Question

Why do alpha particle have a high ionising power?

Answer 1

The atomic number of helium is 2 so an alpha particle carries a double positive charge. Alpha particles don't travel very fast – because they have such a large mass. This means that they tend to collide with plenty of other atoms. Therefore, they cause a lot of ionisation by pulling electrons off the atoms.

Answer 2

alpha particles have double charge and hence more ionising power

simply more charge on particle , more ionising power

hope it is clear to you