Atom sizes and coordination

For example, the lattice parameter a of a-iron is 2.87 A, and in a BCC lattice the atoms are in contact only along the diagonals of the unit cube. The diameter of an iron atom is therefore equal to one half the length of the cube diagonal, or (V3/2)o = 2.48 A. The following formulas give the distance of closest approach in the three common metal structures  

Values of the distance of closest approach, together with the crystal structures and lattice parameters of the elements, are tabulated in Appendix 5. 

This means, for example, that the diameter of an iron atom is greater if the iron is dissolved in FCC copper than if it exists in a crystal of BCC a-iron or is dissolved in BCC vanadium. If it were dissolved in copper, its diameter would be approximately 2.48/0.97, or 2.56 A. 

The size of an atom in a crystal also depends on whether its binding is ionic, covalent, metallic, or van der Waals, and on its state of ionization. The more electrons are removed from a neutral atom the smaller it becomes, as shown strikingly for iron, whose atoms and ions Fe, Fe, Fe have diameters of 2.48, 1.66, and 1.34 A, respectively. 

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