What characteristic classifies elements as metals?

Elements are classified as metals primarily due to their ability to conduct heat and electricity efficiently. This characteristic stems from the structure of metal atoms, which have free electrons that can move easily through the lattice of metal ions. This movement of electrons allows metals to easily transfer energy, making them good conductors.

Furthermore, the presence of these free electrons contributes to other properties commonly associated with metals, such as their malleability and ductility. Metals generally exhibit a shiny appearance and are typically solid at room temperature (with the exception of mercury), all of which are related to their conductive properties.

The other options pertain to characteristics that do not align with metals. For instance, poor conductors of heat and electricity are typically classified as nonmetals, and low melting points along with being insulators are traits associated with certain materials that do not fall into the metal category. Hence, the defining characteristic that sets metals apart is their efficiency in conducting heat and electricity.