Which of the following elements is not a metal?

The element that is not a metal among the choices is helium (He). Helium is a noble gas, which means it is located in group 18 of the periodic table. Noble gases are characterized by their lack of metallic properties, as they generally exist as stable, colorless, odorless gases at room temperature and do not conduct electricity.

In contrast, the other elements listed—aluminum (Al), sodium (Na), and silver (Ag)—are all metals. Metals are typically good conductors of electricity and heat, possess luster, and have higher densities than nonmetals. Aluminum is a lightweight metal used widely in construction and packaging, sodium is a reactive alkali metal, and silver is known for its high electrical conductivity and is often used in jewelry and electronics.

Thus, identifying helium as the non-metal highlights the distinct properties that differentiate metals from nonmetals in the periodic table.