|What is a metal and what is a heavy metal?|
Most of the 110 chemical elements in the periodic table are metals, i.e., at least 70 elements are considered to be metals, while a further 5 are metalloids with both metallic and non-metallic properties. Generally speaking, a metal is a material with high reflectivity and conductivity that can usually be deformed plastically. A metal reflects light like a mirror unless the surface has been corroded. (In fact, mirrors were traditionally made with silver deposited on a glass surface.) The high conductivity of metals, which depends on very mobile electrons, is a critical property for their use in electrical and electronic devices. The variety of shapes produced by different production technologies, such as extrusion and rolling, attests to their plasticity.
Most people use the term "metal" to refer to materials which exhibit the metallic properties mentioned above. The term "metal" also refers, however, to the metallic elements, even when these are combined with other elements to form non-metallic compounds such as salts and oxides, e.g., copper sulphate (a non-metallic crystalline compound) and sodium chloride (table salt). This underscores the fact that the earth is composed mainly of metal compounds, including such non-metallic materials as limestone (calcium carbonate). Moreover, when a metallic element is absorbed by the human body, and more generally in the environment, it is not in its pure metallic form but in compound form.
Metals will display different characteristics depending on temperature, among other factors. For example, tin may exhibit non-metallic characteristics under certain conditions, while, under others, it behaves like a metal. To complicate matters further, arsenic, which is a non-metal, exhibits metallic characteristics under certain conditions.
Alloys are combinations of elements which cannot be readily separated by physical means. Metals can combine in almost any proportion, offering a vast range of alloys which generally show all the characteristics of a metal and are therefore regarded as metals. Alloys can be shaped by casting, machining and plastic forming. They can also be varied by heat treatment to exhibit advantageous mechanical properties (elasticity, strength, etc.).
In general, metals do not combine readily with oxygen, so that in use they maintain their metallic properties. Some metals such as aluminum, which do react with oxygen, are quickly covered by a thin film of oxide which prevents further oxidation and enhances serviceability. These characteristics contribute to product durability and facilitate the repeated recycling of metals.
The metallic elements are often divided into light metals and heavy metals. Among metallurgists, it is common to use the concept "light metal" to mean beryllium, magnesium, aluminum and titanium and their alloys. A similar usage of the concept "heavy metal" to denote a subset of metals is not common. The term heavy metal has apparently arisen as a convenience among policy makers for referring to metals with potential toxicity. Cadmium, mercury, lead and bismuth are often mentioned, partly because human activities have increased their concentration in the environment. Other metals are also associated with health problems. On the other hand, all are not, gold being an example because it cannot be easily absorbed by the body. Moreover, even light metals may cause health problems.
A scientific definition of heavy metal might be based on whether a metal has a lower or higher density than its oxide. The dividing line in the periodic table will then go through titanium, which has several oxides, some of which are heavier and some lighter than pure titanium. Another approach defines heavy metal as one that has a density greater that 6.0 grams per cubic centimetre. Thereby, metals heavier than vanadium would be heavy metals, thus including most of the commercial metals.