Fifth-period elements

The first ionization energies of the elements are plotted in Figure 1.4. There is a characteristic pattern of the values for the elements Li to Ne which is repeated for the elements Na to Ar, and which is repeated yet again for the elements K, Ca and A1 to Kr (the s- and p-block elements of the fourth period). In the latter case, the pattern is interrupted by the values for the 10 transition elements of the d-block. The fourth period pattern is repeated by the fifth period elements, and there is an additional... 

Both the b and k values were estimated by regression analysis from the Taft steric constant Es and the Charton steric constant in an iterative procedure using 96 substituents. The k values are equal to 1.0 for the second period elements, except for the fluorine atom (k = 0.8) k = 1.2,1.3, and 1.7 for the third, fourth, and fifth period elements, respectively. 

As we have seen, the second-period element, fluorine, induces hypervalency in many other p-block elements, but it does not itself display hypervalency. The third and fourth period halogens, i.e. chlorine and bromine, form trifluorides and pentafluorides, XF3 and XF5. The fifth-period element, iodine, forms an unstable trifluoride which decomposes below room temperature, a pentafluoride and a heptafluoride. Iodine also forms a solid trichloride which decomposes on evaporation as in Groups 15 and 16 we find that the greater number of hypervalent derivatives are formed by the fifth-period element. 

Strategy Follow the procedures in Examples 3.4 and 3.5 for drawing Lewis structures and calculating formal charges. Xe is a fifth-period element, so it may need to expand its octet to accommodate aU of the valence electrons. 

Strategy Note that Xe is a fifth-period element. We follow the procedures in Examples... 

Fignre 23.3 compares the covalent radii of the transition elements. The atomic radii increase in going from a fourth-period to a fifth-period element within any column. For example, reading down the Group IIIB elements, you find that the covalent radius of scandium is 144 pm and of yttrium is 162 pm. You would expect an increase of radius from scandium to yttrium because of the addition of a shell of electrons. Continuing down the column of lllB elements, you find a small increase of covalent radius to 169 pm in lanthanum. But all of the remaining elements of the sixth period have nearly the same covalent radii as the corresponding elements in the fifth period. 

Chemical properties and spectroscopic data support the view that in the elements rubidium to xenon, atomic numbers 37-54, the 5s, 4d 5p levels fill up. This is best seen by reference to the modern periodic table p. (i). Note that at the end of the fifth period the n = 4 quantum level contains 18 electrons but still has a vacant set of 4/ orbitals. 

The transition metals, in the center of the periodic table, fill d sublevels. Remember that a d sublevel can hold ten electrons. In the fourth period, the ten elements Sc (Z = 21) through Zn (Z = 30) fill the 3d subleveL In the fifth period, the 4d sublevel is filled by the elements Y (Z = 39) through Cd (Z = 48). The ten transition metals in the sixth period fill the 5d subleveL Elements 103 to 112 in the seventh period are believed to be filling the 6d subleveL... 

The Peierls distortion is not the only possible way to achieve the most stable state for a system. Whether it occurs is a question not only of the band structure itself, but also of the degree of occupation of the bands. For an unoccupied band or for a band occupied only at values around k = 0, it is of no importance how the energy levels are distributed at k = n/a. In a solid, a stabilizing distortion in one direction can cause a destabilization in another direction and may therefore not take place. The stabilizing effect of the Peierls distortion is small for the heavy elements (from the fifth period onward) and can be overcome by other effects. Therefore, undistorted chains and networks are observed mainly among compounds of the heavy elements. 

The structures for spectra of atoms in the fifth period of elements are not so completely analyzed, but insofar as they are known or may be predicted they are represented in table II. 

Systems of Spectral Structures for Ten Elements of the Fifth Period (a) Arc Spectra, Neutral Atoms... 

The fourth and fifth periods are the medium periods. Each of these periods contains 18 elements. 

In the fifth period of the Periodic Table, we find transition metal elements with configurations 4d 5s1 and 4d 5s°. For the corresponding elements in the sixth period, the configurations become 5d -16s2 and 5d 16s1, respectively. This change is due to the aforementioned stabilization of the 6s orbital caused by the relativistic effects. Table 2.4.4 lists the configuration of the elements concerned. 

Comparison of the tetramethyldipnictogens with the isoelectronic dihalogens is particularly informative. Cl2, Br2, and I2 crystallize in isostructural molecular lattices with increasing intermolecular interaction (54,59). For iodine, the atoms are connected intramolecularly at 2.72 A and inter-molecularly in a two-dimensional rectangular net at 3.50 and 3.97 A. The ratio E—E/E—E drops from 1.68 for Cl2 to 1.29 for I2. Only for I2 is there an appreciable intermolecular interaction. Again the brake occurs between the fourth and fifth periods of elements (see Table VIII) (54,59). The... 

Ditellurides, also in the fifth period, seem quite analogous to distibines. Like tetraphenyldistibine (1) the red diphenylditelluride (56) does not associate in the solid state. The closest intermolecular Te---Te contact is 4.255 A, near the van der Waals separation of 4.40 A 61). On the other hand, di(p-methoxyphenyl)ditelluride (57), which has a brown-green metallic luster in the solid, has close intermolecular Te---Te contacts of 3.57 and 3.98 A (62). The ratio Te---Te/Te—Te is 1.32. Just as in the distibines the intermolecular bonding in ditellurides is sensitive to substitution. It is also interesting to note that the intermolecular interaction in ditellurides and dihalogens occurs normal to the metal-metal axis, as well as colinear as in distibines (63). Thus, it is clear that the intermolecular association shown by distibines is a general property of many of the diatomic like compounds of the heavier main group elements. 

Elements of the third to fifth periods are also able to expand their octet shells. New paraelements result in this way which have not so far been mentioned, e.g.,... 

Identify the element in group IB of the fifth period of the periodic table. 

Prepare a diagram such as the one in Figure 2-12(a) for the fifth period in the periodic table, elements Zr through Pd. The configurations in Table 2-7 can be used to determine the crossover points of the lines. Can a diagram be drawn that is completely consistent with the configurations in the table ... 

The periods, or horizontal rows, of the table are numbered on the left hand side from 1 to 7. The first period contains two elements, hydrogen and helium (He), the second period and third period each contain eight elements, while the fourth and fifth periods each contain 18 elements. 

FIGURE 8.1 Variation of atomic radii through the fourth-, fifth-, and sixth-period transition-metal elements. Symbols shown are for the fourth-period elements. 

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