Bond strength, periodic trends

Periodic variations in the surface tension of liquid metals, c1 , are shown in Figure 6.5. The much higher surface tension of rf-block metals compared to the s- and p-block metals suggests that the surface tension relates to the strength of interatomic bonding. Similar periodic trends can be found also for the melting temperature and the enthalpy of vaporization, and the surface tension of liquid metals is strongly... 

As shown in Table 4.55, this quantity increases going down the periodic column in each group, a trend that is known to hold generally throughout the d-block elements. This d-block trend is in contrast to the corresponding main-group trend for bond strengths to diminish down a periodic column. 

The binary acids of non-metals exhibit periodic trends in their acid strength, as shown in Figure 8.5. Two factors are responsible for this trend the electronegativity of the atom that is bonded to hydrogen, and the strength of the bond. 

The binary acids show periodic trends, which are related to electronegativity and bond strength. 

A theoretical foundation for understanding these correlations is found in the calculated bulk electronic structures of the first- and second-row TMS. The electronic environment of the metal surrounded by six sulfur atoms in an octahedral configuration was calculated, using the hypotheses that all the sulfides could be represented by this symmetry as an approximation. There are several electronic factors that appear to be related to catalytic activity the orbital occupation of the HOMO (Highest Occupied Molecular Orbital), the degree of covalency of the metal-sulfur bond, and the metal-sulfur bond strength. These factors were incorporated into an activity parameter (A2), which correlates well with the periodic trends (Fig. 16) (74, 75). This parameter is equal to the product of the number of electrons contained in the... 

Figure 5-8 shows the variation of bond distance with the number of valence electrons in second-period p block homonuclear diatomic molecules. As the number of electrons increases, the number in bonding orbitals also increases, the bond strength becomes greater, and the bond length becomes shorter. This continues up to 10 valence electrons in N2 and then the trend reverses because the additional electrons occupy antibonding orbitals. The ions N2", 02, 02, and 0 are also shown in the figure and follow a similar trend. 

The i-block elements (with the exceptions of H and He) do not have sufficient electrons for hypervalent bonding. Transition metals can use d orbitals in their bonding, and thus do not follow the octet rule. Therefore, with the exception of hydrogen bonding, the central atom in a hypervalent system must be a / -block element. Within the p block, two obvious issues are how the bond strengths change as the central atom A is varied across or down the periodic table. Sufficient data is currently available to begin to define these periodic trends. 

Two factors determine how easily a proton is released from a nonmetal hydride the electronegativity of the central nonmetal (E) and the strength of the E—H bond. Figure 18.11 displays two periodic trends ... 

The trend in directly reflects the trend in the stretching force constant (and bond strength) since the central atom is not moving in this mode. In however, both X and Y atoms are moving, and the mass effect of ihe X atom cannot be ignored completely. Across the periodic table, the stretching frequencies increase as the oxidation state of the central atom becomes higher. Thus we have ... 

The periodic trends discussed above can be readily rationalized in terms of the stabilizing electronic interactions and the destabilizing steric interactions which determine the strength of the M-CO bond. The electronic terms are represented by 7C-back-donation from occupied nd-orbitals on the metal centre to the empty orbital (5a). as well as a-donation from the doubly-occupied Cco orbital (5b) to vacant nd orbitals. The steric terms are dominated by the repulsive four-electron two-orbital interactions between oco and occupied nd orbitals on the metal centre. Our results show that electronic factors are most favourable for the pentacarbonyls where both 7C-... 

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