Which type of radiation is least likely to penetrate human skin?

Alpha particles are the type of radiation least likely to penetrate human skin due to their physical properties. Composed of two protons and two neutrons, alpha particles are relatively large and carry a positive charge. When alpha particles collide with matter, they lose energy quickly because of their mass and charge interaction with electrons in nearby atoms. This results in a very short range before they come to a stop.

In terms of penetration depth, alpha particles can be stopped by just a few centimeters of air or by a layer of human skin, making them unable to penetrate into deeper tissues. While they can cause significant biological harm if ingested or inhaled, outside of the body, they pose minimal risk due to their inability to penetrate the skin.

In contrast, beta particles are smaller and can penetrate further into human tissues, though they can still be stopped by materials like plastic or glass. Gamma rays and X-rays, being forms of electromagnetic radiation, have more energy and greater penetration capabilities, allowing them to pass through human tissue and even dense materials like lead to some extent. Therefore, understanding the properties of these different types of radiation is crucial for evaluating their potential health risks.