Positive ‘core

If a piece of metal, such as silver, is dipping into a solvent, and a positive atomic core is taken from the surface into the solvent, the ion is again surrounded by its electrostatic field but free energy has been lost by the dielectric, and a relatively small amount of work has had to be done. The corresponding potential-energy curve (Fig. 96) is therefore much less steep and has a much shallower minimum than that of Fig. 9a. For large distances d from a plane metal surface this curve is a plot of — c2/4td where t is the dielectric constant of the medium at the temperature considered The curve represents the work done in an isothermal removal of the positive core. 

Electrodes and Galvanic Cells. In connection with Fig. 9 in See. 11 we discussed the removal of a positive atomic core from a metal. The same idea may be applied to any alloy that is a metallic conductor. When, for example, some potassium has been dissolved in liquid mercury, the valence electron from each potassium atom becomes a free electron, and we may discuss the removal of a K+ core from the surface of the amalgam. The work to remove the K+ into a vacuum may be denoted by Ycr When this amalgam is in contact with a solvent, we may consider the escape of a K+ into the solvent. The work Y to remove the positive core into the solvent is much smaller than Yvac. 

The mechanism of electronic polarization operates in all atoms and molecules, since the centre of gravity of the electrons surrounding the positive cores are displaced by the electric field. This effect is extremely fast, and thus effective up to optical frequencies. The dipole moment for this polarization mechanism can be written as follows ... 

The path of least resistance for the electric current is from negative carbon to the positive core. Hence most of the electrons forming the high current density in the arc stream travel from the positive core. At this point the energy is high such that the core vaporizes faster than the shell, thus forming a cup on the face of the positive carbon, which is the main source of light. 

This model takes a more fundamental approach by regarding a molecule as a collection of valence electrons and positive cores. Just as the nature of atomic orbitals derives from the spherical symmetry of the atom, so will the properties of these new molecular orbitals be controlled by the interaction of the valence electrons with the multiple positive centers of these atomic cores. These new orbitals, unlike those of the hybrid model, are delocalized that is, they do not belong to any one atom but extend over the entire region of space that encompasses the bonded atoms. The available (valence) electrons then fill these orbitals from the lowest to the highest, very much as in the Aufbau principle that you learned for working out atomic electron configurations. For small mole-... 

In this technique, determination of Vg also requires calculation of the polarization energy of the positive core, P+. The data in Fig. 5 are best fits to the experimental results obtained in recent studies. 

Photoemission studies have shown that in many cases the formation of a bimetallic bond induces positive core-level shifts for both metals [17,86,87,88,]. This, obviously, is not consistent with a simple metal->metal charge transfer [60,90], The phenomenon could be a consequence of combining inter- and intra-atomic charge redistributions (for example, d-sp rehybridization) induced by bimetallic... 

What soon emerged was a nuclear model of the atom, first proposed by New Zealand-bom physicist Ernest Rutherford. In this view, an element s identity was determined by its atomic number, the amount of positive charge in the very small core nucleus that also contained almost all of the atom s mass. The light electrons were held in orbits by electrostatic attraction to the positive core. 

Figure 2.15 Probability of finding electrons relative to location of the cores for the two standing waves that form when k = n-Kja. In the bottom case, the standing waves distribute the charge over the ion cores, and the attraction between the negative electrons and the positive cores reduces the energy of the system relative to the top situation, where the electrons spend most of their time between the ion cores.

Remember that in Chapter 3 we discussed the idea that an atom has a small positive core (called the nucleus) with negatively charged electrons moving around the nucleus in some way (see Figure 11.1). This concept of a nuclear atom resulted from Ernest Rutherford s experiments in which he bombarded metal foil with a particles. 

If covalent, ionic and metallic bonds are explained in electrical terms, students are better prepared to accept that hydrogen bonds, van der Waals forces, solvent-solute interactions etc. are also types of chemical bonding. Where learners see covalent bonds as electron pairs attracted to two different positive cores, they have a good basis for subsequently learning about electronegativity and bond polarity. 

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