wrote:
snip
I believe the lighter the molecule in the exhaust, the more efficient
the engine? If so, it might be better to make pellets of silicon (MW=
14) as we process the asteroid, charge them, and accelerate them using
an electrostatic engine. Oxygen has a molecular weight of 16, would be
rather corrosive in a high-temp exhaust, and is more useful than
silicon for other purposes.

It's not quite this simple.
The velocity of gas molecules/atoms depends on the temperature and
rises with smaller particles at the same temperature.
This is why smaller is better.
If the particles are liquid, or solid, the mass is irrelevant.