An alpha particle has a much larger volume than a β particle, so when passing by an atom it is more likely to collide with an electron and knock it out of the atom causing the atom to be ionised (meaning it is now an ion) and so more ions are created, making it more ionising than a β particle. Another reason why α particles cause more ionisation is because it has more charge. An α particle has a charge of +2, which is the opposite charge to the electrons orbiting the atoms, so the α particle attracts the electrons and pulls them away from the atom, leaving it with fewer electrons, making it an ion. On the other hand, β particles are electrons, so they have the same charge as the electrons orbiting the atom, so they will only push electrons back towards the atom, meaning it is unlikely that β particles iononise through this method.