Trends within the Group

Color pale yellow green- yellow rust violet 

The trend in oxidation potentials may be considered a composite trend, similar to that described for the E° values of the alkali metals (Chap. 6). For the halogens, the following quantities are involved heats of dissociation of the molecules, electron affinities of the atoms, hydration energies of the ions, heats of vaporization (for bromine and iodine only), and, finally, entropy or randomness effects. Aside from the entropy effects (which turn out to be quite small for the reactions being considered), the reduction of the halogen X to the hydrated ion X at room temperature may be represented in steps as follows  

As with all of the other families, the heavier elements show greater tendency to expand their octets than do the lighter. This is perhaps most strikingly shown by the series of compounds that fluorine forms with the 

Elementary chlorine, bromine, and iodine dissolve in many organic solvents, and the variations in the colors among the iodine solutions in various solvents have been a matter of interest to many workers. (Both bromine solutions and solutions of iodine monochloride show analogous variations, but the effect for iodine is by far the most striking.) In completely nonbasic solvents (for example, CCU and CS2) iodine appears violet, the same color as its vapor but as the basicity of the solvent increases, the iodine color shifts toward orange or brown, presumably because the electronic excitation responsible for the iodine color is made more difficult by approach of electron-rich reagents. As the basicity of the solvent increases, the iodine-to-iodine bond weakens in the organic base, pyridine (reaction c, below), many of the I—I bonds are broken, wrhereas 

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The structural trends within the group II elements can only be understood by including the influence of the valence-d electrons explicitly through the use of non-local pseudopotentials. This is not unexpected considering our earlier discussion in 5.5 of their densities of states. Figure 6.13 shows the... 

The stmctural features in the target compounds are quite diverse and bonding within the target compounds may involve <7- and TT-type interactions. The following chapters are divided by ligand type to allow a more direct comparison of trends within the group of metals. 

All sp metals, except Zn, can be apparently gathered in a single group and there is a clear trend within the group for / CT=o to become more negative as 4> decreases. Thus, AX increases as 4> decreases. Zn has much higher value of AX than the other sp metals with comparable work function. Since X = 5x + the major contribution is probably in 5/. ... 

As noted above, electron ative atoms, such as chlorine, deshield the hydrogen atom of a Cl—C—H unit. The degree of deshielding, as measured by a shift to lower field (larger S value), increases with the electronegativity of the atom. The trend within the group of halogens is shown below. 

Increa sing the bulkiness of the alkyl group from the esterifying alcohol in the ester also restricts the motion of backbone polymer chains past each other, as evidenced by an increase in the T within a series of isomers. In Table 1, note the increase in T of poly(isopropyl methacrylate) over the / -propyl ester and similar trends within the butyl series. The member of the butyl series with the bulkiest alcohol chain, poly(/-butyl methacrylate), has a T (107°C) almost identical to that of poly(methyl methacrylate) (Tg = 105° C), whereas the butyl isomer with the most flexible alcohol chain, poly( -butyl methaciylate), has a T of 20°C. Further increase in the rigidity and bulk of the side chain increases the T. An example is poly(isobomyl methacrylate)... 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

The overall relationship between rate and C-H--C(sp2) distance is shown in Fig. 15. Though there are clear irregularities, the general trend, of reactivity increasing with decreasing distance, is clear and unmistakable an increase of 0.25 A in the distance each hydrogen has to move is associated with a decrease of over 104 times in the rate at which it moves. As the authors point out, the reversal of this trend within small groups of... 

In Ihis chapter the theories developed previously will be used 10 help correlate the important facts of the chemistry of groups 1—12 Much of the chemistry of these elements, in particular the transition metals, has already been included in the chapters on coordination chemistry (Chapters II, 12, and 13). More will be discussed in the chapters on organometaJlic chemistry (Chapter 15), clusters (Chapter 16), and the descriptive biological chemistry of the transition metals (Chapter 19). The present chapter will concentrate on the trends within the series (Sc to Zn, Y to Cd, Lu to Hg, La to Lu, and Ac to Lr), the differences between groups (Ti — Zr - Hf Cu — Ag - Au), and the stable oxidation stales of the various metals. 

Consider the trends within the Periodic Table of the Elements. Suppose we knew (previous experience) that the temperature at which manganese melts (melting point) is 1246°C, and that of rhenium is 3186°C. What is the predicted melting point of the synthetic element technetium Note that these elements are in group VHB. 

The theoretical investigations have shown that the trend in the complex formation and extraction (Equation 32) known for Nb, Ta and Pa turned out to be reversed in going to Db. This could not be predicted by any extrapolation of this property within the group, which would have given a wrong trend, but came as a result of considering real chemical equilibria and calculating relativistically the electronic structure of the complexes. 

In the discussion of the rather unexpected chemical results [42], it was suggested that the chemical properties of the heaviest elements cannot reliably be predicted by simple extrapolations of trends within a group of elements , and that relativistic, quantum chemical calculations for compounds of Nb, Ta, Pa, and Db are needed to understand in detail the differences in the halide complexing of the group-5 elements . 

Based on the trends discussed in Chapter 6, the metallic properties of the elements in group 4A should increase as the atomic number increases. Carbon is a nonmetal silicon and germanium are metalloids tin and lead are metals. With such a wide range of properties, there are few rules that apply to all members of the group. One general trend does apply. The period-2 element, carbon, is not representative of the other elements within the group. 

A general trend within a group, the first ionization energy decreases down the group because in the same direction the atomic radii and principal quantum number n increase. There are only a few exceptions to this trend, and they are found in Groups 13 and 14. 

If we consider the vertical trends, that is, the trends within one group of the elements, we observe an increase in covalent radii. Recall again that the atomic radii increase in the same direction. As we go down a group the valence electrons are found in the atomic orbitals of higher principal quantum number. There are more core (or non-valence) electrons between the valence electrons and the nucleus that shield the valence electrons front the influence of the nucleus. As a consequence, the valence electrons are further away from the nucleus. This means that when a covalent bond is formed, two atomic nuclei are further apart as we go down the group and the covalent radii increase. 

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