Kixium monolayers: A simple alternative to the bubble raft model for close-packed spheres

Abstract

In general chemistry courses, the majority of the chemical examples are gas mixtures or aqueous solutions. However, the most prevalent state of the chemical materials encountered everyday is solid. Thus the recent emphasis on the teaching of solid-state materials in the general chemistry classroom (1).Any discussion of solid-state chemistry involves a description of structure (2). One of the basic structural themes in the solid state is that of close-packed spheres. Atoms are approximated as hard spheres that arrange themselves in such a manner as to reduce voids. The two close-packed structures, hexagonal closest packed (hcp) and cubic closest packed (ccp), and their derivatives are observed in the noble gases and many metals, and also in alloys and ionic and covalent compounds (3). Interestingly, other close packings are possible if the structures aren't periodic (4, 5).The close packing model is therefore a powerful tool for explaining many solid-state structures. To facilitate learning of the close packing model, many demonstrations of closepacking spheres have been developed (6)(7)(8)(9)(10)(11). Most of the models in the literature are static: spheres or circles are packed and stacked by hand and frozen into the familiar hexagonal lattice. Static models, though visually demonstrating the arrangement of spheres in a hexagonal array, don't explore any of the other possible ways of packing spheres. Students may wonder why the instructor has to intervene to force the coins or balls to assume the right shape. A dynamic model, one that both demonstrates the close packed structure visually and spontaneously assumes this structure, is aesthetically more satisfying.Such a dynamic demonstration was exactly what Bragg and Nye reported in 1947 with the bubble raft model. This model has recently been adapted for overhead projection (1, 12). However, the apparatus involved in making the bubble raft demonstration is complex.

Characterizing Crystalline-Vitreous Structures: From Atomically Resolved Silica to Macroscopic Bubble Rafts

“…Bubble rafts provide a 2D model for crystalline materials, simulating grain boundaries and zero-dimensional defects. They have been used previously for teaching purposes. − They have also been used to investigate the behavior of a bulk material under mechanical strain. ,− For example, in 1947, L. Bragg and J. F. Nye used bubble rafts as 2D models to illustrate intercrystalline boundaries and plastic flow in metals . Bubble rafts can be used to simulate amorphous structures as well …”

New Nanotech from an Ancient Material: Chemistry Demonstrations Involving Carbon-Based Soot

“…Objects that have a hydrophobic side and a hydrophilic side are referred to as Janus objects, after the two-faced Roman god. , Very small Janus objects are being studied for their ability to aggregate, or self-assemble, into extended structures. The basis of this assembly is that hydrophobic surfaces from different objects attract, and hydrophilic surfaces from different objects attract. − Demonstrations have been developed involving floating objects at an air–water interface that have only part of their surfaces covered with soot. In this context of superhydrophobic soot demonstrations, Janus objects that are able to roll or tumble easily at the water surface will orient themselves so that their superhydrophobic surface has as little contact with the water as possible.…”

Student Learning about Biomolecular Self-Assembly Using Two Different External Representations

“…Among these are LEGO bricks with magnets attached to them to construct systems that self-assemble in various ways (Campbell et al ., 2001; Jones et al ., 2006). Other described self-assembly models employ capillary forces between objects such as soda straws (Campbell et al ., 2002) or breakfast cereals (Dungey, 2000) on the surface of a liquid to accomplish self-assembly of extended structures. The close packing of atoms in metals has been represented using soap bubbles on a water surface (Geselbracht et al ., 1994).…”