A crystalline solid contains three types of ions, Na⁺, O^2-, and Cl^-. The solid is made up of cubic unit cells that have O^2- ions at each corner, Na⁺ ions at the center of each face, and Cl^- ions at the center of the cells. What is the chemical formula of the compound? What are the coordination numbers for the O^2- and Cl^- ions? If the length of one edge of the unit cell is a, what is the shortest distance from the center of a Na⁺ ion to the center of an O^2- ion? Similarly, what is the shortest distance from the center of a Cl^- ion to the center of an O_2- ion?