In chemistry, a metal (Greek: Metallon) is an element that readily forms ions (cations) and has metallic bonds, and metals are sometimes described as a lattice of positive ions (cations) in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionisation and bonding properties, along with the metalloids and nonmetals. On the periodic table, a diagonal line drawn from boron (B) to polonium (Po) separates the metals from the nonmetals. Elements on this line are metalloids, sometimes called semi-metals; elements to the lower left are metals; elements to the upper right are nonmetals.
Nonmetal elements are more abundant in nature than are metallic elements, but metals in fact constitute most of the periodic table. Some well-known metals are aluminium, copper, gold, iron, lead, silver, titanium, uranium, and zinc.
The allotropes of metals tend to be lustrous, ductile, malleable, and good conductors, while nonmetals generally speaking are brittle (for solid nonmetals), lack luster, and are insulators.
A more modern definition of metals is that they have overlapping conductance and valence bands in their electronic structure. 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-grey reflectiveness of elemental metals.
The properties of conductivity are mainly because each atom exerts only a loose hold on its outermost electrons (valence electrons); thus, the valence electrons form a sort of sea around the close-packed metal nucleii cations.
Most metals are chemically unstable, reacting with oxygen in the air to form oxides over varying timescales (iron rusts over years, potassium burns in seconds, silver tarnishes in months, although this is due to reactions with sulfur, although ozone, which is three atoms of oxygen bound together, can also play a part, as can hydrogen sulfide). The alkali metals react quickest followed by the alkaline earth metals, found in the leftmost two groups of the periodic table. The transition metals take much longer to oxidise (e.g. iron, copper, zinc, nickel), and palladium, platinum and gold do not react with atmospheric oxygen at all (which is why we make shiny jewelry from them). Some metals form a barrier layer of oxide on their surface which cannot be penetrated by further oxygen molecules and thus retain their shiny appearance and good conductivity for many decades (e.g. aluminium, some steels, titanium and more).
Painting or anodising metals are good ways to prevent their oxidation.