Size and Dipole Moment of Water Molecules in Solution

Size and Dipole Moment of Water Molecules in Solution 

In discussions of the radius of a molecule, the only well-defined, exact part of the answer lies in the intemuclear distances (e.g., between H and O for water), but this distance comprises only part of the radius, albeit the main part. All other measures of the radius of a molecule are affected by some aspect of the occupancy of space that is connected with the packing of the molecule, which varies with circumstances. 

In ice (where X-ray measurements of intemuclear distances, d, are more exact than those in the liquid), the 0-0 intemuclear distance is accurately known 280 pm. The distance between the nucleus of oxygen and that of hydrogen (see a space-fitting picture of water, Fig. 2.9) is 138 pm. 

The so-called van der Waals radius of H can be taken to be 52.8 pm (this is the Bohr radius for the ground state of H). The van der Waals radius of O is 115 pm and from these values 168 pm is obtained for the radius of water. 

several other radii between ca. 140 and 193 pm can be obtained by equating 4/3 Ttr to certain volumes (e.g., the close-packed volume of water, or a volume based on the molecular dimensions). Each of these radii is applicable as the circumstances dictate. 

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