Final answer:
1. Alpha particles are the least penetrating form of ionizing radiation; easily stopped by paper or skin. Beta particles are more penetrating but can be blocked by aluminum. Gamma rays are the most penetrating, requiring dense shielding like lead.
Step-by-step explanation:
The form of ionizing radiation that is the least penetrating is alpha particles. These particles are relatively large because they consist of two protons and two neutrons, equivalent to the nucleus of a helium atom. Due to their size and charge, they interact strongly with matter and are stopped by simple barriers such as a sheet of paper, clothing, or even the dead outer layer of human skin. They exhibit high ionizing power because they can strip electrons from atoms they encounter, thus changing the atom's chemical properties. However, their penetration power is the lowest among the types of ionizing radiation when compared to beta particles and gamma rays.
While beta particles can penetrate materials to some extent, they still have significantly less penetrating power than gamma rays. Beta particles can be halted by a few millimeters of aluminum. On the other hand, gamma rays are the most penetrating and can pass through several inches of dense materials like lead. Gamma rays are a type of electromagnetic radiation, similar to X-rays but more powerful, having no mass or charge and high penetration capability.
In summary, among alpha particles, beta particles, and gamma rays, alpha particles are the easiest to shield against due to their minimal penetration capabilities. Proper understanding of the properties of these radiations is vital for ensuring safety when handling radioactive materials.