distinguish three states of matter: gas, liquid, and solid. In the first approximation, the gas state is characterized by the absence of interactions between atoms or molecules, which therefore display statistical disorder. The requirement of maximum of entropy controls this state. Gases are outside the scope of this textbook. In contrast, the more condensed liquid and solid phases are controlled not only by entropy, but also by interparticle interactions. These states are stabilized by a complex interplay between attractive interactions, which are responsible for the condensation of chemical species, and repulsive interactions. The balance yields a local order, defined on some characteristic length. In crystals, this local order reproduces on distances much larger than the interparticle distances, so that one can speak of long-range order. Long-range order of crystalline materials is described in terms of the elements of the group of symmetry under which this order is invariant. Independently of the type of arrangement, all condensed matter media in liquid or solid states present some common features as follows:

• 1 The energies of interaction that stabilize the local order vary in the rang 0.1eV ÷ 10eV per atom or molecule. The order of magnitude that is frquently met is 1 eV.

• 2 The molar volumes are all of the same order of magnitude; typical distances between atoms are of the order of 1 Å.

• 3 Vibration frequencies of atomic bonds are of the order of 10¹³-10¹⁴ Hz.