10. Playing with a Full Deck:
       The Periodic Table
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       The Structures of the Elements

The Group VIA elements have two unpaired electrons and need to make two bonds to neighbors. Oxygen is small enough to make a double bond with a single neighboring atom in gas, but sulfur is too large to do so, and must settle for rings of eight atoms, each connected to two neighbors (lower right). This is the structure of molecules of yellow crystalline sulfur. In the darker, amorphous sulfur allotrope, these rings break open and link together into helical chains. These structures also are the basis for two crystalline allotropes of selenium: red, nonmetallic Se with eight-membered rings, and metallic Se with endless helices. In these Se helices the delocalization of electrons is confined to one chain, and selenium in effect is a "one-dimensional metal." Tellurium has only the metallic chain form, with the chains more closely packed. Polonium carries this compacting farther until the atoms form a simple cubic structure, the only example of such a structure known. (The radioactivity of Po makes it difficult to examine its crystal structure, because the alpha particles emitted during radioactive decay heat the crystal and destroy it.)

Allotropy and the metal-nonmetal borderline vanish in Group V IIA. (Astatine may be a borderline element, but is difficult to study for the same reason Po is.) All of the halogens form simple diatomic molecules and, depending on their size, are gases ( and ) , liquid (), or solids and ) at room temperature. Iodine does have a metallic luster to its crystals, which indicates a less firm grip on electrons than the lighter halogens have. The noble gases in Group 0 all are monatomic.

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