The distance between two atoms is measured by the distance between their nuclei, the internuclear distance.

The most efficient packing of atoms in a plane results when the atoms are arranged in a hexagonal array with each atom touching six nearest neighbors. The number of nearest neighbors is called the coordination number. Thus atoms in a closest packed plane have a coordination number of six. A less efficient arrangement is the square array. Each atom has a coordination number of four in the more open square array.

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