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.