In chemistry, a metal is a material in which the highest occupied energy band (conduction band) is only partially filled with electrons. In terms of physical properties, metals:
Are good conductors of heat and electricity. The electrical conductivity of metals generally decreases with temperature
Are malleable and ductile in their solid state
Show metallic luster
Have high density
Are solids (except mercury)
Have crystal structure in which each atom is surrounded by eight to twelve near neighbors
In terms of their chemical properties, metals:
Metallic characteristics of the elements decrease and non-metallic characteristics increase with the increase of valence electrons. Also metallic characteristics increase with the number of electron shells. Therefore, there is no sharp dividing line between the metals and the non-metals. Of the 113 elements now known, only 17 show primarily non-metallic characteristics (see nonmetal), 7 others are metalloids, and 89 may be classed as metals.
Most metals are found as ores; only a few, such as gold and silver, occur in a native state. Alloys are easily formed because of the nonspecific nondirectional nature of the metallic bond.
Some notable metals
Mercury is exceptional in being a liquid at room temperature. Sodium and potassium are examples of metals that are soft and chemically very reactive; they tarnish quickly in air and are most familiar as their salts. The lightest metal is lithium, which is also very reactive. The heaviest is osmium, which is 22.6 times denser than water and is one of the platinum group, relatively unreactive metals that include ruthenium, rhodium, palladium and iridium.
Malleability and ductility are further metallic characteristics. Gold is the most malleable of all metals; it can be beaten so thin as to be virtually translucent. Some metals have very high melting points and various high-temperature applications: tungsten, with the highest melting point of all at 3,410°C (6,170°F), is employed in light bulb filaments. Aluminum, followed by iron, and the two most abundant and useful of metals. Titanium, although rarely seen as the metal, is more commonly distributed than the more familiar copper, zinc, and lead. Other metals of great economic importance, because of their radioactivity, are uranium and plutonium.
A metal foil is a metal rolled or beaten into a very thin sheet. Commonest is aluminum foil (having largely replaced tin foil), used extensively in packaging and in the home in the storage or cooking of food. Gold foil is used decoratively and in electronics. Lead foil is used as a radiation shield.
A metallic bond is a bond between atoms with similar low electronegativities. Metallic bonding is essentially covalent bonding with delocalized electrons. Metallic nuclei share weakly bonded valence electrons, which form an 'electron gas.'
Metals have many more empty orbits than electrons available to fill them. For example, iron requires 5 electrons to fill its outermost 3d orbital.
Metallically bonded solids share the following properties: low to moderate hardness, malleability, and ductility (weak delocalized bonds); very good thermal conductivity and electrical conductivity (free electrons in cloud); and form crystals with high symmetry (nuclei pack well).
A metallic crystal is the type of crystal formed by most metals, in which the outer electrons are shared by the crystal as a whole. Thus, the positively charged metal ions in the crystal lattice are surrounded by a "gas" of free electrons. These free electrons account for the fact that most metals are good conductors of heat and electricity.
Metal fatigue is deterioration and progressive cracking of metal parts caused by repeated and relatively low stresses. Imperfections in crystal grains, which often occur at notches, screw threads, welding defects, etc., accumulate after numerous cycles in which a small inelastic strain is applied, often leading to eventual failure. Fatigue failure is guarded against by careful design.
'Metals' in astrophysics
In astrophysics, a metal is any element heavier than hydrogen and helium. An object's metallicity is its abundance of such elements, or, often, more specifically, the abundance of iron, which is easy to measure. The terms metal-poor and metal-rich are used to indicate low metallicity and high metallicity, respectively.