Metals (World of Earth Science)
Metals are rarely encountered in their elemental state in nature. They must first be extracted from the ground as an ore, which is then treated to release the metal. Some metals may be extracted from their natural state by electrolysis (for example sodium), while others may need more drastic treatment (such as iron or zinc). Precious metals (e.g., gold, silver, platinum) are relatively rare and as a result have a high value in the marketplace.
There are approximately 90 elements that can be described as metals (the number can fluctuate slightly depending upon the precise definition of a metal used to categorize the elements). Regardless, they all have various characteristics in common ranging from bonding to chemical nature.
In general, metals are elements that conduct electricity, are malleable, and are ductile. Another group of elementshe metalloid or semi-metal elementshare some properties with the metals and some with the nonmetals. There are eight of these elements and they are semiconductors.
Metals are usually solids at standard temperature and pressure (STP). One exception to this is mercury, which is a liquid at STP. As is to be expected from the fact that most metals are solids at STP, the majority of metals also have melting and boiling points that are high.
Electrical current is not the only thing that metals conduct. They are also efficient conductors of heat.
Metals have a shape that can be easily changed by hammering (i.e., they are malleable). Metals are also ductile (i.e., they can be drawn out into a long wire). With the exception of gold and copper, metals are silvery gray in color and all metals take a polish well.
Chemically, the atoms of a metallic element are bonded to their neighbors by metallic bonds, producing a giant metallic lattice structure. Metallic elements have relatively few electrons in their outermost shells. When metallic bonds are formed, these outermost electrons are lost into a pool of free or mobile electrons. Thus, the metallic lattice structure is actually comprised of positive ions packed closely together and a pool of freely moving electrons surrounding them. These free electrons are referred to as delocalized because they are not restricted to orbiting one particular ion or atom. This pool of delocalized electrons allows a metal to conduct an electrical charge because the electrons are free to move. Alloys also have this type of bonding, allowing them to conduct electricity as well. For this same reason metals are also good conductors of heat.
The close packing of the metal ions (the ions are packed as close as they can possibly be) explains the high density of the majority of the metals. A metal with a high molecular mass will have a greater density than one with a lower molecular mass even though their atomic radii may be similar. Lead and aluminum have similar atomic sizes, but lead has a much larger molecular mass and consequently it has a much higher density.
The malleability of metals is due to the regular arrangement of ions within the metallic lattice. The bonds holding the lattice in place are strong, but they are somewhat flexible. Layers of ions can slip over each other without the structure of the molecule being destroyed. This also explains why metals are ductile. Both of these characteristics are more noticeable when the metal is hot. The metallic lattice is also responsible for the appearance of some metals. When a metal is examined under a microscope, it is seen to have a crystalline structure that is made of regions called grains. The smaller the grain size the more closely packed are the ions of the metal and the stronger and harder it is. If hot metal is allowed to cool slowly, the resulting grains are large, making a metal that is easy to shape. This process is known as annealing. When a hot metal is cooled quickly, the crystals produced are small. When this cooling is carried out in water, it is called quenching. Quenching will produce a metal that is strong, hard, and brittle.
Many materials are referred to as metals when in fact they are not. The true metals are actually elements, whereas the false metals are alloysomposites of elements. For example, the element iron is a metal. Steel, however, is not a true metal, since it is an alloy containing a mixture of iron and carbonhe relative ratios of the two materials control the physical characteristics of the product. Alloys have different properties than the materials from which they are produced, so by careful blending the exact properties required can be manufactured.
Metals have a wide range of uses. For example, copper is used to conduct electricity in cables (an excellent conductor and very ductile). Tin is used to coat cans for food storage (non-poisonous and corrosion resistant). Aluminum is used as kitchen foil (high malleability). Iron is used as a fencing material (easily workable and relatively resistant to corrosion). Alloys made from metals have different uses. Steeln alloy of iron and carbons used widely in the construction industry because of it great strength and ease with which it can be initially formed to create specific shaped structural components (e.g., beams, girders, etc.) Solderin and leads used for joining metals together because of its low melting point. Brassopper and zincs fashioned into ornaments, buttons, and screws because of its high strength, low weight, and corrosion resistance.
See also Atomic theory; Atoms; Electricity and magnetism; Minerals