Why do most alpha particles fired through a piece of gold foil emerge almost undeflected, and why do others bounce backward? Most alpha particles pass through with no deflection because the majority of the space in gold is occupied by less massive electrons, but some hit and bounce backward from the dense atomic nuclei. Alpha particles behave like waves and pass freely through gold atoms, but are reflected if they fall on the boundary of two neighboring atoms. Most alpha particles pass through with no deflection because the majority of the space in gold has relatively low density, but some hit and bounce backward from the dense clouds of electrons. Alpha particles behave like waves and can be reflected by gold foil, but most of them pass through because gold has a relatively low reflectivity