why are lead and mercury called heavy metals
Heavy metals 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 5Â g/cm3 is sometimes quoted as a commonly used criterion and is used in the body of this article.
Pencil Prior to 1565 (some sources say as early as 1500), a large deposit of graphite was discovered on the approach to Grey Knotts from the hamlet of Seathwaite in Borrowdale parish, Cumbria, England.[4][5][6][7] This particular deposit of graphite was extremely pure and solid, and it could easily be sawn into sticks. It remains the only large-scale deposit of graphite ever found in this solid form.[8] Chemistry was in its infancy and the substance was thought to be a form of lead. Consequently, it was called plumbago (Latin for "lead ore").[9][10] Because the pencil core is still referred to as "lead", or a "lead", many people have the misconception that the graphite in the pencil is lead,[11] and the black core of pencils is still referred to as lead, even though it never contained the element lead.[12][13][14][15][16][17] The words for pencil in German (bleistift), Irish (peann luaidhe), Arabic (قلم رصاص qalam raṣāṣ), and some other languages literally mean lead pen.