Crystal ionic radius

From the van der Waals radii of Table 2-1 and the ionic crystal radius of Ca2+ of 0.10 nm, we can estimate an approximate distance between the centers of positive and negative charge of 0.25 nm in both cases. It is of interest to apply Coulomb s law to compute the force F between two charged particles which are almost in contact. Let us choose a distance of 0.40 nm (4.0 A) and apply Eq. 2-7. 

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