Xenon is the most common, most popular propellant for ion engines.
But AFAIK, any ionizable gas could do - the ion engine accelerates charged particles, it doesn't really care what these particles are.
Also, Xenon is way down below the periodic table; its atoms quite heavy compared to "competitors", and that means reduction of specific impulse - applying the same energy to a lighter particle you're attaining higher exhaust velocity (at cost of thrust) - and ion engines are all about extreme ISp, so choice of a gas with so heavy particles seems quite counter-intuitive.
What properties of Xenon make it the propellant of choice for ion engines?
