Like cadmium, zinc and lead, mercury is a naturally occurring element known as a "heavy metal" and can be toxic to living organisms. The element's atomic mass is 200.59 grams per mole and its specific gravity is 13.5 times that of water. Mercury has a melting point of -38.9^oC, a boiling point of 357.3^oC, and is the only metal to remain in liquid form at room temperature. Droplets of liquid mercury are shiny and silver-white with a high surface tension, appearing rounded when on flat surfaces. The liquid is highly mobile and droplets combine easily due to low viscosity. The element also combines with other metals such as tin, copper, gold and silver to form mercury alloys known as amalgams. Fortunately, mercury does not form amalgam with iron, which allows for the element to be shipped in standard iron flasks containing 76 pounds, or 34.5 kilograms, of liquid mercury. Mercury has a relatively high vapour pressure and the highest volatility of any metal, vapourizing to become a colourless, odourless gas. The metal is a fair conductor of electricity, but a poor conductor of heat.

Mercury's atomic number is 80. In nature, mercury has 3 possible conditions of electrical charge, or valence states. Elemental mercury (Hg⁰) has no electric charge. Mercury is also found in two positively charged, or cationic, states, Hg²⁺ (mercuric) and Hg¹⁺(mercurous). The mercuric cation is more stable and is generally associated with inorganic molecules, such as sulfur (in the mineral cinnabar), chlorine (mercuric chloride), oxygen and hydroxyl ions. Hg²⁺ is also found in organic (carbon based) substances like dimethylmercury (Me₂Hg), which is far more toxic than inorganic forms of mercury and bioaccumulates in the tissues of living organisms. Since mercury can be adsorbed easily onto small particles of matter, some scientists use the notation Hg(p) to represent elemental mercury attached onto or absorbed into a particle.

Because it is an element, mercury is not biodegradable. It is converted among its various forms through a range of abiotic and biogeochemical transformations and during atmospheric transportation. Although its form and availability to living organisms may change over time, mercury persists in the environment.