Atomic size covalent radius

To accommodate this problem, scientists have come up with several approaches to measuring atomic sizes. A common one is called the covalent radius, which is half the distance between the nuclei of two identical atoms. This technique works well for atoms such as hydrogen or oxygen, both of which readily pair up to form and O2. But how would one determine the covalent radius of a noble gas, which exists only as single atoms ... 

Note that the N3- ion (radius 171 pm) is much larger than the nitrogen atom, for which the covalent radius is only 71 pm. The oxygen atom (radius 72 pm) is approximately half the size of the oxide ion... 

The size of an atom is not a simple concept. An inspection of the wave function for any atom shows that it is asymptotic to infinity, so some practical definition of size is required. There are two ways of assigning sizes to atoms atomic radius and covalent radius. 

The electron cloud around an atomic nucleus makes the concept of atomic size somewhat imprecise, but it is useful to refer to an atomic radius. One can arbitrarily divide the distance between centers of two bonded atoms to arrive at two radii, based on the crude picture that two bonded atoms are spheres in contact. If the bonding is covalent, the radius is called a covalent radius (see Table 8-2) if it is ionic, the radius is an ionic radius (see Table 9-2). The radius for non-bonded atoms may be defined in terms of the distance of closest non-bonding approach such a measure is called the van der Waals radius. These three concepts of size are illustrated in Figure 7-2. 

Atomic orbitals have a disadvantage in that they are diffuse. In solids, large molecules or clusters that are the size of the orbitals are compressed due to the interaction with the neighbors. A measure for the distance between neighbors is given by the so-called covalent radius, r0, and is empirically determined for all atoms. Therefore, it is wise to use orbitals that somehow incorporate this information. To enhance this effect, an additional harmonic potential is added to the atomic Kohn-Sham equations that leads to compressed atomic orbitals, or optimized atomic orbitals (O-LCAO) ... 

In the scale of Allred and Rochow, electronegativity is measured as the electrostatic force exerted at the covalent radius (rcov) of the atom [14,15]. This electronegativity can be written as a function of the screened nuclear charge (Z ) and the atomic size as ... 

We have already remarked that the concept of characteristic covalent radii is subject to a number of qualifications. The first qualification which we would make is that the covalent radius of an atom (unlike its ionic radius) must not be interpreted as implying that the atom is a sphere of that size. The covalent radii are applicable only to... 

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. 

Section 8.4 atomic size (249) metallic radius (249) covalent radius (249) ionization energy (IE) (252) electron affinity (EA) (255)... 

Another radius that can be assigned to an atom is the covalent radius, which relates to the sizes of chemical bonds formed by the atom. For example, in the hydrogen molecule, H2, the atoms are 0.074 nm (0.74 A) apart each hydrogen atom is said to have a covalent radius of 7 x 0.074 = 0.037 nm. The van der Waals and covalent radii are shown in Figure 1.10 for the hydrogen and chlorine molecules. With the use of the two radii, the effective volumes of the molecules can be calculated easily. 

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