In particular, the proportions of the volumes of the atoms which are occupied by the hard incompressible ionic cores assume importance. Though the compound-forming characteristics of beryllium are largely dictated by its small atomic diameter, other factors such as the nature of the bonding which can be exerted by the partner atoms are also significant. The structural characteristics, occurrence, and the stabilities of these phases, and of the others mentioned above, are discussed in terms of atomic radius ratios, the position of the M component in the periodic table, and the value of the univalent ionic radius of the M component, taken as a measure of the extension in space of the hard incompressible ionic core. The atomic radius of a chemical element is a measure of the distance out to which the electron cloud extends from the nucleus. It must be noted, atoms lack a well-defined outer boundary. In the CaCu 5 structure adopted at composition MBe 5, dsub(M-Be) < (rsub(M) + rsub(Be)) and interactions between unlike atoms are significant. The atomic radius of Beryllium atom is 96pm (covalent radius). In the ThMn 12 and NaZn 13 structures adopted in the former case dsub(M-Be) > (rsub(M) + rsub(Be)), (where the atomic radii refer to coordination number 12) and the dsub(Be-Be) distances are contracted.
Using published information it is shown that the structures adopted at the higher Be:M ratios involve different characteristics from those adopted at lower Be:M ratios. At ordinary temperatures, beryllium resists oxidation in air, although its ability to scratch glass is probably due to the formation of a thin layer of the oxide. The results of a survey of the factors affecting the formation of phases of stoichiometry MBe 5, M 2Be 17, MBe 12 and MBe 13 are presented. It has a high permeability to X-rays and when bombarded by alpha particles, as from radium or polonium, neutrons are produced in the amount of about 30 neutrons/million alpha particles.
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