The Metal-Nonmetal Line

The electrical conductivity, a, of a metal is measured in I /L2 m and is the current density divided by the electric field strength. The resistivity, p, is the inverse of the conductivity. The less tightly the metal atom holds onto its electrons and the greater the overlap between its valence orbitals, the smaller the resistivity will be and the more conductive the metal. Therefore, metals with low first ionization energies and small atomic radii will make the best conductors. Among the transition metals, silver is the most conductive, with a room-temperature resistivity of 1.6 X I0 Q m. 

One of the most significant differences between the metals and the nonmetals besides their electrical conductivity is the acid-base properties of their oxides. Metal oxides are basic and react accordingly, as demonstrated by the reactions shown in Equations (5.13)-(5.15). Nonmetal oxides, on the other hand, act as acids, as shown in Equations (5.16)-(5.18). The oxides of the metalloids are amphoteric and can act either as acids or as bases, as shown by Equations (5.19) and (5.20)  

Chemical structures of BeClj and AICI3, showing how both metal ions have tetrahedral coordinations. 

With few exceptions, the metal oxides are ionic solids and react with water to form aqueous ions, the nonmetal oxides are network covalent solids that react with water to make covalent compounds, and the amphoteric oxides of the metalloids form oligomeric polar-covalent solids. Similar relationships hold for the hydrides and fluorides of each element, with the metal forming an ionic solid and the non-metal forming a network covalent solid, although the actual demarcation line varies somewhat depending on the anion. 

Elements to the lower left are metals elements to the upper right ate nonmetals. 

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Diagonal relationships are commonly observed between elements from the second and third series. This periodic trend is especially true for the following pairs of elements Li/Mg, Be/AI, and B/Si. While vertical periodic trends are still predominant, some properties match better along a diagonal. These diagonal periodic trends are no doubt related to the fact that the radius of an atom increases down and to the left in the periodic table, whereas I.E. and E.A increase up and to the right The diagonal nature of the metal-nonmetal line has already been discussed. 

The metal-nonmetal line is the fifth component of our network of interconnected ideas for understanding the periodic table. It is summarized in Figure 9.19, which shows both the diagonal stepwise line in the periodic table and the metalloid elements along that line. A color version of Figure 9.19 is shown inside the front cover of the book. In our future discussions, the metal-nonmetal line component of the network will be represented by the icon shown at left. It symbolically represents this jagged line that separates the metals from the nonmetals. 

To make sense out of the descriptive chemistry of the representative elements, we have defined and discussed the basis of the first five components of a network of interconnected ideas for understanding the periodic table. These organizing principles are (1) the periodic law, (2) the uniqueness principle, (3) the diagonal effect, (4) the inert-pair effect, and (5) the metal-nonmetal line. The definitions of these components are summarized in Table 9.5. The five components are also summarized in Figure 9.20. A color version of this figure is shown on the inside front cover of the text. 

The Metal-Nonmetal Line The division between metals and nonmetals is a stepwise diagonal line. Metals are found to the left of the line, nonmetals to the right of the line, and semimetals along the line. 

Mastering the descriptive chemistry of the main-group or representative elements of the periodic table is a formidable task. In order to bring some order to our study of this topic, we have started to construct a network of interconnected ideas. Five components have been described in this chapter. Three additional components will be defined and described in the next few chapters. The first five components are the periodic law, the uniqueness principle, the diagonal effect, the inert-pair effect, and the metal-nonmetal line. 

The fourth network component, the inert-pair effect, states that the valence ns electrons of main-group metallic elements, particularly those to the right of the second- and third-row transition metals, are less reactive than expected. These relatively inert ns pairs mean that elements such as In, Tl, Sn, Pb, Sb, Bi, and Po often form compounds where the oxidation state is 2 less than the expected group valence. The two major reasons for this effect are (1) larger-than-normal effective nuclear charges in these elements and (2) lower bond energies in their compounds. The fifth network component, the metal-nonmetal line, is just the division of the periodic table into metal, nonmetal, and metalloid regions. 

Sketch the icon that represents the metal-nonmetal line. Briefly explain how the icon symbolically represents the fifth component of the interconnected network of ideas for understanding the periodic table. 

A knowledge of the metal-nonmetal line helps us to make sense of the formal oxidation states in the binary hydrides. Figure 10.5 shows the M-NM line with the nonmetals positioned to its upper right and the metals to its lower left. The range of electronegativities of the nonmetals (2.1 to 4.0) and metals (0.7 to 2.0) are also shown. (The trends in electronegativity and their dependence on effective nuclear... 

These trends in the oxides and hydroxides are expected on the basis of our fifth and sixth network components, the metal-nonmetal line and the acid-base character of the metal (M) and nonmetal (NM) oxides in aqueous solution. 

Consistent with the position of the metal-nonmetal line (and the corresponding acid-base character of metal and nonmetal oxides), boron oxide is an acid anhydride, whereas the oxides of the heavier elements progress from amphoteric to basic in behavior. Boron oxide, then, reacts with water, as shown in Equation (14.2), to produce boric acid, B(OH)3 or H3BO3 ... 

The metal-nonmetal line passes through the heart of the group, with carbon being a bona fide nonmetal and lead a bona fide metal. In between are two metalloids (silicon and germanium) and a borderline metal (tin). The progression in the acid-base character of the oxides of the elements further emphasizes the trend from nonmetal to metal. The formulas of the oxides and halides show the increasing importance of the +2 oxidation state down the group, and this is reinforced by a consideration of standard reduction potentials. 

The Group 5A elements superimposed on the interconnected network of ideas. These include the periodic law, (a) the uniqueness principle, (b) the diagonal effect, (c) the inert-pair effect, (d) the metal-nonmetal line, the acid-base character of the metal (M) and nonmetal (NM) oxides in aqueous solution, the trends in reduction potentials and the variations in dtr-pir bonding that involve elements of the second and third periods. 

For the first time since the alkaline-earth metals, we encounter a group that is not divided by the metal-nonmetal line. Consequently, aU halogens are nonmetals, although iodine and probably astatine do show some signs of metallic character. For example, solid iodine exhibits a metallic luster and, under some conditions, a complexed H cation. At very high pressures, iodine is a conductor of electricity. But these are exceptions. The properties of these elements, including iodine, are indeed consistent with their classification as nonmetals. 

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