<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> 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 96 out of the 118 known chemical 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 / cm is sometimes quoted as a commonly used criterion and is used in the body of this article . </P> <P> 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 including gallium, thallium, and hafnium . </P>

Why are lead and mercury called heavy metals
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