Atomic size variations

The computational bond-length variations in Table 4.53 exhibit the expected periodic trends. Most noticeably, third- and second-series elements for groups 4, 6, and 10 exhibit similar bond lengths, i.e., the post-lanthanide contraction with respect to the ordinary increase of atomic size with increasing Z. 

Variations in ferritin iron cores include the number of iron atoms, composition, and the degree of order (3,6,21-23). Size variations of the iron core range from 1-4500 Fe atoms and appear to be under biological control (e.g. Ref. 15). The distribution of iron core sizes in a particular ferritin preparation can be easily observed after sedimentation of ferritin through a gradient of sucrose. 

The variations in atomic sizes across periods and down groups... 

This section summarizes the variation, across the periods and down the groups of the Periodic Table, of (i) the ionization energies, (ii) the electron attachment energies (electron affinities), (iii) the atomic sizes and (iv) the electronegativity coefficients of the elements. 

As a result there is a steady contraction from left to right The net effect of the top-to-bottom and the lefr-to-right trends is a discontinuous variation in atomic size There is a steady contraction with increasing atomic number until tliere is an increase in the principal quantum number. This causes an abrupt increase in size followed by a further decrease. 

Figure 47 again displays some of the data in Figures 43 and 46. The data are now restricted to complexes of the first-row transition metals from chromium to copper to avoid large variations in central atom size, the 12-coordinate metallic radii being in the range 1.2—1.3 A. 

Transition metals share properties such as electrical conductivity, luster, and malleability with other metals. There is little variation in atomic size, electronegativity, and ionization energy across a period. However, there are differences in properties among these elements, especially physical properties. For example, silver is the best conductor of electricity. Iron and titanium are used as structural materials because of their relative strength. 

It can be argued, however, that (15) and (17) are really not independent of each other, because of the inverse variation of I with atomic size, which can be seen from the asymptotic dependence of p(r) upon radial distance from the nucleus [29,165-167] ... 

From one vibration to another there can be considerable variation and, generally, the displacement of electron rich atoms will influence the spectral intensities more than electron poor atoms. Thus, since polarisability increases with atomic size, Raman spectra involving the heavier atoms are easier to observe, as demonstrated by the weakness of hydrogen s vibrational features in metal hydridocarbonyls ( 11.2.6). It also follows that the optically strong features of a spectmm may mask weaker vibrations of interest, a common problem in intercalate and catalytic systems. 

Ball-and-stick models show the three-dimensional arrangement of atoms clearly, and they are fairly easy to construct. However, the balls are not proportional to the size of atoms. Furthermore, the sticks greatly exaggerate the space between atoms in a molecule. Space-filhng models are more accurate because they show the variation in atomic size. Their drawbacks are that they are time-consuming to put together, and they do not show the three-dimensional positions of atoms very well. We will use mostly the ball-and-stick model in this text. 

As we have seen, the electron confignrations of the elements show a periodic variation with increasing atomic nnmber. Conseqnently, there are also periodic variations in physical and chemical behavior. In this section and the next two, we will examine some physical properties of elements that are in the same group or period and additional properties that inflnence the chemical behavior of the elements. First, let s look at the concept of effective nnclear charge, which has a direct bearing on atomic size and on the tendency for ionization. 

A still more dramatic size variations occurs as a result of the centrifugal barrier effects discussed in chapter 5 since orbital collapse results in deep filling within the atom, in which the outermost electrons are not involved, it is possible for excited states and resonances involving collapsed orbitals in the final states to survive in the solid. 

Compared to the representative elements, the transition metals are remarkable in that little variation in atomic sizes occurs in going from the first to the second and third series. Orbital sizes do not change greatly, and the strength of covalent bonds to ligands remains much more constant (28). This statement means that the atom size factor, which separates out the heavier donor atoms from C, N, O, and F, will not be present for the transition metals. 

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