Final answer:
Mercury is the only metallic element that is liquid at room temperature. It is a toxic metal that was used in thermometers. New energy-efficient light bulbs also contain trace amounts of mercury and should be handled as hazardous waste.
Step-by-step explanation:
Mercury is the only metallic element that is liquid at room temperature. It is a toxic metal that was commonly used in thermometers until its toxicity became known. Mercury switches were also once common, but are no longer used. However, new energy-efficient light bulbs contain trace amounts of mercury and should be handled as hazardous waste.