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 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.
When considering the exact band structure and binding energy of a metal, it is necessary to take into account the positive potential caused by the specific arrangement of the ion cores – which is periodic in crystals. The most important consequence of the periodic potential is the formation of a small band gap at the boundary of the brillouin zone. Mathematically, the potential of the ion cores is treated in the nearly-free electron model. Nonmetals Nonmetal is a term used in chemistry when classifying the chemical elements.
On the basis of their general physical and chemical properties, every element in the periodic table can be termed either a metal or a non-metal. (A few elements with intermediate properties are referred to as metalloids. ) Metalloids Metalloid is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, nearly every element in the periodic table can be termed either a metal or a nonmetal – however a few elements with intermediate properties are referred to as metalloids. In Greek metallon = metal and eidos = sort) There is no rigorous definition of the term, however the following properties are usually considered characteristic of metalloids:
• metalloids often form amphoteric oxides. • metalloids often behave as semiconductors (B,Si,Ge) to semimetals (eg. Sb). The concepts of metalloid and semiconductor should not be confused. Metalloid refers to the properties of certain elements in relation to the periodic table. Semiconductor refers to the physical properties of materials including alloys, compounds) and there is only partial overlap between the two. The following elements are generally considered metalloids: • Boron(B) • Silicon(Si) • Germanium(Ge) • Arsenic(As) • Antimony(Sb) • Tellurium(Te) • Polonium(Po) Some allotropes of elements exhibit more pronounced metal, metalloid or non-metal behavior than others. For example, for the element carbon, its diamond allotrope is clearly non-metallic, however the graphite allotrope displays limited electric conductivity more characteristic of a metalloid.
Phosphorus, tin, selenium and bismuth also have allotropes which display borderline behavior. In the standard layout of the periodic table, metalloids occur along the diagonal line through the p block from boron to astatine. Elements to the upper right of this line display increasing nonmetallic behaviour; elements to the lower left display increasing metallic behaviour. This line is called the “stair-step” or “staircase. ” The poor metals are to the left and down and the nonmetals are to the right and up. In addition, the halogens are found at the right.