Atomic radius/radii nonbonding

To a first approximation, atoms in molecules may be regarded as hard spheres with a segment cut off in the bonding direction, as in the familiar space-filling models. The radius of the atom in a nonbonding direction is called the van der Waals radius. Half the distance between two atoms of the same kind in adjacent molecules at equilibrium is taken as the van der Waals radius (Figure 5.1). In assigning a fixed radius in this way, we assume that atoms... 

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 Wauls 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. 

The electron cloud around an atom makes the concept of atomic size somewhat imprecise. Even so, it is useful to refer to an atomic size or an atomic radius. Operationally, one can divide the experimentally determined distance between the centers of two chemically bonded atoms to arrive at the two atomic radii. If the bonding is covalent (see Chapter 9), the radius is called a covalent radius if the bonding is ionic, the radius is an ionic radius. The radius for a nonbonded situation may also be defined in terms of the distance of closest nonbonding approach and is called a van der Waals radius. These concepts of size are illustrated in Fig. 8-6. 

Table 12-3). It should be noted that the van der Waals radius is the maximum nonbonded radius of an atom, the distance between the nucleus and the effective outside of the atom at a point directly opposite the site of bonding. It is the radius of the atom as set off by a line forming an angle of 180 degrees with the bond direction the atomic radius set off by a line through the nucleus at any other angle must be greater than the covalent radius but less than the van der Waals radius. This may be seen from Figure 9-1, which shows two of the C—Br bonds in a compound...

Covalent and van der Waals radii are other fundamental properties of atoms in molecules that are influenced by nuclear charge and electron distribution. A glance at a molecular model or graphic suggests that most atoms have several different dimensions. There is the distance between each bound atom and also a dimension in any direction in which the atom in not bonded to another atom. The former distance, divided between the two bonded atoms, is called the covalent radius. The nonbonded dimension of an atom or group in a molecule is called the van der Waals radius. This is the distance at which nonbonded atoms begin to experience mutual repulsion. Just short of this distance, the interatomic forces are weakly attractive and are referred to as dispersion or London forces and are attributed to mutual polarization of atoms. 

Table 10.3 shows that the van der Waals b parameter has units of L/mol. This means that we can calculate the sizes of atoms or molecules from the b parameter. Refer back to the discussion in Section 7.3. Is the van der Waals radius we calculate from the b parameter of Table 10.3 more closely assodaled with the bonding or nonbonding atomic radius discussed there Explain. 

A Figure 7.4 Illustration of the distinction between nonbonding and bonding atomic radius. Values of bonding atomic radii are obtained from measurements of interatomic distances in chemical compounds. 

In previous chapters, we saw that the volume of an atom is taken up primarily by its electrons (Chapter 2) occupying quantum-mechanical orbitals (Chapter 7). We also saw that these orbitals do not have a definite boundary but represent only a statistical probability distribution for where the electron is found. So how do we define the size of an atom One way to define atomic radii is to consider the distance between nonbonding atoms that are in direct contact. For example, krypton can be frozen into a solid in which the krypton atoms are touching each other but are not bonded together. The distance between the centers of adjacent krypton atoms—which can be determined from the solid s density—is then twice the radius of a krypton atom. An atomic radius determined in this way is called the nonbonding atomic radius or the van der Waals radius. The van der Waals radius represents the radius of an atom when it is not bonded to another atom. 

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