Van der Waals, metallic, covalent and ionic radii

Van der Waals, metallic, covalent and ionic radii for the s-, p- and first row cZ-block elements... 

A number of useful properties of the Group 1 elements (alkali metals) are given in Table 8. They include ionization potentials and electron affinities Pauling, Allred-Rochow and Allen electronegativities ionic, covalent and van der Waals radii v steric parameters and polarizabilities. It should be noted that the ionic radii, ri, are a linear function of the molar volumes, Vm, and the a values. If they are used as parameters, they cannot distinguish between polarizability and ionic size. 

Chemical structures may be well explained and predicted by means of a set of differently applied constant atomic radii roughly constant for the many cases of a few main types of bond. Thus Pauling (1) used covalent, metallic, van der Waals and ionic radii, aU Structural Radii r chosen to add up to observed Structural Distances D where identical atoms are nearest neighbours D = 2r. But in ionic crystals identical ions are never nearest neighbours hence a special problem arises which forty years ago seemed to have been definitively solved by Goldschmidt (2) and Pauling (/). 

The data include metallic radii, ionic radii, covalent radii, and van der Waals radii. Although the various types of radii cannot be directly compared, the figure does illustrate the periodic trends in sizes. 

It is valuable to be able to predict the internuclear distance of atoms within and between molecules, and so there has been much work done in attempting to set up tables of atomic radii such that the sum of two will reproduce the internuclear distances. Unfortunately there has been a proliferation of these tables and a bewildering array of terms including bonded, nonbonded, ionic, covalent, metallic, and van der Waals radii, as well as the vague term atomic radii. This plethora of radii is a reflection of the necessity of specifying what is being measured by an atomic radius. Nevertheless, it is possible to simplify the treatment of atomic radii without causing unwarranted errors. 

Atomic radius may be calculated self-consistently or it may be determined from experimental structural data. Effective size of an atom varies as a function of its environment and nature of chemical bonding. Several different scales - covalent, ionic, metallic, and Van der Waals radii - are commonly used in crystallography. 

From the early days of structural chemistry there has been considerable interest in discussing bond lengths in terms of radii assigned to the elements, and it has become customary to do this in terms of three sets of radii, applicable to metallic, ionic, and covalent crystals. Distances between non-bonded atoms have been compared with sums of van der Waals radii , assumed to be close to ionic radii. The earliest covalent radii for non-metals were taken as one-half of the M—M distances in molecules or crystals in which M forms % — N bonds N being the number of the Periodic Group), that is, from molecules such as F2, HO-OH, H2N--NH2, P4, Sg, and the crystalline elements of Group IV with the diamond structure. This accounts... 

---