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.
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