Ionic radius alkali metals

Alkali metal alkoxides, r-butyl acetate neat, 45°, 30 min, 98% yield of r-butyl ester from methyl benzoate. The rate constant for the reaction increases with increasing ionic radius of the metal and with decreasing polarity of the solvent. Equilibrium for the reaction is achieved in <10 sec. Other examples eire presented. " ... 

Many of the ionic fiuorides of M, M and M dissolve to give highly conducting solutions due to ready dissociation. Some typical values of the solubility of fiuorides in HF are in Table 17.11 the data show the expected trend towards greater solubility with increase in ionic radius within the alkali metals and alkaline earth metals, and the expected decrease in solubility with increase in ionic charge so that MF > MF2 > MF3. This is dramatically illustrated by AgF which is 155 times more soluble than AgF2 and TIF which is over 7000 times more soluble than TIF3. 

Ionic bond, 287, 288 dipole of, 288 in alkali metal halides, 95 vs. covalent, 287 Ionic character, 287 Ionic crystal, 81, 311 Ionic radius, 355 Ionic solids, 79, 81, 311 electrical conductivity, 80 properties of, 312 solubility in water, 79 stability of, 311... 

The principal product of the reaction of the alkali metals with oxygen varies systematically down the group (Fig. 14.15). Ionic compounds formed from cations and anions of similar radius are commonly found to he more stable than those formed from ions with markedly different radii. Such is the case here. Lithium forms mainly the oxide, Li20. Sodium, which has a larger cation, forms predominantly the very pale yellow sodium peroxide, Na202. Potassium, with an even bigger cation, forms mainly the superoxide, K02, which contains the superoxide ion, O,. ... 

In almost all theoretical studies of AGf , it is postulated or tacitly understood that when an ion is transferred across the 0/W interface, it strips off solvated molecules completely, and hence the crystal ionic radius is usually employed for the calculation of AGfr°. Although Abraham and Liszi [17], in considering the transfer between mutually saturated solvents, were aware of the effects of hydration of ions in organic solvents in which water is quite soluble (e.g., 1-octanol, 1-pentanol, and methylisobutyl ketone), they concluded that in solvents such as NB andl,2-DCE, the solubility of water is rather small and most ions in the water-saturated solvent exist as unhydrated entities. However, even a water-immiscible organic solvent such as NB dissolves a considerable amount of water (e.g., ca. 170mM H2O in NB). In such a medium, hydrophilic ions such as Li, Na, Ca, Ba, CH, and Br are selectively solvated by water. This phenomenon has become apparent since at least 1968 by solvent extraction studies with the Karl-Fischer method [35 5]. Rais et al. [35] and Iwachido and coworkers [36-39] determined hydration numbers, i.e., the number of coextracted water molecules, for alkali and alkaline earth metal... 

The slow development of heavy alkali organometallic chemistry is due to high reactivity, as rationalized by the increase of polar character of the metal-ligand bond due to the reduced polarizing ability of the metals. The increase in ionic character on descending the group of alkali metals is clearly demonstrated by the increase in ionic radii with Li+(0.69A), Na+(0.97A), K+(1.33A), Rb+(1.47A), and Cs+(1.67A), resulting in a radius of Cs+ that is more than double of that of Li+. 

These three structures are the predominant structures of metals, the exceptions being found mainly in such heavy metals as plutonium. Table 6.1 shows the structure in a sequence of the Periodic Groups, and gives a value of the distance of closest approach of two atoms in the metal. This latter may be viewed as representing the atomic size if the atoms are treated as hard spheres. Alternatively it may be treated as an inter-nuclear distance which is determined by the electronic structure of the metal atoms. In the free-electron model of metals, the structure is described as an ordered array of metallic ions immersed in a continuum of free or unbound electrons. A comparison of the ionic radius with the inter-nuclear distance shows that some metals, such as the alkali metals are empty i.e. the ions are small compared with the hard sphere model, while some such as copper are full with the ionic radius being close to the inter-nuclear distance in the metal. A consideration of ionic radii will be made later in the ionic structures of oxides. 

Fig. 12.2 The ratio of radii, k (=ionic radius/covalent radius), for alkali metal eations (M ) and halide anions (X ) in aqueous solutions (Eqs. 12.6a, b). In the right angled triangle, ABC, E and D are the mid points of AB and AC...

Symbol Cs atomic number 55 atomic weight 132.905 a Group lA (Group 1) alkali metal element electron configuration [Xe]6si atomic radius 2.65 A ionic radius (Cs ) 1.84 A ionization potential 3.89 eV valence +1 natural isotope Cs-133 37 artificial isotopes ranging in mass numbers from 112 to 148 and half-lives 17 microseconds (Cs-113) to 2.3x10 years (Cs-135). 

Symbol Rb atomic number 37 atomic weight 85.468 a Group I (Group 1) alkali metal element electron configuration [Kr] 5si valence -i-l atomic radius 2.43A ionic radius, Rb+ 1.48A atomic volume 55.9 cc/g-atom at 20°C ionization potential 4.177 V standard electrode potential Rb+ + e Rb, E° = -2.98V two naturally-occurring isotopes, Rb-85 (72.165%) and Rb-87 (27.835%) Rb-87 radioactive, a beta emitter with a half-bfe 4.88xl0i° year twenty-seven artificial radioactive isotopes in the mass range 74—84, 86, 88-102. 

Symbol Na atomic number 11 atomic weight 22.9898 a Group lA (Group 1) alkali metal element electron configuration [NejSs valence +1 atomic radius 1.85A ionic radius, Na" in crystals 1.02A (for a coordination number 6) ionization potential 5.139 eV standard electrode potential, E°(Na+ + e Na) -2.71 V one naturally-occurring stable isotope, Na-23 (100%) sixteen artificial radioactive isotopes in the mass range 19-22, 24—35 longest-lived radioisotope, Na-22, ti/2 2.605 year shortest-lived isotope Na-35, ti/2 1.5 ms. 

Crowns with Stilbene Fluorophores. Stilbenes undergo cis-trans isomerism on irradiation and this feature is exploited in (3.82). In the case of the frany-isomer, large ionic radius ions such as K+, and Cs, give fluorescent complexes whilst the smaller Lb and Na " cause quenching. UV irradiation to the dy-isomer causes a considerable change in the conformation of the crown ether substructure and increases its ability to extract alkali metals, including Lb and Na+, from water into benzene. 

Atomic and ionic radius of some important alkali and transition metals. 

AHa for the Adsorption of Alkali Metals. If an alkali metal atom is located at an infinite distance from a metal surface at zero potential, then the heat of adsorption comprises the work done in (1) transferring an electron from the atom to the metal, and (2) bringing the positive ion to its equiUbrium distance from the metal surface (127). In the first step, the energy change is (e0 — el), where is the work function of the metal and I is the ionization potential of the alkali metal atom. In the second, the force of attraction on the positive ion at a distance d from the metal surface, i.e., the electrostatic image force, is e /4d hence, the heat Uberated is e /4do, where do is the equilibrium distance of the adsorbed ion from the metal surface. This distance is often assumed to be equal to the ionic radius, which is 1.83 A. for the Na ion. The initial heat of adsorption, therefore, is... 

As concerns the spatial fit of host and guest, 44 forms the most stable complex with K+ (Figure 3.1) [10], since its radius of ca. 138 pm is approximately equal to the ionic radius of the guest. The dependence of stability constants of the complexes of 47, 48 and 44 with alkali metal cations on the ion diameters is shown in Figure 3.2. The complicated character of the depicted relations indicates that more factors (e.g., solvent effect) are at play in the ions recognition. 

These ions all have the same number of electrons as the neon atom, as is true of the negative ions of the previous period N3, O2-, F". From N3 to Cl7+ the nuclear charge rises from seven to seventeen, resulting in a steady contraction of the electron cloud, or, in other words, a contraction of the ionic radius. Now let us compare the ions in a column of the system, for example, the alkali metals... 

The formation of a metal structure from free atoms must be associated with ionization, from which it follows that a high ionization energy in an element prevents it. Metallic properties are therefore found in the alkali- and alkaline-earth elements. Boron, the first element in the third group, is hardly metallic in this group the element with the smallest ionic radius loses its metallic character. 

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