Lower atomic mass gives better Isp for chemical rockets, for which the exhaust velocity is proportional to the temperature of the exhaust divided by the square

Lower atomic mass gives better Isp for chemical rockets, for which the exhaust velocity is proportional to the temperature of the exhaust divided by the square root of the atomic mass of the exhaust product. However, these satellites are using ion propulsion. In ion engines, the exhaust velocity is constant, so a higher atomic mass allows each atom / molecule to carry more energy. This makes higher mass fuels like Xenon better for ion engines. (Radon would be better still but the radioactivity poses problems for various systems.)