Heavy metals are generally defined as metals with relatively high densities, atomic weights, or atomic numbers. The criteria used, and whether metalloids are included, vary depending on the author and context. In metallurgy, for example, a heavy metal may be defined on the basis of density, whereas in physics the distinguishing criterion might be atomic number, while a chemist would likely be more concerned with chemical behaviour. More specific definitions have been published but none of these have been widely accepted. The definitions surveyed in this article encompass up to 92 elements; only mercury, lead and bismuth meet all of them. Despite this lack of agreement the term (plural or singular) is widely used in science. A density of more than 5 g/cm3 is sometimes quoted as a commonly used criterion and is used in the body of this article. The earliest known metals—common metals such as iron, copper, and tin, and precious metals such as silver, gold, and platinum—are heavy metals. From 1809 onwards, light metals, such as magnesium, aluminium, and titanium, were discovered, as well as less well-known heavy metals like gallium, thallium, and hafnium. Some heavy metals are either essential nutrients (typically iron, cobalt, and zinc), or relatively harmless (such as ruthenium, silver, and indium) but can be toxic in large amounts or certain forms. Other heavy metals, like cadmium, mercury, and lead, are notably toxic. Potential sources of heavy metal poisoning include mining and industrial wastes, agricultural runoff, occupational exposure, and contact with lead-based paints. Physical and chemical characterisations of heavy metals need to be treated with caution as the metals involved are not always consistently defined. As well as being relatively dense, heavy metals tend to be less reactive than lighter metals and have much less soluble sulfides and hydroxides. While it is relatively easy to distinguish a heavy metal such as tungsten from a lighter metal such as sodium, a few heavy metals such as zinc, mercury, and lead have some of the characteristics of lighter metals, and lighter metals such as beryllium, scandium, and titanium have some of the characteristics of heavier metals. Heavy metals are relatively scarce in the Earth's crust but are present in many aspects of modern life. They are used in, for example, golf clubs, cars, antiseptics, self-cleaning ovens, plastics, solar panels, mobile phones, and particle accelerators.

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