Sunday, May 20, 2012

The Structure of Matter

       The matter which occupies the space may be solid, liquid or gaseous. The molecules and atoms, of which all the substances are composed, are not at all elements but are themselves made up of simpler entities. We know this because, we, up to certain extent, are successful in breaking atoms and studying the resulting products. For instance, such particles of atom are obtained by causing ultraviolet light to fall on cold metal surfaces, such particles are spontaneously ejected from the radioactive elements. So such particles are obtained from many different substances under widely varying conditions.
       In fact, according to the modern electron theory, matter is composed of the three fundamental particles, which are invisible to bare eyes. These are the neutron, the proton and the electron. The proton is positively charged and the electron is negatively charged. The neutron is uncharged i.e. electrically neutral in nature possessing no charge. The mass of neutron and proton is same while the electron is very light, almost 1/1840 th  the mass of neutron. The following table gives the information about these three particles.
       There is no difference between an electron of copper and an electron of aluminium or an electron of any other element. Similarly the neutrons and protons of various atoms are characteristicwise identical in nature. Then why do various elements behave differently ? This is because of the difference in the arrangement of elecrtrons, protons and neutrons of which each atom is composed. Let us see the structure of an atom.
1.1 Structure of an Atom
       The atom have a planetary type of structure, according to classical Bohr Model.
       All the protons and neutrons are bound together at the centre of an atom, which is called Nucleus. While all the the electrons are moving around the nucleus. So nucleus can be though of as a central sun, about which electrons revolve in a particular fashion like the planets.
       In a normal atom the number of protons is equal to the number of electrons. As neutron is electrically neutral, an atom as a whole is electrically neutral. The number of protons in an atom is called as its atomic number. While the atomic weight is approximately equal to the total number of protons and neutrons in the nucleus of an atom.
       The electrons which are revolving round the nucleus, do not move in the same orbit. The electrons are arranged in the different orbits or shells at fixed distances from the nucleus. Each shell can contain a fixed number of electrons. In general, a shell can contain a maximum of 2n2 electrons where n is the number of the shell. The first shell can occupy maximum of two electrons (2 x 12) while the second shell can occupy maximum of eight electrons (2 x 22) and so on.
       Each shell has an energy level associated with it. The closer an electron is to the nucleus, the stronger are the forces that bind it to the nucleus. So the first shell which is closet to the nucleus is always under the tremendous force of attraction. While the shell which is farthest from the nucleus is under very weak force of attraction. The electrons revolving in the last shell i.e. farthest from the nucleus are very loosely bound to the nucleus. Such electrons in the outermost shell are responsible for the electrical and chemical characteristics of an atom.
Key Point : The outermost shell is called the valance shell and the electrons in this shell are called valence electrons.
       The exception to the '2n2' rule stated above is that the outermost shell in an atom cannot accommodate more than eight electrons. The valance electrons revolving in the outermost shell are said to be having highest energy level. The amount of energy required to extract the valance electron from the outer shell is very less.
Key Point : Each shell has energy level associated with it. Closer the shell to the nucleus, more tightly it is bound the nucleus and possesses lower energy level.
        Thus energy level of shell one is lowermost while the energy level of valence shell is highest. More energy level indicates that the electrons of that are loosely bound to the nucleus. Hence valence electrons are loosely bound to the nucleus as having highest energy level. The concept of energy level is shown in the Fig.1.
Fig. 1 Concept of energy level
       When an atom absorbs energy from a heat source or from light or due to high atmospheric temperature, the energy levels of the electrons are raised. When such an additional energy is imparted to the electrons, the electrons move to the next orbit which is farther from the nucleus. If such an energy is imparted to a valance electron, it tries to jump to the next orbit. But as a valence electron is in the outermost orbit, actually it gets completely removed from the force of attraction of the nucleus.
Key Points :
1) An electron which is not subjected to the force of attraction of the nucleus is called a free electron. Such free electrons are basically responsible to the flow of current.
1.2 Structure of Semiconductor Materials
       The semiconductor materials such as Ge and Si have four electrons in their valence shell i.e. outermost shell. The Fig. 2 shows atomic structure of the semiconductor materials, germanium and silicon.
Fig. 2 Atomic structure of germanium and silicon atoms
       The germanium has a nucleus with 32 protons. The electrons are distributed as follows : 2 electrons in the first orbit, 8 in the second orbit, and 18 in the third. The remaining four electrons are in the outer or valence orbit.
       The silicon has nucleus with 14 protons and 14 electrons. The remaining four electrons are in the outermost orbit.
Key Point : When there are four electrons in the outermost orbit, the semiconductor material is referred to as pure or intrinsic semiconductor.
       These intrinsic materials can be manufactured to a very high purity level. The impurity levels are of the order of 1 part in billion.
1.2.1 Properties of Intrinsic Materials
       The important properties of such intrinsic semiconductor materials are,
1. High purity level.
2. The characteristics can be changed as per the requirement by the process of adding impurity called doping.
3. The characteristics can be changed significantly by the application of heat and light.
4. Most suitable for heat sensitive and light sensitive devices.
1.3 Ionization
       If an electron is extracted from the outermost shell of an atom then the overall negative charge of that atom decreases as it looses negative charge in the form of an electron. But the number of protons remains same hence positive charge remains same. So atom as a whole looses its electrical neutral nature and becomes positively charged. Such an atom is called positive ion. Similarly by any means if an electrically neutral atom gains an additional electron then it becomes negatively charged and called negative ion. Thus by loosing or gaining electrons, an atom gets converted into a charged ion. This process of loosing or gaining an electron, which converts electrically neutral atom to a charged ion is called ionization.

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