Oxides of the heavier elements

Sulfur trioxide, for which the structure S=0 in resonance with 

The sulfur bond angles, about 125° for the two unshared oxygen 

Bonds with Partial Double-Bond Character 

—The structure of the molecules P40 and As406. Large circles represent phosphorus or arsenic atoms, small circles oxygen atoms. 

—The structure of the molecules P4O10 and P4O6S4, showing the positions of attachment of the four oxygen or sulfur atoms to the P4O6 framework. 

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The oxides of the heavier elements (Ca, Sr, Ba), made by decomposition of their carbonates at temperatures of about 900°C have appreciable semiconductivity, and strong electron emission at high temperatures. Single crystals are generally grown by vapor or solution techniques, because of the high melting points and reactivity of the compounds (e.g., BaO melts at 2196°C). 

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 ... 

Fig.6.1 depicts the Ebsworth diagram of N and P in acidic solution as well as N in basic solution. The top two elements in this group N and P are typical non-metals. The metallic character, which appears in the heavier elements, increases down the group, although the conductivity of solid Bi is not high. The typical oxidation numbers of all the members of the group are +3 and +5 but the stability of the +3 state in Bi is greater than the +S state (inert pair effect). The chemistry of N and P is dominated by covalent bond formation. On the other hand, ionic compounds of Bi(III) are the common bismuth compounds. The oxides of N and P are acidic in nature (except the neutral N2O and NO), the amphoteric nature becomes apparent in the oxides of the heavier elements. 

The effect of the CFSE is expected to be even more marked in the case of the heavier elements because for them the crystal field splittings are much greater. As a result the +3 state is the most important one for both Rh and Ir and [M(H20)6] are the only simple aquo ions formed by these elements. With rr-acceptor ligands the +1 oxidation state is also well known for Rh and Ir. It is noticeable, however, that the similarity of these two heavier elements is less than is the case earlier in the transition series and, although rhodium resembles iridium more than cobalt, nevertheless there are significant differences. One example is provided by the +4 oxidation state which occurs to an appreciable extent in iridium but not in rhodium. (The ease with which Ir, Ir sometimes occurs... 

Magnetic properties. Whereas a simple interpretation of magnetic susceptibilities of tlie compounds of first transition series elements usually gives the number of unpaired electrons and hence the oxidation state and d orbital configuration, more complex behavior is often encountered in compounds of the heavier elements. 

The composition and properties of the oxides and oxoacids of the heavier elements of the group are generally similar to those formed by the third-period element. 

The iodides of the alkaU metals and those of the heavier alkaline earths are resistant to oxygen on heating, but most others can be roasted to oxide in air and oxygen. The vapors of the most volatile iodides, such as those of aluminum and titanium(II) actually bum in air. The iodides resemble the sulfides in this respect, with the important difference that the iodine is volatilized, not as an oxide, but as the free element, which can be recovered as such. Chlorine and bromine readily displace iodine from the iodides, converting them to the corresponding chlorides and bromides. 

The ricji oxoacid chemistry of sulfur (pp. 705-21) is not paralleled by the heavier elements of the group. The redox relationships have already been summarized (p. 755). Apart from the dark-brown hydrated monoxide Po(OH)2 , which precipitates when alkali is added to a freshly prepared solution of Po(ll), only compounds in the +4 and +6 oxidation states are known. 

Group 13/III is the first group of the p block. Its members have an ns np1 electron configuration (Table 14.5), and so we expect a maximum oxidation number of +3. The oxidation numbers of B and A1 are +3 in almost all their compounds. However, the heavier elements in the group are more likely to keep their s-electrons (the inert-pair effect, Section 1.19) so the oxidation number +1 becomes increasingly important down the group, and thallium(I) compounds are as common as... 

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