While nanoparticle polarizability increases linearly with volume, the polarizability per atom was found to, in general, decrease as particle size increases, as shown in Fig. . This is due in part to the larger fraction of surface atoms in smaller particles, and the distinct contribution from surface and core atoms to properties such as α. In addition to the downward trend, there are large fluctuations from particle to particle, with more elongated particles displaying larger polarizabilities than their higher-symmetry neighbors. We note that the elongated particles also tend to have a larger percentage of surface atoms. Thus, instead of considering only the number of atoms or electrons in a particle, we consider the electron delocalization volume. This volume depends on the electronic structure of the particle [see Fig. (a)], and accounts for the distinction between surface (larger volume) and core (smaller volume) atoms. We used a least squares procedure to fit the data in Fig. (a) to Eq. 7, assuming spherical volumes. The fit assigned the particles a relative permittivity of 12.9. While dependent on the precise definition of particle volume, this value is a consistent characteristic of all particles surveyed when using our volume definition, yielding R 2 = 0.997.