ATOMIC RADII OF THE ElEMENTS The simple model of an atom as a hard sphere that can approach The Cambridge Crystallographic Data Center also makes use only to a fixed distance from another atom to which it is not bond- of a set of “covalent radii” to determine which atoms in a crystal ed has proved useful in interpreting crystal structures and other are bonded to each other . Thus two atoms A and B are judged to molecular properties . The term van der Waals radius, rvdw, was be connected by a covalent bond if their separation falls within a originally introduced by Pauling as a measure of this atomic size . tolerance of ±0 .4 Å of the sum rcov (A) + rcov (B) . The covalent radii Thus in a closely packed structure two non-bonded atoms A and are given in the fourth column of the table . B will be separated by the sum of their van der Waals radii rvdw (A) and rvdw (B) . The set of van der Waals radii proposed by Pauling References was refined by Bondi (Reference 1) based on crystallographic data, gas kinetic collision cross sections, and liquid state properties . The 1 . Bondi, A ., J. Phys. Chem. 68, 441, 1964 . non-bonded contact distances predicted from the recommended 2 . Rowland, R . S . and Taylor, R ., J. Phys. Chem. 100, 7384, 1996 . 3 . Cambridge Crystallographic Data Center, www .ccdc .cam .ac .uk/prod- r of Bondi have been compared with actual data in the collec- vdw ucts/csd/radii/ tion of the Cambridge Crystallographic Data Center by Rowland and Taylor (Reference 2) and modified slightly . Their recommend- ed van der Waals radii are given in the third column of this table . Element Symbol rvdw/Å rcov/Å Element Symbol rvdw/Å rcov/Å Element Symbol rvdw/Å rcov/Å Actinium Ac 1 .88 Hafnium Hf 1 .57 Promethium Pm 1 .80 Aluminum Al 1 .35 Helium He 1 .40 Protactinium Pa 1 .61 Americium Am 1 .51 Holmium Ho 1 .74 Radium Ra 1 .90 Antimony Sb 1 .46 Hydrogen H 1 .09 0 .23 Rhenium Re 1 .35 Argon Ar 1 .88 1 .51 Indium In 1 .93 1 .63 Rhodium Rh 1 .45 Arsenic As 1 .85 1 .21 Iodine I 1 .98 1 .40 Rubidium Rb 1 .47 Astatine At 1 .21 Iridium Ir 1 .32 Ruthenium Ru 1 .40 Barium Ba 1 .34 Iron Fe 1 .34 Samarium Sm 1 .80 Berkelium Bk 1 .54 Krypton Kr 2 .02 Scandium Sc 1 .44 Beryllium Be 0 .35 Lanthanum La 1 .87 Selenium Se 1 .90 1 .22 Bismuth Bi 1 .54 Lead Pb 2 .02 1 .54 Silicon Si 2 .10 1 .20 Boron B 0 .83 Lithium Li 1 .82 0 .68 Silver Ag 1 .72 1 .59 Bromine Br 1 .85 1 .21 Lutetium Lu 1 .72 Sodium Na 2 .27 0 .97 Cadmium Cd 1 .58 1 .69 Magnesium Mg 1 .73 1 .10 Strontium Sr 1 .12 Cesium Cs 1 .67 Manganese Mn 1 .35 Sulfur S 1 .80 1 .02 Calcium Ca 0 .99 Mercury Hg 1 .55 1 .70 Tantalum Ta 1 .43 Californium Cf 1 .83 Molybdenum Mo 1 .47 Technetium Tc 1 .35 Carbon C 1 .70 0 .68 Neodymium Nd 1 .81 Tellurium Te 2 .06 1 .47 Cerium Ce 1 .83 Neon Ne 1 .54 Terbium Tb 1 .76 Chlorine Cl 1 .75 0 .99 Neptunium Np 1 .55 Thallium Tl 1 .96 1 .55 Chromium Cr 1 .35 Nickel Ni 1 .63 Thorium Th 1 .79 Cobalt Co 1 .33 Niobium Nb 1 .48 Thulium Tm 1 .72 Copper Cu 1 .40 1 .52 Nitrogen N 1 .55 0 .68 Tin Sn 2 .17 1 .46 Curium Cm 0 .99 Osmium Os 1 .37 Titanium Ti 1 .47 Dysprosium Dy 1 .75 Oxygen O 1 .52 0 .68 Tungsten W 1 .37 Erbium Er 1 .73 Palladium Pd 1 .63 Uranium U 1 .86 1 .58 Europium Eu 1 .99 Phosphorus P 1 .80 1 .05 Vanadium V 1 .33 Fluorine F 1 .47 0 .64 Platinum Pt 1 .72 Xenon Xe 2 .16 Gadolinium Gd 1 .79 Plutonium Pu 1 .53 Ytterbium Yb 1 .94 Gallium Ga 1 .87 1 .22 Polonium Po 1 .68 Yttrium Y 1 .78 Germanium Ge 1 .17 Potassium K 2 .75 1 .33 Zinc Zn 1 .39 1 .45 Gold Au 1 .66 Praseodymium Pr 1 .82 Zirconium Zr 1 .56 9-49.