267With some irregularities, atoms therefore become smaller, ionization energy increases,
and there is a gradual change in character, across a period, from strongly metallic, to
weakly metallic, to weakly nonmetallic, to strongly nonmetallic elements.
Going down a main group periodic table column, the effect of increasing nuclear charge
is generally outweighed by the effect of additional electrons being further away from the
nucleus. With some irregularities, atoms therefore become larger, ionization energy falls,
and metallic character increases. The combined effect of these competing horizontal and
vertical trends is that the location of the metal-nonmetal transition zone shifts to the right
in going down a period.
A related effect can be seen in other diagonal similarities that occur between some
elements and their lower right neighbors, such as lithium-magnesium, beryllium-
aluminum, carbon-phosphorus, and nitrogen-sulfur.
Other Metalloids
Given there is no agreed definition of a metalloid, some other elements are occasionally
classified as such. These elements include hydrogen, beryllium, nitrogen, phosphorus,
sulfur, zinc, gallium, tin, iodine, lead, bismuth and radon. The term metalloid has also been
used to refer to:
Elements that exhibit metallic luster and electrical conductivity, and that are also
amphoteric. Arsenic, antimony, vanadium, chromium, molybdenum, tungsten, tin,
lead and aluminum are examples.
Elements that are otherwise sometimes referred to as poor metals.
Nonmetallic elements (for example, nitrogen; carbon) that can form alloys with, or
modify the properties of, metals.
Heavy Metals
A heavy metal is a member of a loosely defined subset of elements that exhibit metallic
properties. It mainly includes the transition metals, some metalloids, lanthanides, and
actinides. Many different definitions have been proposed—some based on density, some
on atomic number or atomic weight, and some on chemical properties or toxicity. The term
heavy metal has been called a "misinterpretation" in an IUPAC technical report due to the
contradictory definitions and its lack of a "coherent scientific basis".
There is an alternative term toxic metal, for which no consensus of exact definition exists
either. As discussed below, depending on context, heavy metal can include elements
lighter than carbon and can exclude some of the heaviest metals. Heavy metals occur
naturally in the ecosystem with large variations in concentration. In modern times,
anthropogenic sources of heavy metals, i.e. pollution, have been introduced to the
ecosystem. Waste-derived fuels are especially prone to contain heavy metals, so heavy
metals are a concern in consideration of waste as fuel.
Motivations for controlling heavy metal concentrations in gas streams are diverse. Some
of them are dangerous to health or to the environment (e.g. mercury, cadmium, lead,
chromium), some may cause corrosion (e.g. zinc, lead), some are harmful in other ways
(e.g. arsenic may pollute catalysts).
Within the European community the eleven elements of highest concern are arsenic,
cadmium, cobalt, chromium, copper, mercury, manganese, nickel, lead, tin, and thallium,
the emissions of which are regulated in waste incinerators.