Atomic spacing refers to the distance between the nuclei of atoms in a material. This space is extremely large compared to the size of the atomic nucleus, and is related to the chemical bonds which bind atoms together. 
The spacing between atoms in most ordered solids is on the order of a few angstroms (a few tenths of a nanometer). In very low density gasses (for example, in outer space) the average distance between atoms can be as large as a meter.
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