Metals and semimetals

Metals and semimetals

Author: 
BME-ABÉT

A metal is a chemical element that is a good conductor of both electricity and heat and forms cations and ionic bonds with non-metals. In chemistry, a metal is an element, compound, or alloy characterized by high electrical conductivity. In a metal, atoms readily lose electrons to form positive ions (cations). Those ions are surrounded by delocalized electrons, which are responsible for the conductivity. The solid thus produced is held by electrostatic interactions between the ions and the electron cloud, which are called metallic bonds.

Metals occupy the bulk of the periodic table, while non-metallic elements can only be found on the right-hand-side of the Periodic Table of the Elements. A diagonal line drawn from boron (B) to polonium (Po) separates the metals from the nonmetals. Most elements on this line are metalloids, sometimes called semiconductors. This is due to the fact that these elements exhibit electrical properties common to both conductors and insulators. Elements to the lower left of this division line are called metals, while elements to the upper right of the division line are called non-metals.

An alternative definition of metal refers to the band theory. If one fills the energy bands of a material with available electrons and ends up with a top band partly filled then the material is a metal. This definition opens up the category for metallic polymers and other organic metals, which have been made by researchers and employed in high-tech devices. These synthetic materials often have the characteristic silvery gray reflectiveness (luster) of elemental metals.

The reducing series of the metals is the following:
K, Na, Ca, Mg, Cr, Al, Zn, Fe,H, Cu, Hg, Ag, Au, Pt

The most well known semi-metals are the followings:
Bor (B)
Silicium (Si)
Germanium (Ge)
Arsenic (As)
Antimon (Sb)
Tellur (Te)
Polonium (Po)
Astacium (At)

Fom the biological point of view, metals can be
1.
non toxic and essential: these are not hazardous in the existing concentrations: B, Na, K, Mg, Ca, Fe;
2. essential and toxic: they may have concentrations in the practice both in the essential and the toxic concentration ranges: Cu, Zn, Mn, Co, Mo;
3. toxic metals, which are not essential, these are: As, Cd, Sb, Pb, Hg, Cr, Ni.

Metals cause more and more problem in the environment, because the mined out, processed and used metals are diffusely dispersed in the waters, sediments and soils as well as in the tissues of the organisms. Background concentrations are continuously increasing and the genetic response of the biota is more and more prevalent. Living organisms are able to concentrate (bioaccumulate) metals in their tissues and it may cause a biomagnification through the food chain.