Final answer:
Alpha radiation is the slowest moving and least penetrating type of radiation, easily stopped by a sheet of paper or human skin, whereas gamma radiation is the most penetrating.
Step-by-step explanation:
The type of radiation emitted by radioactive nuclei that is the slowest moving and least penetrating is alpha radiation. Alpha particles consist of two protons and two neutrons, which makes them relatively large and heavy compared to other forms of radiation. Due to their size and the fact that they carry a positive charge, they interact strongly with matter and lose their energy quickly. This interaction limits their ability to penetrate materials, making them the least capable of penetrating compared to beta, neutron, and gamma radiation.
Alpha particles can be stopped by a simple sheet of paper or even the outer layer of human skin. In contrast, gamma radiation is the most penetrating, requiring dense materials like lead to be effectively shielded against. Beta particles are more penetrating than alpha particles but less so than gamma rays, and they can be stopped by a thin layer of metal, such as aluminum.