• Define an atom
• Explain the structure and composition of an atom
• Differentiate between conductors, insulators and semi conductors
1.1 ATOM
Defination: An atom is the smallest indivisible particle which all matter is made of up.
1.2 STRUCTURE AND COMPOSITION OF AN ATOM
According to the electron theory of matter, an atom is made up of tiny particles. The central part of the atom is known as the nucleus. The nucleus of the atom consists of particles called protons and neutrons held together in a compact form by a binding energy. The outer portion of the atom is made up of orbiting particles called electrons. Electrons normally move about the centre of an atom in paths which are referred to as shells orbits. Each of these shells can contain only a certain maximum number of electrons. The central part of the atom shows its nucleus. We should note that protons are positively charged particles while electrons are negatively charged particles, since opposite charges attract, the negatively charged electrons are attracted to the positively charged protons in the nucleus. Consequently, this electric force of attraction holds the electrons in their orbit. The number of orbiting electrons normally equals the number of protons in the nucleus. For that reason an atom is said to be neutrally charged.
1.3 CONDUCTORS, INSULATORS AND SEMICONDUCTOR
Conductor In a conductor, electric current can flow freely, in an insulator it cannot. Metals such as copper typify conductors, while most non-metallic solids are said to be good insulators, having extremely high resistance to the flow of charge through them. "Conductor" implies that the outer electrons of the atoms are loosely bound and free to move through the material. Most atoms hold on to their electrons tightly and are insulators. In copper, the valence electrons are essentially free and strongly repel each other. Any external influence which moves one of them will cause a repulsion of other electrons which propagates, "domino fashion" through the conductor.
Types of conductors and their properties
Many types of materials can conduct electricity.
Take these materials listed here and put them in order from lightest to heaviest with respect to their densities, that is, weight per unit volume:
aluminum, copper, gold, iron, lead, mercury, silver, water, zinc aluminum copper
Insulator
An Insulator is a material or object which resists the flow of heat (thermal insulators) or electric charge (electrical insulators).
The term insulator has the same meaning as the term dielectric , but the two terms are used in different contexts. The opposite of insulators are conductors which permit the flow of charge. Semiconductors and semiconductors , are strictly speaking also insulators, since they prevent the flow of electric charge at low temperatures, unless doped with atoms that release extra charges to carry the current). However, some materials (such as silicon dioxide) are very nearly perfect electrical insulators, which allow flash memory technology. A much larger class of materials, (for example rubber and many plastics) are "good enough" insulators to be used for home and office wiring (into the hundreds of volts ) without noticeable loss of safety or efficiency.
Semiconductor
A semiconductor is a material with a conductance that is intermediate between that of an insulator and conductor.
A semiconductor behaves as an insulator at very low temperature,, and has an appreciable conductance at room temperature.
A semiconductor can be distinguished from a
conductor by the fact that, at abssolute zero, the uppermost filled electron energy filled in a semiconductor, but only partially filled in a conductor. The distinction between a
semiconductor and an insulator is slightly more arbitrary. A semiconductor has a band gap which is small enough such that its conduction band is appreciably thermally populated with electrons at room temperature, whilst an insulator has a band gap which is too wide for there to be appreciable thermal electrons in its conduction band at room temperature.
Fig 1.1: Band structure of a semiconductor showing a full valence band and an empty conduction band. The Fermi level lies within the forbidden band gap
In the parlance of solid-state physics, semiconductors (and insulators) are defined as solids in which at absolute zero (0 K), the uppermost band of occupied electron energy states, known as the valence band, is completely full. Or, to put it another way, the Fermi energy of the electrons lies within the forbidden band gap. The Fermi energy, or Fermi level can be thought of as the energy up to which available electron states are occupied at absolute zero.
At room temperatures, there is some smearing of the energy distribution of the electrons, such that a small, but not insignificant number have enough energy to cross the energy band gap into the conduction band. These electrons which have enough energy to be in the conduction band have broken free of the covalent bonds between neighbouring atoms in the solid, and are free to move around, and hence conduct charge. The covalent bonds from which these excited electrons have come now have missing electrons, or holes which are free to move around as well. (The holes themselves don't actually move, but a neighbouring electron can move to fill the hole, leaving a hole at the place it has just come from, and in this way the holes appear to move.)
It is an important distinction between conductors and semiconductors that, in semiconductors, movement of charge (current) is facilitated by both electrons and holes. Contrast this to a conductor where the Fermi level lies within the conduction band, such that the band is only half filled with electrons. In this case, only a small amount of energy is needed for the electrons to find other unoccupied states to move into, and hence for current to flow.
The ease with which electrons in a semiconductor can be excited from the valence band to the conduction band depends on the band gap between the bands, and it is the size of this energy bandgap that serves as an arbitrary dividing line between semiconductors and insulators. Materials with a bandgap energy of less than about 3 electron volts are generally considered semiconductors, while those with a greater bandgap energy are considered insulators..
The current-carrying electrons in the conduction band are known as "free electrons," although they are often simply called "electrons" if context allows this usage to be clear. The holes in the valence band behave very much like positively-charged counterparts of electrons, and they are usually treated as if they are real charged particles.
Source:
UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION REVITALISATION PROJECT-PHASE II
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