Periodic Anomalies of the Nonmetals and Posttransition Metals

It is generally assumed that the properties of the various families of the periodic chart change smoothly from less metallic (or more electronegative) at the top of the family lo more metallic (or less electronegative) at the bottom of the family. Certainly for the extremes of the chart—the alkali metals on the left and the halogens and noble gases on the right—this is true the ionization potentials, for example, vary in a rather monotonous way. This is not true for certain central parts of the chart, however. 

Reluctance of Fourth-Row Nonmetals to Exhibit Maximum Valence 

There is a definite tendency for the nonmetals of the fourth row—As, Se. and Br—to be unstable in their maximum oxidation stale. For example, the synthesis of arsenic pentachloride eluded chemists until comparatively recently.29 although both PCI, and SbCI, arc stable. The only stable arsenic penlahatide is AsF, AsCI, decomposes at -SO °C. and AsBr, and Asl, are still unknown. 

In Group VIA (16) the same phenomenon is encountered. Selenium trioxide is thermodynamically unstable relative to sulfur trioxide and tellurium trioxide. The enthalpies of formation of SF6, SeF6, and TeF6 are - 1210. -1117. and -1320 kJ mol-1, respectively. This indicates comparable bond energies for S—F and Te—F bonds (317 and 330 kJ mol-1, respectively), which are more stable than Se—F bonds (285 kJ mol 1). 

The best known exceptions to the general reluctance of bromine to accept a +7 oxidation state are perbromic acid and the perbromate ion, which were unknown prior to 1968 (see Chapter 17). Their subsequent synthesis has made their nonexistence somewhat less crucial as a topic of immediate concern to inorganic chemists, but bromine certainly continues the trend started by arsenic and selenium. Thus the perbromate ion is a stronger oxidizing agent than either perchlorate or periodate. 

---