Atomic Radii. II

Abstract

X-ray data show that the two assumptions usually involved in previous calculations of atomic radii from crystal structures—namely (1) that the distance between adjacent atomic centers in crystals equals the sum of the two atomic radii, and (2) that the radii of atoms of the same element in different crystals are equal—in many cases cannot be correct. Assuming that a valence electron pair can be assigned a definite position and defining the atomic radius as the distance from the nucleus to a valence electron pair, it is evident that (1) is only true when there is a valence pair on the straight line joining the centers of the adjacent atoms, and (2) should be true, or nearly so, only when the atoms of the given element are structurally similar and similarly surrounded in the two crystals. The probable arrangements of valence electrons in crystals, as deduced elsewhere by the author, are used in deciding as to the validity of the above assumptions in particular cases.