What dense metal is used in car batteries and radiation shielding?

Lead is the correct answer because it is widely used in both car batteries and radiation shielding due to its density and effectiveness. In car batteries, particularly lead-acid batteries, lead serves as both the positive and negative plates, facilitating efficient electron transfer and storage of electrical energy. The construction of these batteries relies on lead's ability to generate a high voltage and manage electrical currents effectively.

Moreover, lead's high atomic number makes it an excellent choice for radiation shielding. It is particularly effective at blocking gamma rays and X-rays, making it a common material used in radiation shielding in medical facilities and nuclear applications. This property is attributed to its density; as denser materials are better at absorbing and scattering radiation.

While other materials mentioned, such as mercury, aluminum, and zinc, have specific uses in different applications, they do not possess both the dense nature needed for effective radiation shielding and the functional qualities required for batteries to the same extent as lead.