Atomic refraction, additivity

The molecular refraction rmol is an additive function, i.e. it is a function which is equal to the sum of the atomic refraction of carbon, rCi and that of hydrogen, rn, each multiplied by its number of atoms in the average" molecule ... 

The refraction of molecules may be calculated by the addition of the appropriate atomic refractions. It is necessary to add to the sum so obtained additional quantities corresponding to the multiplicity of the bonds involved. For a large number of molecules, the calculated and measured values of the molecular refraction agree, within the limits of experimental error. In molecules containing a conjugated system of double bonds, however, deviations from the additivity rule occur, the experimental value always being greater than the calculated. The difference, may... 

The measure of polarizability was provided by the abundant data on molecular, ionic and atomic refractions. Refractions in chemistry seem to be of historical importance today. Their long use and the sophisticated level they reached in chemistry provide important hints. Refraction has been recognized as a molecular volume, thus discussion of softness may be reduced to the level of molecular or atomic dimensions, as indeed it was later proposed (Sect. 1.2). Refractions have also been known to be additive, atomic refractions are transferable between molecules. This was but the first indication that atomic softness that should roughly parallel refractions (polarizability), may also be defined for bonded atoms. Examples of atomic and ionic refractions are given in Tables 1 and 2. The increasing order of refraction indeed reflects the expected hardness sequence, the borderline between soft and hard species can be drawn at ca. 2-3 cm /mol. 

The comparison of the covalent and ionic refractions of the same elements shows that the addition or removal of the valence electrons influences the atomic refraction most. The changes of atomic refractions due to bond ionicity in molecules and crystals have been calculated as... 

The molecular refraction is an additive function of atomic refractions. Fluorine has the lowest atomic refraction (Table 13.1) of the atoms used to build compounds or polymers for coating fiber surfaces. Consequently, the refractive index and polarizability of fluorocarbons are lower than those of the corresponding hydrocarbons and chlorocarbons. Because dispersion forces constitute the major part of their cohesive forces, their surface energy is low. However, a low-surface energy is not the sole criterion for an effective soil retardant. The dispersion forces are effective only over short distances. A finish deformable by soil particles provides a larger area in close contact with soil than a finish that does not flow on impact. Hence, hardness of finish is also important. Soiling can also be affected by a liquid film on fibers, such as oil or fat, which can fill the voids between the surfaces of soil and fiber and act as an adhesive. 

The molecular refraction is a constant frequently quoted for individual chemical compounds, and is of considerable value as evidence of constitution, since it is generally true that the molecular refraction of a compound is composed additively of the refractive powers of the atoms contained in the-mmolecular refraction is the value obtained by multiplying the refractive power by the molecular weight. 

In all cases the dielectric constant used is that of the pure solvent. Neglect of the solute is usually justified by its low concentration and the assumption that any necessary correction would be additive. In at least a few cases where the first two expressions have been employed the linearity of the results is to some extent dependent on how closely the refractive index of the solute meets the conditions n2 = 2.0 or 2.5 a situation not always recognized by the investigators. In one instance attempts have been made to clarify the role of solvent reaction field by examining solutes with different dipole moment orientations relative to the bonds involving coupled atoms. 

Due to the atomic structure of the 4 f-shell there are narrow and sharp absorption and/or emission lines in the visible range v ch may be used in various ways. In addition the oxides and/or oxide systems in glasses provide a high index of refraction with low dispersion. 

Optical Properties. The addition of lanthanum oxide to PZT has a rather remarkable effect on the optical transparency, especially when the amount of lanthanum exceeds seven atom percent. Thin polished plates characteristically transmit about 67% of the incident light. When broadband antireflection coatings are applied to the major surfaces, this transmission is increased to greater than 98%. Surface reflection losses are a function of the index of refraction (n = 2.5) of the PLZT. 

Because of their high refractive index, these materials possess desirable optical and thermal properties, in addition to being impervious to a variety of chemicals [5]. Analogous reactions involving carbon disulfide and epoxides have provided extremely interesting examples of atom-exchange polymerization processes. 

The molar refractivity is expressed in cm3 moT1 as is V. It is fairly temperature independent and is additive in the constituent atoms of the molecules of the solvents, and some structural features, with good accuracy. The infinite frequency value of the molar refractivity is 1-2% smaller than RD. The... 

The refractive index of a compound is a property of some significance in regard to molecular constitution. The molar refraction, defined by Eq. (29-14), is a constitutive and additive property for a given compound it may be approximated by the sum of contributions of individual atoms, double bonds, aromatic rings, and other structural features. 

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