> [xenon is] great for in-space propulsion because it’s fairly heavy (so you get more ooomph per atom)

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.