Trends in Metallic Behavior

It s important to realize, however, that an element s properties may not fall neatly into our categories. For instance, the nonmetal carbon in the form of graphite is a good electrical conductor. Iodine, another nonmetal, is a shiny solid. Gallium and cesium are metals that melt at temperatures below body temperature, and mercury is a liquid at room temperature. And iron is quite brittle. Despite such exceptions, we can make several generalizations about metallic behavior. 

As we move across a period, it becomes more difficult to lose an electron (IE increases) and easier to gain one (EA becomes more negative). Therefore, with regard to monatomic ions, elements at the left tend to form cations and those at the right tend to form anions. The typical decrease in metallic behavior across a period is clear among the elements in Period 3. Sodium and magnesium are metals. Sodium is shiny when freshly cut under mineral oil, but it loses an electron so readily to O2 that, if cut in air, its surface is coated immediately with a dull oxide. These metals exist naturally as Na and Mg ions in oceans, minerals, and 

The gradation in metallic behavior among the elements is depicted as a gradation in shading from bottom left to top right, with arrows showing the direction of increase. (Hydrogen appears next to helium h this periodic table.) 

Acid-Base Behavior of the Element Oxides Metals are also distinguished from nonmetals by the acid-base behavior of their oxides in water  

Some metals and many metalloids form oxides that are amphoteric they can act as acids or as bases in water. 

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Trends in Metallic Behavior Properties of Monatomic Ions... 

Describe the general properties of metals and nonmetals and understand how trends in metallic behavior relate to ion formation, oxide acidity, and magnetic behavior understand the relation between atomic and ionic size and write ion electron configurations ( 8.5) (SPs 8.6-8.8) (EPs 8.47-8.65)... 

Trends in Metallic Behavior 265 Properties of Monatomic Ions 266 CHAPTER REVIEW GUIDE 271 PROBLEMS 272... 

In the preceding chapter we looked at the elements of the third row in the periodic table to see what systematic changes occur in properties when electrons are added to the outer orbitals of the atom. We saw that there was a decided trend from metallic behavior to nonmetallic, from base-forming to acid-forming, from simple ionic compounds to simple molecular compounds. These trends are conveniently discussed... 

Within the general trend in the behavior across the actinide series, their alloys, and their metallic compounds from superconductors to local moment magnets, the only serious irregularity occurs in some plutonium compounds. These compounds should be magnetic but turn out to be temperature independent paramagnets. 

In their electrochemical surface properties, a number of metals (lead, tin, cadmium, and others) resemble mercury, whereas other metals of the platinum group resemble platinum itself. Within each of these groups, trends in the behavior observed coincide qualitatively, sometimes even semiquantitatively. Some of the differences between mercury and other. y- or p-metals are due to their solid state. Among the platinum group metals, palladium is exceptional, since strong bulk absorption of hydrogen is observed here in addition to surface adsorption, an effect that makes it difficult to study the surface itself. 

Alloy stability is always of concern in heterogeneous catalysis, but in electrocatalysis there are new mechanisms for destabilizing alloys, namely electrochemical dissolution or corrosion. Greeley and Norskov developed an intuitive and simple thermodynamic framework for estimating the stability of alloy surfaces in electrochemical environments. " Their scheme is essentially an extension of an atomistic thermodynamic approach that uses chemical potentials to determine stability to one that uses electrochemical potentials to determine stability. They estimate the electrochemical potentials using total energies calculated within DFT and ideal solution behavior of the ions to consider concentration and pH effects. Within this formalism they are able to estimate the dissolution potential of metals in alloys. They further compared the trends in dissolution behavior to trends in segregation behavior and... 

In the following, we will attempt to derive some trends in the behavior of rare earths and actinides alloyed with transition metals ... 

Ion formation is only one pattern of chemical behavior. Many other chemical trends can be traced ultimately to valence electron configurations, but we need the description of chemical bonding that appears in Chapters 9 and 10 to explain such periodic properties. Nevertheless, we can relate important patterns in chemical behavior to the ability of some elements to form ions. One example is the subdivision of the periodic table into metals, nonmetals, and metalloids, first introduced in Chapter 1. 

A knowledge of the behavior of d orbitals is essential to understand the differences and trends in reactivity of the transition metals. The width of the d band decreases as the band is filled when going to the right in the periodic table since the molecular orbitals become ever more localized and the overlap decreases. Eventually, as in copper, the d band is completely filled, lying just below the Fermi level, while in zinc it lowers further in energy and becomes a so-called core level, localized on the individual atoms. If we look down through the transition metal series 3d, 4d, and 5d we see that the d band broadens since the orbitals get ever larger and therefore the overlap increases. 

Copper clusters, as reported by the Rice group(lc), do not react with hydrogen. Hydrogen chemisorption on copper surfaces is also an activated process. Surface beam scattering experiments place this barrier between 4-7 kcal/mole(33). This large value is consistent with the activated nature oT hydrogen chemisorption on metal clusters, and the trend toward bulk behavior for relatively small clusters (>25 atoms in size). 

Carbon monoxide eventually dissociates at room temperature on all but some of the group 8-10 (VII IB) metals (44). This dissociation occurs only for metal surfaces which form sufficiently strong metal-carbon plus metal-oxygen bonds to break the 257 kcal/mole CO bond. The known values for gas phase metal atoms predict the same trend(48). The similarity in the behavior of surfaces and atoms... 

The general procedure for constructing Lewis-like diagrams for transition-metal species can best be illustrated by representative examples. From Table 4.1 one can recognize that the first transition series (Sc-Zn) includes a disproportionate number of exceptional cases compared with later series, and illustrative examples will therefore be drawn primarily from the third transition series (La-Hg). (The somewhat anomalous behavior of the first transition series and general vertical trends in the d-block elements will be discussed in Section 4.10.)... 

While considering trends in further investigations, one has to pay special attention to the effect of electroreflection. So far, this effect has been used to obtain information on the structure of the near-the-surface region of a semiconductor, but the electroreflection method makes it possible, in principle, to study electrode reactions, adsorption, and the properties of thin surface layers. Let us note in this respect an important role of objects with semiconducting properties for electrochemistry and photoelectrochemistry as a whole. Here we mean oxide and other films, polylayers of adsorbed organic substances, and other materials on the surface of metallic electrodes. Anomalies in the electrochemical behavior of such systems are frequently explained by their semiconductor nature. Yet, there is a barrier between electrochemistry and photoelectrochemistry of crystalline semiconductors with electronic conductivity, on the one hand, and electrochemistry of oxide films, which usually are amorphous and have appreciable ionic conductivity, on the other hand. To overcome this barrier is the task of further investigations. 

Scheme I and, in more detail, Table 4 represent the trend of ionic radii of these large cations which prefer formal coordination numbers in the range of 8-12 [77]. For example, considering the effective Ln(III) radii for 9-co-ordination, a discrepancy of 0.164 A allows the steric fine-tuning of the metal center [60]. The structural implications of the lanthanide contraction can be visually illustrated by the well-examined homoleptic cyclopentadienyl derivatives (Fig. 2) [78], Three structure types are observed, depending on the size of the central metal atom A, [( j5—Cp)2Ln(ji— 5 rf — Cp)] x, 1 < % < 2 B Ln(fj5 —Cp)3 C, [fo -CpJjLnCi- 1 ff1—Cp)], these exhibit coordination numbers of 11 (10), 9, and 8, respectively. Also a small change in ligand substitution leads to a change in coordination behavior and number (10), as...

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