Single-particle longitudinal motion and pairwise lateral motion was investigated while the particles were excited by an oscillating electric field directed normally to an electrode proximate to the particles. The electrode was polarized over a range of potential insufficient to drive electrochemical reactions, a range called the "ideally polarizable region". The particles' motion was qualitatively dependent on the choice of electrolyte despite the absence of electrochemical reactions. As when electrochemical reactions were not explicitly excluded, the phase angle theta between particle height and electric field correlated with the particles separation or aggregation during excitation. A simple harmonic oscillator model of the particles' response, including colloidal and hydrodynamic forces and including the Basset force not previously cited in this context, showed how theta can increase from 0 degrees at low frequencies, cross 90 at similar to 100 Hz, and then increase to 180 degrees as frequency was increased. The model captured the essence of experimental observations discussed here and in earlier works. This is the first a priori prediction of theta for this problem.