LESSON 1. Electron- Energy bond in solids- Insulators-semiconductors and metals.
An atom is defined as the smallest particle into which an element can be divided and still retain the chemical properties of that element. Atom basically consists of electrons, proton and neutrons. The proton and neutron is altogether called as nucleus. The nucleus is the central portion of an atom. It is having positive charge. Around the nucleus small negatively charged particles called electrons revolve in different paths. The different paths are called either orbits or shells.
An electron is a negatively charged particle. It is moving around the nucleus. It is having negligible mass. Hence it is very mobile.
Charge of an electron = 1.6025 X 10-19 coulomb
Mass of an electron = 9.106 X 10-31 kg
The ratio of charge mass of an electron = 1.77 X 1011 coulombs / kg
Energy of electron
The electrons which are moving around the nucleus posses two types of energy.
- Kinetic energy due to its motion
- Potential energy due to the charge on the nucleus.
Hence, the total energy of the electrons are the sum of these two energies. The total energy increases as its distance from the nucleus increases. Thus the electrons in the last orbit posses very high energy as compared to the electrons in the inner orbits.
The central portion of an atom is called nucleus. It contains protons and neutrons. The proton is a particle carrying a positive charge. The positive charge is numerically equal to the negative charge of an electron. The neutron has no charge but it is having the same mass as the proton. So, the nucleus of an atom is positively charged, The mass of an electron is (1/1845 that of a porton) negligible. Hence the neutron and proton constitute the entire weight of an atom. This is called atomic weight. In an atom, the number of electrons are equal to the number of protons. This number is called as atomic number. The negative charges of the electrons are exactly balanced by the positive charges of the protons. Therefore net charge of an atom is zero.
The electrons are revolving around the nucleus in different orbits. Each orbit have different energy level. The electrons moving in the outermost orbit have highest potential energy. They can be easily disturbed by external influences. These electrons are known as free electrons or valance electrons. The last orbit is known as valance orbit. The electrons moving in the inner orbits which are closely situated to nucleus have very low potential energy. They are much influenced by the central portion nucleus. Hence, they cannot be much disturbed by external influences. They are known as bound electrons.
Electron arrangement in atoms
In general, electrons reside in groups of orbits called shells, The shells are elliptically shaped and are assumed to be located an fixed intervals. Thus the shells are arranged insteps that corresponding to fixed energy levels.
The shells and the number of electrons required to fill them may be predicted by the employment of Pauli's Exclusion Principle. Simply this principle specifies that each shall may contain no more than 2n2 electrons where 'n' corresponds to the small number starting with the one closest to the nucleus.
Starting with the shell closest to the nucleus and progressing outward, the shell are labeled K,L,M,N,O,P and Q respectively. The shells are considered to be full or complete when they contain the following quantities of electrons : 2 in K shell, 8 in the L shell, 18 in the M shell and 32 in the N shell. The formula 2n2 can be used to determine the number of electrons only in the four shells closest to the nucleus of an atom. Succeeding shells have as maximum number of electrons; O –shell-18 electrons. P shell-12 electrons, and Q shell –2 electrons.
Each of the shell is a major shell and can be divided into sub shells. They are labled S,P, d and f A sub-shell exists at a given energy (level that is, at a given distance from the nucleus.)
Like the major shells, the subshells are also limited as to the number of electrons which they can contain. Thus the S subshell is complete when it contains two electrons, the P subshell when it contain six, the d subshell when it contains ten and the last subshell f when it contains fourteen electrons.
The K shell can contain not more than two electrons, it must have one sub shell, the S subshell. The M shell is composed of 3 sub shells; S, P and d. The relationship exists between shells and sub shells upto and including the N shell. In such a way, the electrons are arranged in an atom.
Energy band in solids
- Conduction band
- Forbidden energy gap
Electrons in the outer most orbit of an atom are called valence electrons. The range of energies possessed by valence electrons is known as valence band. This band may be completely or partially filled. It is the highest occupied band.
(ii) Conduction band
The electrons which left the valence band are called free electrons. The band occupied by these electrons is called the conduction band. This band is next in the valence band. It may either be empty or partially filled with electrons. In the conduction band, electrons move freely and conduct electric current through the solid.
(iii) Forbidden energy gap
The valence band and conduction band are separated by a gap on the energy band diagram is known as forbidden energy gap. There is no allowed energy state in the forbidden energy gap. If the width of the forbidden energy gap is greater means the valence electrons are tightly bounded to the nucleus and vice versa.
Insulators, Semiconductors and Metals
A semiconductor is a substance which conductivity lies in between these two extremes A material may be placed in one of these three class depending upon its energy band structure.
Insulator is a substance through which the passage of current is not allowed. The electrons in the valence band are bound very tightly to their parent atoms. Hence it requires very large electric field to remove them from their nuclei, in terms of energy band, the valence band is full, the conduction band is empty and the forbidden energy gap is very large between them. This is shown in Fig. 1.1(a). Therefore a very high electric field is required to carry the electron from the filled valence band into the empty conduction band. At higher temperature some electrons may go to the conduction band and in turn the insulator resistance decreases i.e., an insulator has negative temperature co-efficient of resistance.
A semiconductor is a substance whose conductivity lies in between conductors and insulators.
At low temperature, the valence band is completely full and conduction band is completely empty. Hence a semiconductor behaves as an insulator at low temperatures. As the temperature is increased, more valence electrons cross over to the conduction band and the conductivity increases. Hence electrical conductivity of the semiconductor increases, with rising temperature i,e., a semi conductor has negative temperature co-efficient of resistance. In terms of energy band the valence band is filled and the conduction band is empty. Moreover, the energy gap between valence and conduction band is very small as shown in Fig. 1.1(b). Hence, it requires smaller electrical field to push the electron from the valence band to the conduction band.
Conductor is a substance which easily allows the passage of electric current through it. It is because there are plenty of free electrons available in a conductor. In terms of energy band the valence and the conduction bands overlap each other as shown in Fig 1.1(c). In fact, there is no physical distinction between the two bands. A slight potential difference across a conductor is quite enough to cause the electrons to constitute electric current. There is no structure to establish holes since the forbidden energy gap is absent. Hence total current in such conductor is simply due to the flow of electrons.
Conduction in metals
In metals, atoms are kept very close to each other in a regular fashion, called crystal lattice. It is a repeating arrangement of atoms with in a crystal. The physical properties of a material are to a great extend depend upon the lattice structure of the material.
The atoms kept in the lattice have inter atomic reactions. Therefore the electrons in a particular orbit which have little bit difference in energy levels form large number of lines. That lines are regarded as continuous band of energy. A band is partially filled means that there is some free electrons. These free electrons move freely from one atom to another or a random manner inside the metal.
Depends upon the metal, one or two electrons are free to move in this manner. When an electric field is applied to a metal the free electrons are pulled towards the positive electrode. This drift towards the positive electrode by the external applied electrical field is superimposed on the random motion of the electrons due to thermal energy. Thus it constitutes an electric current that means, the material conduct electricity.