What is defined as a heavy metal?

A heavy metal is typically defined as a metallic element that is toxic at low concentrations. This classification is important in various fields, including environmental science and toxicology, as these metals can pose significant health risks even when present in small amounts. Common examples of heavy metals include lead, mercury, and cadmium, which can lead to serious health and environmental problems.

Identifying metals as heavy metals is based on their potential toxicity rather than their weight or physical state at room temperature. Therefore, elements that are gaseous (like the first option) or considered non-toxic (as in the second option) do not fit within the heavy metal category. The notion of lightweight metals (the fourth option) also does not apply; metals with low atomic mass can still be highly toxic, while some heavier metals may not be harmful at all. Thus, the clear definition hinges on the toxicological properties of the metals, affirming why the correct choice defines heavy metals accurately.