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

The next two, Ca and Sr, have close packing at room temperature, but change to body centering above 600°C; the largest alkaline earth, Ba, always uses the open bcp structure. The metals in Groups IIIA and IVA have a more-or-less distorted cp structure because the forces between atoms are more covalent than the forces in Group IA and IIA metals. The borderline with nonmetals is not far off.

The first nonmetal in Row 2, boron, is encountered at the top of Group IIIA. It has a complex three-dimensional covalent cage structure. The no-man's-land between metals and nonmetals is especially dramatic in Groups IVA, VA, and VIA, with the lighter elements being nonmetals, the heavier elements at the bottom of the table metals, and a transition zone separating them. In this zone, one element often has two different crystal forms, or allotropes, one metallic and the other nonmetallic. The nonmetallic allotrope is held together by covalent bonds with all electrons localized, whereas the metallic form usually will show the dark color and metallic luster that indicate the presence of mobile electrons.

Body-centered packing (bcp)


Close packing (cp)

(a) Cubic

(b) Hexagonal

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