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

We can use the structures of the elements as a means of surveying the entire table. The diagram at the left shows the structures of the representative elements, and the transition metals are similar to the metals shown.

The metals all are packed spheres of positive ions held together by mobile electrons. In close packing (cp), the densest way of packing marbles or other spherical objects, each atom has twelve nearest neighbors touching it, and 74% of the volume of the metal is occupied by atoms. (There are two main kinds of close packing, cubic and hexagonal, which differ only in the way that the individual close-packed layers of atoms are stacked.) In a slightly less dense form, bodycentered packing (bcp), each atom has eight neighbors in contact with it in the directions of the corners of a cube, and the atoms occupy only 68% of the total volume.

Close packing is favored in metals with small atoms and many electrons to hold them; body-centered packing is found in metals with larger atoms and fewer electrons to bind them together. It is not uncommon for a metal to show a close-packed structure at low temperatures, and change to a body-centered structure at higher temperatures when the atoms are vibrating more freely. The alkali metals in Group IA all have the bcp structure at room temperature; but the smallest two, Li and Na, change to close packing below -200°C. Among the alkaline earths (Group IIA), which are held together by two electrons per atom, the smallest two, Be and Mg, always use the dense cp structure.

Body-centered packing (bcp)


Close packing (cp)

(a) Cubic

(b) Hexagonal

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