TextBox#1HeavyMetalinChemistry

In popular culture the term “heavy metal” has a very precise definition – energetic and highly amplified electronic rock music (7). In chemistry the term “heavy metal” is ambiguous and is often confusing and misleading. In chemistry it is quite simple to separate elements into metals, nonmetals, and metalloids based on their physical/chemical characteristics and location on the periodic table . The most basic classification of the elements on the periodic table is into two groups: metals and nonmetals (see Figure 1). Figure1:ThePeriodicTable–Metalsandonmetals Taken from Chemical Principals 5th edition by Steven Zumdahl, 2005, Houghton Mifflin Company, Boston MA

The one single physical property that best distinguishes a metal from a nonmetal on the periodic table is conductivity – the ability of a substance to allow for the movement of an electrically charged particle from point A to point B through itself (8). Metals have high conductivity, or low resistance, to the passage of electricity and heat while nonmetals have low conductivity or high resistance to the passage of electricity and heat (8). The one single chemical property that best distinguishes a metal from a nonmetal on the periodic table is a metals tendency to give up electrons to form a positive ion (charged species) due to the low ionization energies associated with metal atoms (9). Nonmetal atoms in contrast have the tendency to gain electrons and form negative ions (9). Nonmetals have large ionization energies and negative electron affinities (9). The afore mentioned properties of metals and nonmetals provide the scientist with a very clear and definitive definition of what a metal is as well as what a nonmetal is. An additional term that often presents itself in the scientific literature suggestive of a subdivision of the metals is the term “heavy metal”. This term is used often in scientific literature and is found in the title of published articles from prestigious chemical journals as such as Analytical Chemistry published by the American Chemical Society (10).

TextBox#1HeavyMetalinChemistry(Cont.)

The interesting problem associated with the use of this term is that if you attempt to look up a definition for the term in a general chemistry text, inorganic chemistry text, organic chemistry text, IUPAC Compendium of Chemical Terminology, and/or the American Chemical Society’s web site you will not find one. You will not find a definition for the term “heavy metal” because one single definition for the term doesn’t actually exist. The term is completely subjective and the way it is defined in the scientific community depends on who you ask.

“Heavy metal” is a totally subjective term. Alkali metal, alkaline earth metals, and transition metals all have very precise definitions whereas heavy metal does not. The term heavy metal has been used to describe metals that: have a density greater than values that range from 3.5 g/cm 3 to 7.0 g/cm 3, have an atomic weight greater than sodium, have an atomic number beyond 20, and are generally toxic to animals (11). Unfortunately the list of definitions for a heavy metal goes on and on and is not limited to the definitions provided above.

Zinc is a metal that has a density of 7.140 g/cm 3, has an atomic weight greater than sodium and has an atomic number greater than 20. So by certain standards zinc could be considered a heavy metal. Zinc is also an essential element necessary for sustaining life. The fact that zinc is necessary for sustain life would not qualify this element as a heavy metal by the standard that heavy metals are generally toxic to animals. The fact that zinc does not fit all of the various criteria for the definition of a heavy metal leads to a slightly confusing situation.

The two metals found in the same group (group 12 or 2B) below zinc on the periodic table (Cd and Hg) are considered to be quite toxic (poisonous) to animals and in toxicology (the science and study of the noxious effects of chemical substances on living systems) they are also considered to be “heavy” metals. Published literature cites these metals as being “heavy” along with Pb and sometimes even arsenic. Arsenic is classified as a metalloid on the periodic table because As exhibits both metallic and nonmetallic properties under certain circumstances. Arsenic does not qualify as a metal and yet can be referred to as a “toxic metal” or “heavy metal” in published literature. Once again this leads to a confusing situation.

In conclusion it should be noted that the term heavy metal really has no clear or precise definition. The classification of a metal must be based on its physical and chemical properties and the term “heavy metal” is really a pseudoscientific term with no real clear meaning or definition.