Which type of radiation has the least penetrating power?

Alpha particles possess the least penetrating power among the types of radiation listed. This is primarily due to their large mass and positive charge, which result in a strong interaction with matter. When alpha particles encounter materials, they are readily stopped by just a few centimeters of air or by the outer layer of human skin. Their inability to penetrate deeply into materials makes them less harmful externally, although they can be quite dangerous if ingested or inhaled, as they can cause significant damage to tissues from within.

In contrast, beta particles, being lighter and less charged than alpha particles, can penetrate further into materials, typically requiring denser substances or thicker layers to stop them effectively. Gamma rays and X-rays are even more penetrating, as they are forms of electromagnetic radiation with no mass and no charge, allowing them to easily traverse materials, including human tissue, making them much more penetrative than alpha and beta particles.