Sodium covalent radius

J0 The point is made early in this chapter that comparisons oi atomic radii should be restricted to the same type of radii van der Waab vs. van der Waals. covalent vs. covalent, etc. In class one day a student asked. Which is larger, the van der Waals radius of neon or the covalent radius of sodium (I) Was that a nonsensical question.7 Discuss. 

Sodium, with a covalent radius of 157 pm, can occupy all the available voids of both structures, but the type II host lattice is the more stable. Experimental data agree with this argument type I clathrates are formed in milder conditions than type II. This type II structure formed with small sodium atoms is unique and has no hydrate equivalent both voids are occupied stoichiometrically and a large range of composition is observed 3 < x < 22. 

Sodium, the smallest of the alkali-metal considered here, with a covalent radius of 0.157 nm, can enter the type I and type II structures and occupy the two kinds of available cages. However, the type I was obtained in milder experimental... 

The sodium atom has an outer 3s electron and a neon core. Since the 3s electron is the outermost electron, and since it is shielded from the nuclear charge by the core electrons, it contributes greatly to the size of the sodium atom. The sodium cation, having lost the outermost 35 electron, has only the neon core and carries a charge of 1+. Without the 35 electron, the sodium cation (ionic radius = 95 pm) becomes much smaller than the sodium atom (covalent radius = 186 pm). The trend is the same with all cations and their atoms, as shown in Figure 8.12 t. 

Atomic radii are represented by the solid arrows. The covalent radius is based on the diatomic molecule Na2(g), found only in gaseous sodium. The metallic radius is based on adjacent atoms in solid sodium, Na(s). The value of the ionic radius of Na is obtained by the comparative method described in the text. 

The structure of sodium thallide NaTl can be understood as a diamond-like framework of T1 atoms, whose vacant sites are completely filled with Na atoms. Figure 13.7.2(a) shows the structure of NaTl, in which the Tl-Tl covalent bonds are represented by solid lines. The T1 atom has three valence electrons, which are insufficient for the construction of a stable diamond framework. The deficit can be partially compensated by the introduction of Na atoms. The effective radius of the Na atom is considerably smaller than that in pure metallic sodium. 

The experimental value of the internuclear distance in gaseous sodium chloride is 2 51 A the sum of the covalent radii is 2-53 A whereas the sum of the ionic radii is 2 48 A. In obtaining this figure, allowance has been made for the decrease, by about ii per cent, of the ionic radii in the gaseous molecule compared with that in the ionic lattice. A similar result is obtained in the case of sodium bromide and iodide. Since the radius of the positive ion is less, and that of the negative ion is greater, than the corresponding covalent radii, it would appear that differences have been compensated. 

Opposite to rebound reactions is the reaction Na + Ch — NaCl + Cl which proceeds via the spectator stripping mechanism. In this case, the crossing between the nonreactive covalent Na-Cl2 curve and the Na+Cl ion-pair curve, which promotes the reaction, occurs at a large distance [Re = 5.22 A, when using the chlorine adiabatic electron affinity in Magee s equation). This distance increases to 22.3 A when sodium is excited to the 3p P level. One would expect an increased reaction cross-section, but this is not observed because electron transfers at such large distance are inefficient. The overlap between the sodium HOMO and the CI2 LUMO is very small at these distances. As a result, when the crossing radius increases substantially, there is only a small effect on the dynamics of the reaction [164, 165]. 

The intense nucleophilic reactivity of these compormds is ascribed to the presence of nearly a full unit of negative charge on one of the carbon atoms. n-Butyl-lithium, on the other hand, is a colourless liquid which readily dissolves in saturated hydrocarbons, as a hexamer (Bu°Li) , believed to have an electron-deficient covalent constitution though the lithium-carbon bonds are certainly highly polar. The more covalent character of lithium alkyls is likely to be due mainly to the smaller radius and higher polarizing power of lithium relative to sodium. 

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