or are they 1/8 of a sphere at the corners and 1/4 of a sphere at the edge?
In solid chemistry, 'spheres' are totally symbolic. A sphere is used to denote an ion/atom in that particular space. Microscopically, no definite or predicted shape for these entities exist. According to the Atomic orbital theory, the region of a molecule we denote is basically a region where the probability of finding a part of the molecule is quite high. We denote such a region by a sphere.
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