CLUSTER

Atoms and molecules are the smallest forms of matter typically encountered under normal conditions and are in that sense the basic building blocks of the material world. There are phenomena, such as lightning and electric discharges of other kinds, that allow free electrons to be observed, but these are exceptional occurrences. It is of course in its gaseous state that matter is encountered at its atomic or molecular level; in gases each molecule is an independent entity, only occasionally and briefly colliding with another molecule or with a confining wall. In contrast to the free-molecule character of gases, the condensed phases of matteras liquids, crystalline solids, and glasses are calleddepend for their properties on the constant proximity of all their constituent atoms. The extent to which the identities of the molecular constituents are maintained varies widely in these condensed forms of matter. Weakly bound solids, such as solid carbon dioxide (dry ice), or their liquid counterparts, such as liquid air or liquid nitrogen, are made up of molecules whose properties differ only slightly from the properties of the same molecules in gaseous form; such solids or liquids are simply molecules packed tightly enough to be in constant contact. These are called van der Waals solids or liquids, after Johannes D. van der Waals, the Dutch physicist who described the weak forces that just manage to hold these materials together if they are cold enough. In other solids, like diamond, graphite, silicon, or quartz, the individual atoms retain their identity, but there are no identifiable molecules in their structures. The forces between the constituent atoms are roughly as strong as the forces that hold atoms together in the strongly bound covalent molecules that make up most common substances. Negatively charged electrons act as a glue to hold the positively charged nuclei together and are more or less confined to the vicinity of the so-called home-base nuclei with which they are associated; they are not free to roam through the entire solid or liquid. These materials are said to be covalently bound and are electrical insulators. They are best described as neutral atoms held together by covalent bonds and are essentially one giant molecule. Another kind of bonding found in condensed matter is exhibited by sodium chloride, ordinary table salt, which is composed of positive sodium ions (Na+) and negative chloride ions (Cl-). Such ionic compounds are held together by the mutual attraction of the oppositely charged ions; because of their locations, these attractions are stronger than the repulsions of the ions with like charges. Each ion in an ionic crystal is surrounded by nearest neighbours of opposite charge. The consequence is that the binding energies of ionic compounds are large, comparable to those of strongly bound covalent substances. Metallic bonding is another type of binding found in condensed matter. Electrons moving between the positive atomic cores (i.e., the nuclei plus inner-shell, tightly bound electrons) form an electron cloud; the attractions between the positive cores and the negative charges that make up the cloud hold metals together. Metals differ from covalently bound insulators in that those electrons responsible for the cohesion of the metals move freely throughout the metal when given the slightest extra energy. For example, under the influence of the electric field produced in a copper wire when its ends are connected to the terminals of a battery, electrons move through the wire from the end connected to the battery's negative pole toward the end connected to its positive pole. An electric field applied to a metal generates an electric current, but the same electric field applied to a covalent insulator does not (see below Comparison with other forms of matter). The net binding forces between electrons and atomic cores of a metal are comparable in strength to those that hold ionic compounds together. As mentioned above, liquids constitute a condensed or dense phase of matter, but their atomic arrangement differs from that of solids. In a liquid the constituent atoms are only slightly farther apart than they are in a solid, but that small difference is significant enough to allow the atoms or molecules that constitute a liquid to move around and to assume a full range of geometric configurations. Atoms of the same kind can trade places and can wander through the liquid by the random-walk process called diffusion. In general, materials that can form solids can also form liquids, but some, such as carbon dioxide, can only enter the liquid state under excess pressure. At least one substance, helium, can form a liquid while having no known solid form. Materials that form solids and liquids can exhibit another form, one that may be solidlike or liquidlike but that has properties somewhat different from those of the bulk. This is the form of matter consisting of exceedingly small particles that are called clusters. Clusters are aggregates of atoms, molecules, or ions that adhere together under forces like those that bind the atoms, ions, or molecules of bulk matter; because of the manner in which they are prepared, clusters remain as tiny particles at least during the course of an experiment. There are clusters held together by van der Waals forces, by ionic forces, by covalent bonds, and by metallic bonds. Despite the similarity of the forces that bind both clusters and bulk matter, one of the fascinating aspects of clusters is that their properties differ from those of the corresponding bulk material; that characteristic affords the opportunity to learn about the properties of bulk matter by studying how, as the number of constituent particles increases, the properties of clusters evolve into those of bulk matter. For example, a cluster of 20 or 30 atoms typically has a melting point far lower than that of the corresponding bulk. The electrical properties of clusters also differ in some instances from those of the bulk matter: clusters of only a few atoms of mercury are insulators, held together by weak van der Waals forces, but clusters of hundreds of mercury atoms are metallic. One of the puzzles posed by clusters is the question of how properties of small clusters evolve with size into properties of bulk matter. Additional reading Michael A. Duncan and Dennis H. Roubray, Microclusters, Scientific American, 261(6):110115 (December 1989), provides a general introduction and survey for nonscientists. Works presenting the results of recent research include R. Stephen Berry, When the Melting and Freezing Points Are Not the Same, Scientific American, 263(2):6872, 74 (August 1990), written for nonscientists, a description of the melting and freezing of clusters and their relation to bulk melting and freezing; and several collections of conference proceedings: P. Jena, B.K. Rao, and S.N. Khanna (eds.), Physics and Chemistry of Small Clusters (1987), covering a wide variety of topics within cluster science; S. Sugano, Y. Nishina, and S. Ohnishi (eds.), Microclusters (1987); G. Scoles (ed.), The Chemical Physics of Atomic and Molecular Clusters (1990), at the graduate-student level; S. Sugano, Microcluster Physics (1991), accessible to scientifically literate readers; and Zeitschrift fr Physik, part D, vol. 19 and 20 (1991) and vol. 26 (1993), the proceedings of the international conferences on small particles and inorganic clusters held in 1990 and 1992, respectively. R. Stephen Berry