Which of the following have little penetrating power and are normally a hazard to health only in the form of internal radiation?

Alpha particles are indeed characterized by their limited penetrating power. They consist of two protons and two neutrons, which makes them relatively large and heavy compared to other types of radiation. Because of this size, alpha particles can be easily stopped by a sheet of paper or even the outer layer of human skin. Their ability to penetrate materials is very low.

However, when alpha-emitting substances are ingested or inhaled, they can present significant health risks. Inside the body, these particles can interact with biological tissues at a very close range, causing significant damage to cells and potentially leading to serious health issues such as cancer. This potential for harm is primarily why alpha radiation is considered hazardous primarily in the context of internal exposure, rather than external exposure. Since alpha particles cannot penetrate the skin, their health risks are not typically a concern unless the source of radiation is inside the body.

This characteristic distinguishes alpha particles from other types of radiation, like gamma rays or X-rays, which can penetrate deeply into materials and tissues, posing an external hazard.