A linear chain is made up of sodium and chlorine atoms alternately placed with neighboring equilibrium positions separated by 0.281 nm. A Na atom has a mass of 3.82x10-26 Kg and a Cl atom has a mass of 5.89x10-26 Kg. Suppose the atoms interact electrostatically, with each sodium atom having a net charge of e and each chlorine of -e. Estimate the maximum and minimum angular frequency in each branch of the dispersion relation (i. e., acoustical and optical branches).