Nonmetal covalent radius

The value of the effective van der Waals radius of an atom in a crystal depends on the strength of the attractive forces holding the molecules together, and also on the orientation of the contact relative to the covalent bond or bonds formed by the atom (as discussed below) it is accordingly much more variable than the corresponding covalent radius. In Table 7-20 there are given the ionic radii of nonmet llic elements for use as van der Waals radii. They have been rounded off... 

Figure 8.8 shows two common definitions of atomic size. The metallic radius is one-half the distance between nuclei of adjacent atoms in a crystal of the element we typically use this definition for metals. For elements commonly occurring as molecules, mostly nonmetals, we define atomic size by the covalent radius, one-half the distance between nuclei of identical covalently bonded atoms. 

Also, the ionic radii change with the oxidation state of the ion, with the increase of the oxidation state leading to a decrease in ionic radius. Figures 2.8,2.9, and 2.10 compare the atomic radius, ionic radius, and the covalent radius for periods 1 and 2, and period 3 and period 4 elements, respectively (Brezeanu et al. 1990 Whitten et al. 1988 Housecroft and Constable 1997). It can be seen from these figures that covalent radii follow the same general trend as the ionic radii they have even smaller values than the ionic radii for metals and higher values for the nonmetals. 

Covalent radius. Used for elements occurring as molecules, mostly nonmetals, it is one-half the shortest distance between nuclei of bouded atoms (Figure 8.7B). 

Atomic radius is the distance between an atom s nucleus and its valence shell. The atomic radius of a metal atom is defined as the metallic radius, which is one-half the distance between adjacent, identical nuclei in a metal solid. The atomic radius of a nonmetal is defined as the covalent radius, which is one-half the distance between adjacent, identical nuclei in a molecule. In general, atomic radii decrease from left to right across a period of the periodic table and increase fiwm top to bottom down a group. 

Another way to define the size of an atom, called the bonding atomic radius or covalent radius, is defined differently for nonmetals and metals, as follows ... 

Atomic radius refers to metallic radius for metals and covalent radius for nonmetals. Ionization energies refer to first ionization energy. Metallic character relates generally to the ability to lose electrons, and nonmetallic character to the ability to gain electrons. 

For an interstitial alloy to form, the solute atoms must have a much smaller bonding atomic radius than the solvent atoms. Typically, the interstitial element is a nonmetal that makes covalent bonds to the neighboring metal atoms. The presence of the extra bonds provided by the interstitial component causes the metal lattice to become harder, stronger, and less ductile. For example, steel, which is much harder and stronger than pure iron, is an alloy of iron that contains up to 3% carbon. Other elements may be added to form alloy steels. Vanadium and chromium may be added to impart strength, for instance, and to increase resistance to fatigue and corrosion. 

In principle, the vdW radius of an atom must depend on its effective charge. However, Pauling [82] found that vdW radius of a nonmetal coincide with its anionic radius, inasmuch as the bonded atom presents the same face to the outside world in directions away from its bond as the ion does in aU direction and used this rule to suggest vdW radii for some elements. He also noticed that vdW radii exceed the covalent radii (r) of the same elements by S 0.8 A Bondi used this rule in constructing his system of radii (he assumed S = 0.76 A), as well as some other physical properties (e.g. critical volumes). This value can be deduced from covalent radii using the equations... 

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