In Pearson's theory, soft acids and bases have high values for this quantity, which is higher for iodide than for fluoride.
Normal modes that transform as a quadratic function of position will change this quantity. It's not volume, but this quantity squared, over radius to the sixth, times the harmonic mean of ionization energy, equals the long-range interaction energy in an ideal gas. This quantity must change for a vibration to be observed in Raman spectroscopy. In Pearson's theory, soft acids and bases have high values for this quantity, which is higher for iodide than for fluoride. (*)  London dispersion forces scale with this property, which is roughly proportional to molecular surface area. This quantity is the ratio of induced dipole moment to the external electric field. For 10 points, name this quantity symbolized alpha, which measures how easy it is to distort an electron cloud.
Normal modes that transform as a quadratic function of position will change this quantity.
electric  polarizability  [or word forms; or  alpha  until it is read; prompt on molecular  size  or  volume or similar answers] 