With Radon it might even be conceivably possible (not sure how hard it is to get and if any restrictions apply because of its radioactivity), and it would work for a few years, because it has a half-life of 3.825 days (EDIT: this is of course complete nonsense, the "." is a decimal point, so it will only work for a few days). In the quantities needed for a gas tube (and as long as it stays in the tube!), I guess it should also be relatively safe, but I'm not an expert. Apparently it produces red light when used in a tube. Oganesson however has a half-life of 0.7 ms, so, aside from how expensive it would be to synthesize enough of it, it's doesn't stay around long enough for any experiments...
More specifically, for a given exhaust velocity and grid spacing, the space charge limited thrust density (thrust/area) of an ion engine scales as the square of the mass/charge ratio of the ions. So you really want heavy singly charged ions. This is completely unlike thermal rockets, where you want low molecular weight exhaust gases.
Plasma engines that accelerate a quasi-neutral plasma aren't subject to space charge limits, but even there heavy ions help because they reduce the energy used in ionizing the propellant per unit propellant mass.
[0]https://sci.esa.int/web/bepicolombo/-/60642-bepicolombo-mtm-after-xenon-loading
The BepiColombo number is similar, I think, to the amount of xenon made annually in nuclear reactors (where it occurs in spent fuel as the result of fission.)
In practice I think a combination of argon and CO2 is typically used for inert gas welding of steel.
About the only place I can think of in a plasma engine where you'd want to use light elements is if the engine is thermal: making a confined plasma, heating it to very high temperature, then expanding it through a magnetic nozzle. There, you'd want to use hydrogen to minimize radiative losses from the hot plasma, especially vs. using a plasma containing partially ionized atoms of higher atomic number elements; these can radiate fiercely.