Final answer:
Materials with higher densities are effective at stopping X-rays because they absorb more X-ray photons resulting in darker images. The energy of the X-ray photon also plays a role, as higher energy photons can penetrate more deeply into material.
Step-by-step explanation:
The properties of materials that make them effective at stopping X-rays are primarily related to the density of the material and the energy of the X-ray photon. Materials with higher density, like bones, absorb more X-rays and therefore create darker shadows on the X-ray images. The higher the energy of the X-ray photon, the more penetrating power it has, thus requiring greater density to be effectively stopped. This is why X-rays are excellent for imaging dense structures in the body such as bones or identifying breaks and tumors that differ in density from their surroundings.