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There are, however, some metals, like manganese, mercury, gallium, and indium, that do not crystallise in the usual simple structures of metals but in much more complex structures.
FCC and hexagonal close-packed structures represent alternate ways of packing identical atoms as tightly as possible.
There is only one way in which a single layer of spheres of equal size may be packed as tightly as possible, and a second layer must be placed on top of the first so that its spheres will fit into the hollows between every three spheres in the layer below. But a third layer can be added in two different ways, both leading to the same tightness of packing but yielding totally different structures.
In both types of close packing each atom is surrounded by 12 others.
The structure of NaCl is characteristic also to many chlorides and oxides such as MgO, CaO, SrO and BaO.
This structure can be considered as arising from the penetration of the two FCC lattices. One is the sodium ion lattice and the other is the chlorine ion lattice.
The resultant structure gives 8 chlorine ions at the corners and 6 ions on the faces = 8 x + 6 x ½ = 4; similarly, 12 sodium ions located at the edges of the cube and one ion in the centre = 12 x ¼ + 1 = 4.
The total number of chlorine and sodium ions is 8, or 4 NaCl molecules.