Carbon ionic crystal radius

Figure 7.3 plots the ratio of crystal radius versus charge for selected ions. Oxyanions—sulfate, selenate, phosphate, arsenate, borate, molybdate, carbonate, and silicate—are represented by their central cations S6+, Se6+, P5+, As5+, B3+, Mo4+, C4+, and Si4+. The ions fall into three behavioral groups. Ions of high ionic potential, the alkali and alkaline earth cations and the halide anions, large univalent and divalent ions, are highly water soluble, easily weatherable, and leach readily from soils to the sea over geologic time. 

It should not be inferred that the crystal structures described so far apply to only binary compounds. Either the cation or anion may be a polyatomic species. For example, many ammonium compounds have crystal structures that are identical to those of the corresponding rubidium or potassium compounds because the radius NH4+ ion (148 pm) is similar to that of K+ (133 pm) or Rb+ (148 pm). Both NO j and CO, have ionic radii (189 and 185 pm, respectively) that are very close to that of Cl- (181 pm), so many nitrates and carbonates have structures identical to the corresponding chloride compounds. Keep in mind that the structures shown so far are general types that are not necessarily restricted to binary compounds or the compounds from which they are named. 

The above speculation [21] may be extended to include the related quaternary ammonium compounds such as xylocholine (XXXIX). It is probable that the volumes of the guanidinium ion and the trimethylammonium group are similar. The ionic radius of the guanidinium ion (IX) is about 3A the ionic radius of the tetramethylammonium ion has been estimated [300] to be 3-4A, although rather smaller values have also been proposed [301-303]. Crystallographic analyses of muscarine iodide [304], choline chloride [305] and acetylcholine bromide [306] have revealed that the carbon to nitrogen distance is about l-SA, and that a hydrogen bond (C-H-0 distance 2-87-3 07A) exists in the crystals of these compounds. 

The ionic radius of cesium (1.69 A) is near that of benzene (1.39 A from center to the carbon atoms or 2.47 A to the hydrogen atoms) and the cyclohexa-l,3-dienyI radical hence extensive electrostatic interaction is anticipated between the corresponding anions on intimate contact with cesium cations. Such similarity in size may facilitate close packing in the crystal lattice cesium benzenide appears to be very insoluble in tetrahy-drofuran. 

Calcium, strontium and barium each have slightly different impacts on the properties of the perovskite. This is due, in part, to the difference in ionic radii of the ions. Calcium has a radius of 1.36 A, whereas the radii of strontium and barium are 1.44 and 1.65, respectively. Thus barium-containing perovskites have a more open crystal lattice and thus have a higher ionic conductivity compared to strontium- and calcium-based materials. However, barium is more reactive towards C02-containing gas mixtures and forms barium carbonate very readily. 

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