is determined by the distance between the centers of adjacent atoms or ions in a crystal structure.

This distance is not fixed because electron positions are not fixed and therefore, the radii really represent the area around the nucleus where there is a good probability of finding orbiting electrons.

Radii may vary for a particular element or ion due to external influences such as:
1. pressure and temperature
   • high pressure/low temperature - smaller radius
   • low pressure/high temperature - larger radius

2. number and charge of coordinating ions
3. bond type

Effective radii vary between different elements or ions according to the following rules:

1. in a given group (column in periodic table), ionic radii increase with increasing atomic number because the number of filled shells progressively increases

2. for positive ions of the same electronic structure but increasing atomic number - ionic radii decreases because of increasing nuclear charge

3. for an element that can exist in several valence states, the ionic radii decrease with increasing positive charge

Interionic Radii or "Bond Length" is calculated using Coulomb's Law: F=k[(q1*q2)/r2] where: F = atrractive force k = constant q = charge on ion r = interionic radius

Click Here for a Periodic Table of the Elements with Atomic and Ionic Radii

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