|The wake of an alpha particle|
An alpha particle is some 7300 times heavier than an electron. It undergoes practically no change in direction when it ionises other atoms. It leaves behind it, strewn around its straight path, a wake of ejected electrons and ionised atoms. Occasionally, when the electron is released at great speed, it too develops the ability to ionise. Finally, alpha particles do sometimes collide with other atomic nuclei, resulting in a deflection at large angles. These collisions are very rare, but were what allowed Ernest Rutherford to deduce the existence of the nucleus.