Question

why do alpha particles have high ionising power?

Answer 1

alpha particles are big, strong, and active! An alpha particle is about 16,000 times more massive than an electron and has twice their charge, but of opposite sign. Therefore, as alpha particles go through a material, they're ripping electrons off of the atoms and molecules within that material left and right without breaking a sweat, ionizing all of those poor atoms and molecules in the process.