Half-good conductors

Unfortunately, the term semiconductor refers not just to half-good conductors, which might be confusing. Semiconductors most often refers to those materials that are the basis for devices that are steerable in a certain fashion such as diodes, transistors, etc. ie, the basics for computers. We leave the details aside. 

The counter electrode should be a good electrical conductor. This condition is clear if the counter electrode is not a good conductor, it means that a poor amount of current can pass the interface at the counter electrode, which will then determine the measured current of the electrochemical cell. Therefore, the measured current will provide information about the properties of the counter electrode and not about the half reaction occurring at the surface of the working electrode. 

While in the very good conductors, such as the monatomic metals, both in the alkali metals of the A-subgroups and in those of the B-subgroups copper, silver and gold, the band is only half-filled, with the elements at the limit of the metals and metalloids the transition to the insulators takes place. 

There have been many attempts to correlate the outstanding chemisorptive and catalytic properties of the Groups 8-10 metals with the presence of an incomplete d-band or unfilled d-orbitals. According to the Band Theory, electrical conduction requires excitation to energy levels above the Fermi surface, so that substances that have only completely filled bands will be insulators. A metal such as magnesium for example is a good conductor because it possesses a partly filled hybrid sp band. By the same token, it is easier to carry a full bottle of mercury than a half-full one, because it doesn t slop about so much. 

Overlap of 3s and 3p bands is not necessary to explain the ability of Na or of any other Group lA metal to conduct electricity, as the half-filled 3s band is sufficient for this. In the Group 2A metals, however, such overlap is important. Consider a crystal of magnesium as an example. The 3s atomic orbital of an isolated Mg atom is filled with two electrons. Thus, without this overlap, the 3s band in a crystal of Mg is also filled. Mg is a good conductor at room temperature because the highest energy electrons are able to move readily into vacant orbitals in the 3p band (Figure 13-34). 

Lithium is one of the alkali metals and it is the lightest of all the metallic elements, with a density about half that of water. When first made or freshly cut, lithium has the luster and color of bright silver, but it tarnishes rapidly in moist air. It is soft and malleable, can be readily extruded into thin foils, and is a good conductor of electricity. Table 14.3 lists some of the physical properties of lithium. -" ... 

Since Fe3+ is a reactant in the cathode half-reaction, Fe(N03)3 would be a good electrolyte for the cathode compartment. The cathode can be any electrical conductor that doesn t react with the ions in the solution. A platinum wire is a common inert electrode. (Iron metal can t be used because it would react directly with Fe3+, thus short-circuiting the cell.) The salt bridge contains NaN03/ but any inert electrolyte would do. Electrons flow through the wire from the iron anode (—) to the platinum cathode ( + ). Anions move from the cathode compartment toward the anode while cations migrate from the anode compartment toward the cathode. 

You ve heard electrochemistry of corrosion as a lecture I shouldn t spend much time on it but I d like to describe some electrochemical effects for film formers. First the general principles. If you put a good electronic conductor (a metal) in an aqueous solution, you will typically find that an electrical potential is developed between the piece of conductor and the solution. When ions of the metal enter the solution and leave extra electrons behind a negative potential is developed. All oxidation reactions occurring on the surface are expected to produce this result. Similarly, reduction reactions that use electrons from the metal are expected to produce a more positive potential in the metal. The solution potential of the metal influences the rate of an electrochemical half-cell reaction in accordance with Le Chatelier s Principle, so it is possible to predict through the use of the Nernst Equation the potential that will exist when the only significantly rapid reactions are the oxidation and reduction parts of a reversible reaction. When more than one potentially reversible process occurs, the rate of oxidation will be expected to exceed the rate of reduction for at least one and the converse for at least one. At... 

Without the overlap of energy bands, the periodic properties of metals could not be explained. In the absence of the d- and p-bands, we would expect the s-band to be half-filled for the alkali metals (group lA) and completely filled for the alkaline-earth metals (group 2A). If that were true, metals like magnesium, calcium, and strontium would not be good electrical and thermal conductors, in disagreement with experimental observations. 

Ideally, the two extreme states of this material should correspond to the C0O2/UC0O2 couple. To express the Nernst law In this material In a simplified manner, one can consider only the equilibrium of lithium ions which keep their oxidation number -i-l. This implies that the insertion material is a sufficiently good electronic conductor for the Insertion limit to be ruled by the ionic insertion sites. The redox half-reaction can then be written ... 

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