Shielding of nuclear charge

As the atom radii increase down group 1, the outermost electron is further from the nucleus (1) and more shielded from the nuclear charge, so is held less tightly (despite the increasing number of protons in the nucleus of each atom) (1). Always answer in terms of factors such as distance, shielding and nuclear charge. 

For the crystal composed of ions, the equality given in Eq. (9.33) is no longer valid, because the two ionic charge distributions, which partly shield the nuclear charges, are different. For the free-ion crystal, the values of the potential at the nuclear positions in NaF, evaluated according to Section 9.3.2, are... 

The greater electron affinity of PtF, compared with WF correlates with the greater nuclear charge of the platinum and with the poor shielding of this charge from the ligands by the, formally... 

Electron configuration of cation. The d electrons are less effective in shielding the nuclear charge than the s ox p electrons and are thus more polarizing. Thus ions with d electrons tend to form more covalent bonds. For example, Ca and Hg have very similar radii (114 and 116 pm, respectively) and yet the salts of Hg have lower melting points than those of Ca — HgCl2 melts at 276°C, whereas CaCb melts at 782°C. 

This approximation is similar to the point-charge-potential simplifications introduced [10] for core-other-core and core-other-nucleus interactions, showing that the net effect of the core interaction energy is to shield the nuclear charges in the internuclear repulsion energy.) Hence the E of Eq. (9) becomes... 

Recall from Chapter 2 that a gradual change in properties as we move in any direction in the periodic table is called a periodic trend. Most periodic trends can be understood from the perspective of the simplified shell model, and underlying most trends are two important concepts inner-shell shielding effectiw nuclear charge. 

It is well estabUshed from chemical experience that most chemical properties of molecules and soUds are determined by the valence electrons of the constituent atoms. The core states are weakly affected by changes in chemical bonding. The effect of core electrons is principally to shield the nuclear charges and to provide an effective potential for the valence electrons. The main reason for the limited role of the core electrons is the spatial separation of the core and valence shells that originates from the comparatively strong binding of the core electrons to the nucleus. This effect is illustrated in Fig. 8.2, [470], where the radial orbitals of the oxygen atom are plotted. The spatial separation of the Is core state from the valence 2s, 2p states and... 

For the actinides, the situation is significantly different. Indeed, the shielding of their 5/orbitals by the partially filled 6s and 6p orbitals is not as complete for the first actinides. The result is that the/orbitals have sufficient radial extension to cover the ligand orbital up to a certain extent. As a consequence, various oxidation states and partially covalent metal-ligand bonds are found for the first actinides. This covalent character remains minor, however, and the ionic character dominates. The accumulation of nuclear charges provokes the contraction of the 5/orbitals as one moves forward in the actinide series. Again, the oxidation state III dominates and the complexes are completely ionic for the later actinides as for the lanthanides. 

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