Which radiation type is least able to penetrate human skin?

Alpha particles are the least capable of penetrating human skin primarily due to their large mass and positive charge. Composed of two protons and two neutrons, they have a relatively low energy compared to other types of radiation. Their size and charge result in a high likelihood of interaction with matter, which leads to rapid energy loss as they collide with atoms in the skin. Consequently, alpha particles can be stopped by just a few centimeters of air or even the outer layer of human skin, making them relatively harmless unless they enter the body through ingestion or inhalation.

In contrast, gamma rays and X-rays are forms of electromagnetic radiation that have no mass or charge, allowing them to penetrate much more deeply into human tissues. Beta particles, which are high-energy electrons or positrons, are more penetrating than alpha particles but still have limitations; they can typically travel through skin but are less penetrating than gamma rays and X-rays. Thus, alpha particles represent the least penetrating type of radiation when considering direct contact with human skin.