About Ion Thruster Engines - physics

According to NASA,the most common propellant used in ion propulsion is the heavy neutral gas Xenon,which is easily ionized and has a high atomic mass,thus generating a desirable level of thrust when ions are accelerated.The thrusters work by using an electrical charge to accelerate ions from Xenon fuel to a speed 7-10 times the speed of chemical engines.An ion is defined as an atom or molecule that has gained or lost one of its valence eletrons,giving it a net positive or negative electrical charge.So it is a charged entity,as opposed to a neutral one.