ELECTRICITY: Conductors and Insulators

ATOMS and MOLECULES zAn atom is the smallest unit of an element that has the properties of the element. zA molecule is the smallest unit of a compound or the form of an element in which atoms bind together that has the properties of the compound or element.

CONDUCTORS, INSULATORS, and SEMI-CONDUCTORS  When some atoms combine to form solids, one or more electrons are often liberated and can move with ease through the material. Electrons are easily liberated in some materials, which are known as conductors. Metals, particularly copper and silver, are good conductors.  Materials in which the electrons are tightly bound to the atoms are known as insulators, nonconductors, or dielectrics. Glass, rubber, and dry wood are examples of these materials. zA third kind of material is a solid in which a relatively small number of electrons can be freed from their atoms in such a manner as to leave a “hole” where each electron had been. The hole, representing the absence of a negative electron, behaves as though it were positively charged. An electric field will cause both negative electrons and positive holes to move through the material, thus producing a current of electricity. Such a solid, called a semiconductor, generally has a higher resistance to the flow of current than a conductor such as copper but a lower resistance than an insulator such as glass.

METALLIC BONDS Silver, a typical metal, consists of a regular array of silver atoms that have each lost an electron to form a silver ion. The negative electrons distribute themselves throughout the entire piece of metal and form non-directional bonds between the positive silver ions. This arrangement, known as metallic bonding, accounts for the characteristic properties of metals: They are good electrical conductors because the electrons are free to move from one place to another, and they are malleable (as shown here) because the positive ions are held together by non-directional forces.

METALS zThe structure of the atom, in particular the configuration of the electron cloud, is responsible for the obvious physical differences between metals and nonmetals.  Metals have a characteristic luster, are opaque, can be hammered and drawn into various shapes, and conduct electricity. Nonmetal elements, on the other hand, are often gases, and, if solid, nonmetals are generally brittle, sometimes transparent, and do not conduct electricity. zThe atoms of metals have outer shells that contain few electrons and are nowhere near filled (and therefore lack the stability of a noble gas). As a result, all metals tend to easily lose some of these outer electrons. zThis means, chemically, that metals tend to form positively charged ions, or positively charged atoms or molecules, when they enter into chemical combination. zPhysically, the fact that the outer shells of metal atoms are unfilled means that these “loose” electrons can flow and enable metals to conduct electricity; this fact also accounts for the mechanical properties of metals.

NONMETALS

xThree objects demonstrate the way in which electrical charges affect conductors and nonconductors. xA negatively charged rod, A, affects the way charges are distributed in a nearby conductor, B, and a nonconductor, C. xA positive charge is induced on the sides of B and C that are nearest A; a negative charge is induced on the sides of B and C that are farthest from A. xIn the conductor, B, the separation of charge involves the entire object because the electrons are free to move. In the nonconductor, C, the separation of charge is limited to the way in which the electrons redistribute themselves within an atom. xThis effect is most noticeable if the nonconductor is close to the charged object.