# How do alpha particles cause ionisation?

An $\alpha$-particle is effectively a helium nucleus, that is 2 protons and two neutrons- without its two valence electrons.
As it is highly electron-deficient it will rip electrons from other molecules or atoms to become a stable helium atom, but in taking electrons from another molecule or atom, the $\alpha$-particle turns that molecule or atom into a cation- a positively charged particle because it just lost its electrons.
The $\alpha$-particle is considered as being strongly ionizing but has a very low penetration power, and can be stopped by a sheet of paper, or even by the air (As the particle will rip electrons from the oxygen and nitrogen in the air).