Penetration and shielding

Penetration and shielding account for the general form of the periodic table and the physical and chemical properties of the elements. The two effects underlie all the varied properties of the elements and hence their contributions to biological systems. 

As a consequence of penetration and shielding, the energies of orbitals in the same shell of a many-electron atom lie in the order s p d f. The individual orbitals of a given subshell (such as the three p orbitals of the p subshell) remain degenerate because they all have the same radial characteristics md so experience the same effective nuclear charge. 

Now let us consider the arrangement of the electrons in Li (Z = 3). In the ground state, the H atomic orbital is fully occupied and the third electron could occupy either a 2s or 2p orbital. Which arrangement will possess the lower energy An electron in a 2s or 2p atomic orbital experiences the effective charge, of a nucleus partly shieldedhy the ly 

A set of empirical rules (Slater s rules) for estimating the effective nuclear charges experienced by electrons in different atomic orbitals is described in Box 1.6. 

In 1869 and 1870 respectively, Dmitri Mendeleev and Lothar Meyer stated that the properties of the elements can be represented as periodic functions of their atomic weights, and set out their ideas in the form of a periodic table. As new elements have been discovered, the original form of 

Effective nuclear charges, Z ff, experienced by electrons in different atomic orbitals may be estimated using Slater s rules. These rules are based on experimental data for electron promotion and ionization energies, and Z ff is determined from the equation  

Electrons in any group higher in this sequence than the electron under consideration contribute nothing to S. 

Atomic Element Ground state Atomic Element Ground state 

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The effects of penetration and shielding can be large. A 4s-electron generally has a much lower energy than that of a 4p- or 4d-electron it may even have lower energy than that of a 3d-electron of the same atom (see Fig. 1.41). The precise ordering of orbitals depends on the number of electrons in the atom, as we shall see in the next section. 

In a many-electron atom, because of the effects of penetration and shielding, the order of energies of orbitals in a given shell is s < p < d < f. 

Ground state electronic conhgurations experimental data Penetration and shielding... 

Determine whether each statement regarding penetration and shielding is true or false. (Assume that all lower energy orbitals are fully occupied.)... 

Before examining the rules for assigning electrons to the various orbitals, we will first discuss the concepts of penetration and shielding. These concepts will help us explain why, in a multielectron atom, orbitals with different values of F within a principal shell have different energies. 

X-Radiation like y-radiation is eleetromagnetie in nature. It ean be emitted when p-partieles reaet with atoms. More often it is eleetrieahy generated by aeeelerating eleetrons in a vaeuum tube. The latter souree ean be switehed off. X-rays are extremely penetrating and are merely attenuated by distanee and shielding. 

Salt addition to the subphase has a strong influence on monolayer formation, too. The effect of salt was studied by spreading particles la on an aqueous KCl solution of different salt concentration, with the pH of the subphase always being 5. If no salt is present at pH 5, the particles simply disappear into the subphase, as discussed earlier. However, the presence of salt causes the metal ions to penetrate the particle shell and shield the ionic groups electrostatically. Consequently, the particles become less hydrophilic and monolayer formation is improved, as indicated by the larger value of Aq. As shown in Figure 6a, a KCl concentration of 10 moles is sufficient to cause formation of a stable particle layer even at pH 5. 

As a result of the presence of one or more maxima near the nucleus, s orbitals are very penetrating and are somewhat less shielded by inner-shell electrons than are orbitals with larger values of L In turn, (hey tend to shield somewliat better than other orbitals. Orbitals with high l values, such as A and f orbitals, arc much less penetrating and are far poorer at shielding. 

Leather was sometimes treated with wax to make it harder and more water-resistant. The most effective method was to immerse leather sheets or objects in hot liquid wax for some period of time, long enough for the wax to penetrate all the way through the skin. Leather items too big for this treatment could be coated with wax, but this was not as effective since the wax was then only on the surface. Waxed leather was commonly used for containers liquids, such as buckets, bottles, and cups. It was also used to make protective armor and shields for both people and horses. 

In a derailment there is a tendency for couplers to move vertically and come apart. The free coupler then becomes a battering ram that can easily breach a tank car. Shelf couplers prevent separation, and end shields on tank cars can prevent penetration and spill, and are now required by DOT under recommendation of the National Transportation Safety Board. (5) Such improvements were developed by the Association of American Railroads in 1970, but implementation has not been pushed. 

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