Which of the following elements is considered a metal?

Sodium is considered a metal because it is located in Group 1 of the periodic table, where the alkali metals are found. Metals are characterized by specific properties including high electrical and thermal conductivity, malleability, ductility, and a shiny appearance. Sodium, with its metallic luster and the ability to conduct electricity and heat, exemplifies these characteristics.

In contrast, helium is a noble gas and is located in Group 18; it is non-metallic and does not share the properties typical of metals. Chlorine, situated in Group 17, is a halogen and is also classified as a nonmetal, known for its reactivity. Oxygen, found in Group 16, is another nonmetal, essential for combustion and respiration, and displays very different properties from metals, such as forming gases and covalent compounds rather than conducting electricity. Thus, sodium stands out among the options provided as a representative of metallic elements.