With a charged, heavy ion, you accelerate it in an electrical field and shoot it out the rear of your rocket. The thrust you get depends on the mass of the atom, its terminal velocity when it leaves the accelerating potential, and the number of ions in the flow...in other words the current. Modern ion engines can achieve accelerations of only a few hundredths of the acceleration of gravity, and are used in many satellites for 'station keeping' to keep them in the same orbit with gentle nudges every once in a while. These engines need simply a source of electricity...solar panels etc, and a supply of ions to accelerate, and in principle you can keep them running continuously for months to get large, interesting accelerations for interplanetary travel. None have been used in that way though. It is just a matter of time.