Metallic character

Metallic character increases from top to bottom in a gronp and decreases from left to right within a period. 

Metalloids are elements with properties intermediate between those of metals and nonmetals. Becanse the definition of metallic character depends on a combination of properties, there may be some variation in the elements identified as metalloids in different sonrces. Astatine (At), for example, is listed as a metalloid in some sonrces and a nonmetal in others. 

Malleability is the property that allows metals to be pounded into thin sheets. Ductility, the capacity to be drawn out into wires, is another characteristic of metals, 

4 Which element, K or Ca, will have the greater IE, which will have the greater / , and which will have the greater EAi  

As we move to the right across a row (or period) in the periodic table, metallic character decreases. 

As we move down a column in the periodic table, ionization energy decreases, making electrons more likely to be lost in chemical reactions. Therefore, 

The rare-earth metals, elements 58 to 71, lying between lanthanum and hafnium, have partially filled 4f orbitals. They are not given places in the form shown but may be accommodated by further expanding the Periodic Table. All of these elements have a principal valence of +3 and certain of them may have +2 or +4 valences also. Descriptions of the chemistry of these metals appear in a number of reference books and more advanced works. 

Already familiar to the student is the crude but important generalization, Metallic character of the elements in the periodic classification decreases in progressing from left to right in the table but increases in progressing from top to bottom. The term metallic character is a rough and qualitative combination of a number of more specific properties—for example, electrical and thermal conductivity, oxidation potential, basicity of the respective oxide, etc. 

TRENDS WITHIN THE PERIODIC TABLE THE RakH OASES 

The above generalization loses its usefulness if applied to the transition elements. One may say that magnesium is more metallic than silicon., which is, in turn, more metallic than sulfur we would, however, have some trouble in showing that chromium was more metallic than iron, which was, in turn, more metallic than zinc. Similarly, metallic character in Group Vb may be said to decrease in the order  

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Figure Bl.19.5. Tunnelling/-Fcurves acquired across a defect on Si(lOO). Away from the defect a bandgap can be seen. Over the defect itself, the bandgap disappears, suggesting that it possesses metallic character.

A century ago, Mendeltef used his new periodic table to predict the properties of ekasilicon , later identified as germanium. Some of the predicted properties were metallic character and high m.p. for the element formation of an oxide MOj and of a volatile chloride MCI4. 

Antimony(V) oxide can be prepared by treating antimony with concentrated nitric acid. It is an oxidising agent and when gently heated loses oxygen to form the trioxide. (The change in oxidation state stability shown by antimony should be noted since it corresponds to increasing metallic character.)... 

The elements, sulphur, selenium and tellurium form both di- and tri-oxides. The dioxides reflect the increasing metallic character of... 

Tellurium chemistry is often significantly different in comparison with selenium chemistry owing to the larger size and metallic character of tellurium (Section 1.1). As an illustration, the cyclocondensation of... 

Reactions yielding compounds of the type ArwXCl3 w (X=As, Sb, or Bi) are closely related to the Nesmeyanov reaction. Bismuth has the most metallic character of that group of elements. Waters s experiments with bulk bismuth (1937, 1939) did... 

The low ionization energies of elements at the lower left of the periodic table account for their metallic character. A block of metal consists of a collection of cations of the element surrounded by a sea of valence electrons that the atoms have lost (Fig. 1.53). Only elements with low ionization energies—the members of the s block, the d block, the f block, and the lower left of the p block—can form metallic solids, because only they can lose electrons easily. 

Metals typically form basic oxides and nonmetals typically form acidic oxides, but what about the elements that lie on the diagonal frontier between the metals and nonmetals Along this frontier from beryllium to polonium, metallic character blends into nonmetallic character, and the oxides of these elements have both acidic and basic character (Fig. 10.7). Substances that react with both acids and bases are classified as amphoteric, from the Greek word for both. For example, aluminum oxide, A1203, is amphoteric. It reacts with acids ... 

The elements show increasing metallic character down the group (Table 14.6). Carbon has definite nonmetallic properties it forms covalent compounds with nonmetals and ionic compounds with metals. The oxides of carbon and silicon are acidic. Germanium is a typical metalloid in that it exhibits metallic or nonmetallic properties according to the other element present in the compound. Tin and, even more so, lead have definite metallic properties. However, even though tin is classified as a metal, it is not far from the metalloids in the periodic table, and it does have some amphoteric properties. For example, tin reacts with both hot concentrated hydrochloric acid and hot alkali ... 

Metallic character increases significantly down Group 14/TV. 

Metallic character increases down Group 16/Vl as electronegativity decreases. 

The Niccolite Structure. The substances which crystallize with the niccolite structure (B8) are compounds of transition elements with S, Se, Te, As, Sb, Bi, or Sn. The physical properties of the substances indicate that the crystals are not ionic, and this is substantiated by the lack of agreement with the structural rules for ionic crystals. Thus each metal atom is surrounded by an octahedron of non-metal atoms but these octahedra share faces, and the edges of the shared faces are longer than other edges (rather than shorter, as in ionic crystals). Hence we conclude that the bonds are covalent, with probably some metallic character also. 

Our consideration of Eq. 5 accordingly accounts in a general way for the atomic arrangements of most of the metals, in addition to providing a criterion for metallic character. 

A requirement for metallic character is that unsynchronized resonance of covalent bonds occur, which means that M and M° have an unoccupied orbital available to accept an additional bond, changing them to M° and M, respectively. M does not need the extra orbital, because it cannot change to M2-. A hyperelectronic metal is one in which the number of outer electrons is greater than the number of outer orbitals, not including the metallic orbital. An example is metallic tin, with 14 outer electrons and 9 outer orbitals (6j, three 6p, five 5d). Sn+ and Sn° have five unshared electron pairs, and Sn has six. Sn+ and Sn form three covalent bonds, and Sn° forms two. Sn+ and Sn° have a metallic orbital, and Sn does not. They may be represented as... 

Furthermore, the strongly metallic character of selenium weakens the C-Se bond and thus favors reactions involving opening of the ring. The basicity of the three heterocycles is approximately in the same order, the nitrogen atom of selenazole and thiazole possessing much the same properties as the heteroatom of pyridine. Of the two carbon atoms ortho to nitrogen, that is, the 2-carbon and the 4-carbon, only the one in the 2-position is fairly active as a result of its interaction with selenium or sulfur. The 4- and 5-positions of thiazole and selenazole are more susceptible to electrophilic substitution than the 3- and 5-positions of pyridine. This is particularly true of the 5-position of selenazole. Thus it can be said that the 2- and 5-positions of the selenazoles and thiazoles... 

Electrical-conductivity measurements (F16, F18, V13) confirmed the metallic character of graphite-AsFg. The second stage has a peak, a-axis conductivity of 6.3 x 10 cm , i.e., marginally higher than... 

Resistivity measurements on single crystals of AuTe2l and AuTe2Br established the metallic character of the conductivity. Single crystals... 

The brass-colored PdTel consists of Jahn-Teller distorted PdTe2/2l4/4 octahedra, which are interconnected by common edges and corners to afford a loose, spatial network. The compound is considered to be ionic, containing Pd and Te , although the observed diamagnetism, electronic conduction, and color suggest some metallic character. 

When particles are arranged in an FCC structure, as shown in Figure 3, the I V) curve shows a linear ohmic behavior (Fig. 9C). The detected current, above the site point, markedly increases compared to data obtained with a monolayer made of nanocrystals (Fig. 9C). Of course, the dIldV(Y) curve is flat (inset Fig. 9C). This shows a metallic character without Coulomb blockade or staircases. There is an ohmic connection through multilayers of nanoparticles. This effect cannot be attributed to coalescence of nanocrystals on the gold substrate, for the following reasons ... 

Basic physical properties of sulfur, selenium, and tellurium are indicated in Table 1.3. Downward the sulfur sub-group, the metallic character increases from sulfur to polonium, so that whereas there exist various non-metallic allotropic states of elementary sulfur, only one allotropic form of selenium is (semi)metallic, and the (semi)metallic form of tellurium is the most common for this element. Polonium is a typical metal. Physically, this trend is reflected in the electrical properties of the elements oxygen and sulfur are insulators, selenium and tellurium behave as semiconductors, and polonium is a typical metallic conductor. The temperature coefficient of resistivity for S, Se, and Te is negative, which is usually considered... 

The trend toward more metallic character of the elements in Group 16 is complete at polonium, which has two allotropes, both with typically metallic structures a-cubic, which converts at 36 °C to P-rhombohedral (m.p. 254 "C). 

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