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 (e.g., iron rusts over years, potassium burns in seconds). 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). Others, like palladium, platinum and gold, do not react with the atmosphere at all. 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).
Painting or anodising metals are good ways to prevent their corrosion.
An alloy is a mixture with metallic properties that contains at least one metal element. Examples of alloys are steel (iron and carbon), brass (copper and zinc), bronze (copper and tin), and duralumin (aluminium and copper). Alloys specially designed for highly demanding applications, such as jet engines, may contain more than ten elements.
Traditionally, metals have certain characteristic physical properties: they are usually shiny (they have "lustre"), have a high density, are ductile and malleable, usually have a high melting point, are usually hard, and conduct electricity and heat well. However, this is mainly because the low density, soft, low melting point metals happen to be reactive and we rarely encounter them in their elemental, metallic form. Metals are also sonorous, which means that they conduct sound well.
The oxides of metals are basic; those of nonmetals are acidic.