Most metals are chemically reactive, reacting with oxygen in the air to form oxides over changing timescales (for example iron rusts over years and 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. Examples: 4Na + O2 → 2Na2O (sodium oxide)
2Ca + O2 → 2CaO (calcium oxide)
4Al + 3O2 → 2Al2O3 (aluminium oxide)
The transition metals take much longer to oxidize (such as 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 (like aluminium, some steels, and titanium). The oxides of metals are basic (as opposed to those of nonmetals, which are acidic), although this may be considered a rule of thumb, rather than a fact. 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.
Physical Porperties of Metals
Traditionally, metals have certain characteristic physical properties: they are usually shiny (they have metallic luster), have a high density, are ductile and malleable, usually have a high melting point, are usually hard, are usually a solid at room temperature and conduct electricity, heat and sound well. While there are several metals that are low density, soft, and have low melting points, these (the alkali and alkaline earth metals) are extremely reactive, and are rarely encountered in their elemental, metallic form. The electrical and thermal...