Metals are sometimes described as a lattice of positive ions surrounded by a cloud of delocalized
electrons. They are one of the three groups of elements as distinguished by their ionization 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. Most
elements on this line are metalloids, sometimes called semi-metals; elements to the lower left are
metals; elements to the upper right are nonmetals.
Metals are usually inclined to form cations through electron loss, reacting with oxygen in the air
to form oxides over changing timescales (iron rusts over years, while potassium burns in
seconds). Examples:
4Na + O2 → 2Na2O (sodium oxide)
2Ca + O2 → 2CaO (calcium oxide)
4Al + 3O2 → 2Al2O3 (aluminium oxide)
Metals in general have superior electric and thermal conductivity, high luster and density, and
the ability to be deformed under stress without cleaving.While there are several metals that have
low density, hardness, and melting points, the alkali and alkaline earth metals are extremely
reactive, and are rarely encountered in their elemental, metallic form.
The majority of metals have higher densities than the majority of nonmetals. Nonetheless, there
is wide variation in the densities of metals; lithium is the least dense solid element and osmium is
the densest. The metals of groups I A and II A are referred to as the light metals because they are
exceptions to this generalization. The high density of most metals is due to the tightly-packed
crystal lattice of the metallic structure. The strength of metallic bonds for different metals
reaches a maximum around the center of the transition series, as those elements have large
amounts of delocalized electrons in a metallic bond.
The nondirectional nature of metallic bonding is thought to be the primary reason for the
malleability of metal. Planes of atoms in a metal are able to slide across one another under stress,
accounting for the ability of a crystal to deform without shattering.
When the planes of an ionic bond are slid past one another, the resultant change in location shifts
ions of the same charge into close proximity, resulting in the cleavage of the crystal. Covalently
bonded crystals can only be deformed by breaking the bonds between atoms, thereby resulting in
fragmentation of the crystal.
The electrical and thermal conductivity of metals originate from the fact that in the metallic
bond, the outer electrons of the metal atoms form a gas of nearly free electrons, moving as an
electron gas in a background of positive charge formed by the ion cores. Good mathematical
predictions for electrical conductivity, as well as the electrons' contribution to the heat capacity
and heat conductivity of metals can be calculated from the free electron model, which does not
take the detailed structure of the ion lattice into account.
Alloys
An alloy is a mixture of two or more elements in solid solution in which the major component is
a metal. Most pure metals are either too soft, brittle or chemically reactive for practical use.
Combining different ratios of metals as alloys modifies the properties of pure metals to produce
desirable characteristics. The aim of making alloys is generally to make them less brittle, harder,
resistant to corrosion, or have a more desirable color and luster. Examples of alloys are steel
(iron and carbon), brass (copper and zinc), bronze (copper and tin), and duralumin (aluminum
and copper). Alloys specially designed for highly demanding applications, such as jet engines,
may contain more than ten elements.
An alloy is a partial or complete solid solution of one or more elements in a metallic matrix.
Complete solid solution alloys give single solid phase microstructure, while partial solutions give
two or more phases that may be homogeneous in distribution depending on thermal (heat
treatment) history. Alloys usually have different properties from those of the component
elements.
Alloying one metal with other metal(s) or non metal(s) often enhances its properties. For
instance, steel is stronger than iron, its primary element. The physical properties, such as density,
reactivity, Young's modulus, and electrical and thermal conductivity, of an alloy may not differ
greatly from those of its elements, but engineering properties, such as tensile strength and shear
strength may be substantially different from those of the constituent materials. This is sometimes
due to the sizes of the atoms in the alloy, since larger atoms exert a compressive force on
neighboring atoms, and smaller atoms exert a tensile force on their neighbors, helping the alloy
resist deformation. Alloys may exhibit marked differences in behavior even when small amounts
of one element occur. For example, impurities in semi-conducting ferromagnetic alloys lead to
different properties, as first predicted by White, Hogan, Suhl, Tian Abrie and Nakamura. Some
alloys are made by melting and mixing two or more metals. Brass is an alloy made from copper
and zinc. Bronze, used for bearings, statues, ornaments and church bells, is an alloy of tin and
copper.
Unlike pure metals, most alloys do not have a single melting point. Instead, they have a melting
range in which the material is a mixture of solid and liquid phases. The temperature at which
melting begins is called the solidus and the temperature when melting is complete is called the
liquidus. However, for most alloys there is a particular proportion of constituents which give
them a single melting point or (rarely) two. This is called the alloy's eutectic mixture.