Size, atomic ionic

The large size of the unit of structure and the complexity of the chemical formula make the deduction of the atomic arrangement from X-ray data alone impractical if not impossible. We consequently make use also of arguments based on analogy with other structures, semi-empirical structural rules regarding ionic sizes and ionic environments, etc., with ultimate recourse to the stochastic1) method, which has already... 

In contrast to the ionic complexes of sodium, potassium, calcium, magnesium, barium, and cadmium, the ease with which transition metal complexes are formed (high constant of complex formation) can partly be attributed to the suitably sized atomic radii of the corresponding metals. Incorporated into the space provided by the comparatively rigid phthalocyanine ring, these metals fit best. An unfavorable volume ratio between the space within the phthalocyanine ring and the inserted metal, as is the case with the manganese complex, results in a low complex stability. 

The alkali metals share many common features, yet differences in size, atomic number, ionization potential, and solvation energy leads to each element maintaining individual chemical characteristics. Among K, Na, and Li compounds, potassium compounds are more ionic and more nucleophilic. Potassium ions form loose or solvent-separated ion pairs with counteranions in polar solvents. Large potassium cations tend to stabilize delocalized (soft) anions in transition states. In contrast, lithium compounds are more covalent, more soluble in nonpolar solvents, usually existing as aggregates (tetramers and hexamers) in the form of tight ion pairs. Small lithium cations stabilize localized (hard) counteranions (see Lithium and lithium compounds). Sodium chemistry is intermediate between that of potassium and lithium (see Sodium and sodium alloys). 

It is, of course, impossible to measure the absolute size of an isolated atom its electron cloud extends to infinity. It is possible to calculate the radius within which (say) 95% of its total electron cloud is confined but most measures of atomic/ionic size are based upon experimental measurements of internuclear distances in molecules and crystals. This means that the measurement is dependent on the nature of the bonding in the species concerned, and is a property of the atom or ion under scrutiny in a particular substance or group of substances. This must always be borne in mind in making use of tabulated radii of atoms or ions. The most important dictum to remember is that radii are significant only insofar as they reproduce experimental internuclear distances when added together. The absolute significance of a radius is highly suspect,... 

I Be sure to distinguish between atomic size and ionic size. 

Like the atomic size, the ionic radius varies a little with the way the ions are arranged in the crystal. The figures given in Table 9 are for 6 co-ordination, as in the NaCl structure, where each ion is surrounded octahedrally by 6 others of opposite charge (Fig. 30). 

Carry out calculations relating atomic arrangement, density, unit cell size, and ionic or... 

Ionic Size vs. Atomic Size The ionic radius is an estimate of the size of an ion in a crystalline ionic compound. You can picture it as one ion s portion of the distance between the nuclei of neighboring ions in the solid (Figure 8.21). From the relation between effective nuclear charge and atomic size, we can predict the size of an ion relative to its parent atom ... 

In the construction of aquaphilic migration forms can participate practically all elements of the Mendeleyev Table except for noble gases. Their properties depend first of all on the properties of elements of their composition. Major in this case are such properties of elements as their size, electronegativity and ionization energy of individual atoms, which define charge, size and ionic potential of migration forms. 

Can you imagine what types of substances would crystallize in either of these closest-packing schemes Hint Because we have been talking about spheres of identical size, the substances are not ionic compounds. Compounds contain more than one element and therefore have more than one size atom. 

Size-enhanced ionicity of copper and oxygen contributes to the 2p energy shift of CuO nanosohd [36]. Oxygen atoms bond more strongly to the Cu atoms in a nanosohd than they do inside the bulk. 

Because the electrons in a metal become delocalized, it may seem strange that the hard sphere size of the ions cores consistent with the measured density of the solid is larger than the ionic diameter used to calculate the size of ionic compoimds. The explanation is that a delocalized electron is not completely lost from the ion core as it is when transferred to an electronegative atom. The delocalized electrons are shared among all of the ion cores so the effective size of the ion core is less diminished when its valence electrons delocalized. 

To date there is no evidence that sodium forms any chloride other than NaCl indeed the electronic theory of valency predicts that Na" and CU, with their noble gas configurations, are likely to be the most stable ionic species. However, since some noble gas atoms can lose electrons to form cations (p. 354) we cannot rely fully on this theory. We therefore need to examine the evidence provided by energetic data. Let us consider the formation of a number of possible ionic compounds and first, the formation of sodium dichloride , NaCl2. The energy diagram for the formation of this hypothetical compound follows the pattern of that for NaCl but an additional endothermic step is added for the second ionisation energy of sodium. The lattice energy is calculated on the assumption that the compound is ionic and that Na is comparable in size with Mg ". The data are summarised below (standard enthalpies in kJ) ... 

Before we examine the polymerization process itself, it is essential to understand the behavior of the emulsifier molecules. This class of substances is characterized by molecules which possess a polar or ionic group or head and a hydrocarbon chain or tail. The latter is often in the 10-20 carbon atom size range. Dodecyl sulfate ions, from sodium dodecyl sulfate, are typical ionic emulsifiers. These molecules have the following properties which are pertinent to the present discussion ... 

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

It is shown that solute atoms differing in size from those of the solvent (carbon, in fact) can relieve hydrostatic stresses in a crystal and will thus migrate to the regions where they can relieve the most stress. As a result they will cluster round dislocations forming atmospheres similar to the ionic atmospheres of the Debye- Huckel theory ofeleeti oly tes. The conditions of formation and properties of these atmospheres are examined and the theory is applied to problems of precipitation, creep and the yield point."... 

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