metal

Definition: In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions (cations), and form metallic bonds between each other.

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.

Oxidation

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)

The transition metals (such as iron, copper, zinc, and nickel) take much longer to oxidize. 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 (like aluminium, some steels, and titanium).

The oxides of metals are generally basic (as opposed to those of nonmetals, which are acidic.

Painting, anodising or plating metals are good ways to prevent their corrosion. However, a more reactive metal in the electrochemical series must be chosen for coating, especially when chipping of the coating is expected.

Water and the two metals form an electrochemical cell, and if the coating is less reactive than the coatee, the coating actually promotes corrosion.

See also

 periodic table
 biometals