Electron Theory: Principles & Applications
Table of Contents
Electron Theory and Atom:
Electron Theory: Principles & Applications- Anything which tends to occupy space, and has a weight, is known as a matter. There are three types of matter, i.e., solid, liquid, and gas. It has been proved as a result of modern research that everything found in the universe, has been formed from a matter, and matter itself consists of small particulars, which are known as molecules. These molecules are also further composed of small particles, which are known as atoms (the substance which is formed as a result of a combination of two or more atoms, is called a molecule). The substances, the molecules of which are composed of the same atoms, are known as elements. And, the substances, which consist of dissimilar atoms, are known as compounds. So far, 105 elements have been discovered, whereas the number of compounds is actually unlimited. These elements have been bifurcated into several groups according to their chemical characteristics as well as atomic numbers. Thus, the smallest particle, which can sustain an independent existence, is known as an atom.
Structure of an Atom
According to the modern theory regarding the structure of an atom, the atoms of all substances consist of three fundamental particles, which are known as electron, proton, and neutron. An atom consists of the following segments;
- Internal Segment or portion
- External Segment or Portion
Internal Segment or portion
The central or interior part of an atom is known as the nucleus of that atom. The nucleus consists of two types of particles, one of which is called Proton while the other one is called Neutron. There is a positive charge on a proton (in other words, that particle of an atom, which bears a positive charge, is known as a proton), whereas there is no charge of any kind on a neutron (i.e., a neutron is such a particle, which is electrically neutral). Both these two types of particles are combined closely together in the nucleus through some extremely powerful forces.
External Segment or Portion
The external part of an atom consists of electrons. An electron is an extremely light, and tiny type of particle, which turns around the nucleus of an atom in almost an elliptical-shaped shell. There is a negative charge on an electron (The quantity of this charge tends to be equivalent to 1.602 x 10 -19 Coulomb). An electron is about 1 / 1840 times lighter compared to a proton (In other words, protons or neutrons are 1840 times heavier than an electron). Remember, that an electron has a mass of 9.11 x 10-28 grams, whereas it has a radius of about 1.9 x 10-15 meters.
The weight of any atom basically depends on the number of protons and neutrons existing in the nucleus of that atom, whereas electrons move around the nucleus of an atom under the influence of a centrifugal force in specific types of elliptical, or circular orbits or shells. This has been illustrated in figure 1.1 (a). The number of electrons and protons existing in any atom always remains equal, and a dissimilar charge always exists in both. That’s there is a positive charge on a proton, whereas a negative charge on an electron. As such, having mutually equal and inverse charge, electrons neither drop out from the atom nor do they infiltrate into the nucleus. Rather, they remain in a balanced state. That’s why an atom tends to be electrically neutral.
It has been proved from the aforementioned discussion that the positive charge on a proton equals the negative charge on an electron and that the number of protons and electrons in an atom, tends to be equal. Therefore, every atom ends to be electrically neutral. The total number of protons existing in the nucleus of an atom of an element, or the total number of freely moving electrons around the nucleus in distinct orbits, is known as the “Atomic Number” of that element. Whereas, the total number of neutrons and protons existing in the nucleus of an atom or the total weight of a nucleus, is known as the “Atomic Weight” or “Atomic Mass” of that particular element.
Electron Shells
We know that electrons rotate around the nucleus in their specific orbits or shells. These orbits are known as energy levels or shells. These shells are normally denoted as K. L. M. N. The shell closest to the nucleus tends to be “K”. After “K”, “L” is closest and as such, this process moves on.
A specific number of electrons exist in a shell, which is determined by the formula 2n2. That’s the maximum number of electrons in any shell can be 2n2. Here, “n” means the number of shells or orbits. In other words, every shell has a quantum number, which is represented by “n”. The number or price of ‘n” for the “K” shell turns out to be 1. Similarly, the price of “n” for L, M, and N shells will be 2, 3, and 4 respectively. For example,
The number of electrons in “K” shell = 2n2 = 2 (1)2 = 2 electrons
The number of electrons in “L” shell = 2n2 = 2 (2)2 = 8 electrons
The number of electrons in “M” shell = 2n2 = 2(3)2 = 18 electrons
Remember that there may be a maximum of seven shells or orbits in an atom. However, the distribution of electrons in an atom will take place according to the following conditions;
(i). The rule of 2n2 does not apply to the outermost shell
(ii). The number of electrons in the outermost shell cannot exceed 8
(iii). The number of electrons in the penultimate shell will not exceed 18
(iv). The number of electrons in any shell will not exceed 32.
Remember that every orbit of an atom will have a specific radius. Similarly, the second or “L” orbit will also have a distinct radius, which will be slightly bigger as compared to “K”. No electron in this atom can revolve around these two orbits. As such, every atom has certain specific orbits, which are known as ‘Allowed Orbits”. As no orbit is possible apart from these allowed orbits, therefore, the orbit lying midway in the allowed orbit is known as “Forbidden Orbit”.
Energy Levels
There is a specific energy of every electron on an orbit, therefore every orbit has a distinct energy level (Allowed Energy Level). For instance, the energy level of the “K” orbit tends to be the least, and the electrons present in its orbit, turn out to be closely connected to the nucleus owing to their proximity to the nucleus. The energy of electrons in this particular orbit tends to be less compared to the energy level of electrons in other orbits. The energy level of the outermost orbit tends to be maximum, and due to their larger distance from the nucleus, electrons in such orbits are not firmly connected to the nucleus. The energy level of electrons in this orbit is higher compared to the electrons in other orbits. See figure 1.1 (b).
Figure 1.1 (b) – Electrons shells K, L, M, and sub-shells s, p, d … around an atom
Structure of Electron shells
The atom of every element has a distinct atomic number, which can reflect the number of protons and electrons existing in the atom of that particular element. For example, the simplest form of an atom is hydrogen (the atomic number of hydrogens is one). Its nucleus contains just one proton, and only one electron rotates in its orbit. This has been demonstrated in figure 1.2 (a).
Figure 1.2 (a) – Hydrogen atom
Figure 1.2 (b) – Helium atom
The nucleus of a helium atom comprises two protons and two neutrons, and there are two electrons in its orbit. This has been illustrated in figure 1.2 (b).
The atomic number of a Carbon atom tends to be 6. That’s the number of protons in its nucleus tends to be 6, whereas its “K” shell has 2 while its “L” shell has 4 electrons. This has been illustrated in
Figure 1.3 – illustration of a carbon atom
Now take the example of a copper atom. A copper atom consists of 29 protons and 35 neutrons. As the number of protons in an atom tends to be equal to the number of electrons, therefore, a copper atom consists of 29 electrons, which are distributed in different shells according to the following method. Also see figure;
The number of electrons in K shell = 2n2 = 2(1)2 = 2 electrons
The number of electrons in L shell = 2n2 = 2 (n)2 = 8 electrons
The number of electrons in M shell = 2n2 = 2 (3)2 = 18 electrons
The number of electrons in N shell = (2 + 8 + 18) – 29 = 1 electron
Figure 1.4 – the copper atom
We know that there should be 8 electrons in the last shell of an atom. However, there are 4 electrons in the last shell of carbon, whereas just one electron in the last shell of copper. It means that the last shells of carbon and copper are incomplete. It has to be remembered that the last copper shell is less stable as compared to the last carbon shell. That’s, one electron existing in the last copper shell can easily be segregated from the atom through the exertion of a little energy.
Valence Electrons
The number of electrons present in the outer shell of an atom is called valence electrons, while the last or outer shell of an atom, is known as the valence orbit or valence shell.
We know that there is a maximum of 8 electrons in the outer shell of an atom. In other words, the maximum number of valence electrons can be 8. Moreover, the electrical characteristics of any metal can also be determined through the number of valence electrons existing in an atom.
All materials are divided into conductors, insulators, and semiconductors. If the number of valence electrons in an atom is less than 4, that material is known as a conductor. For example, copper, sodium, magnesium, and aluminum because they consist of 1, 1, 2, and 3 valence electrons respectively. If the number of valence electrons in an atom is greater than 4, that material is known as an insulator. For example, Nitrogen, Sulphur, and Neon, because they consist of 5, 6, and 8 valence electrons, respectively. If the number of valence electrons existing in the valence shell of an atom is 4, such a material is known as a semi-conductor. For example, carbon, silicon, and germanium are semiconductors, because each one of these consists of four valence electrons. Remember that the number of valence electrons in an atom is also called the valency of that atom.
Importance of Valence Electrons
Valence electrons are of tremendous importance. The valency of an atom reflects how easily an atom can transfer its valence electron to some other atom, or how easily can it get a valence electron from some other atom.
Atoms, in which the number of electrons is less than the conceivable number (i.e., 8), can very easily emit their valence electrons. For example, there are a total of 4 orbits in a copper atom, and the distribution of electrons in these orbits takes place like this; There are 2 electrons in the first orbit, 8 electrons in 2nd, 18 electrons in 3rd, and just one electron in the 4th orbit. Since the 4th orbit is the last, and further 7 electrons are required in order to accomplish it (i.e., a total of 8 electrons are required) therefore, the copper atom can emit its solitary valence electron quite easily, instead of obtaining further 7 atoms.
An atom, in which the number of electrons in the valence orbit is closer to a probable number, does not emit its valence electrons. Rather, they complete their valence orbit, by getting electrons from any other atom. For example, there are just two orbits in the atom of Nitrogen, and the distribution of electrons happens as such; two electrons in the first orbit and 6 electrons in the 2nd and last orbit. As the 2nd orbit is the last, and 8 electrons are required to accomplish it, therefore instead of emitting valence electrons, this atom tries to obtain a further two electrons from any other atom.
Free Electrons
The valence electrons which are connected to the nucleus weakly, are called free electrons. Normally, all electrons of an atom remain connected to it. However, atoms (elements) having a very small number of electrons (2 or 3) in their last orbit, can easily emit their electrons through the application of a little energy or potential. For example, copper or silver, etc. As valence electrons of copper and silver are very loosely attached to the nucleus. Therefore, these electrons shuffle from one atom to the second and from the second to the third atom. When the valence electron of an atom enters from one atom to another, then an electron of the second atom moves to the third atom. Although, no electron isolates from an atom at the same time, even then these electrons move freely randomly, and irregularly. These electrons are called free electrons. The free or random motion of such electrons in some type of copper wire has been illustrated in figure 1.4 (b).
Figure 1.4 (b) – Random motion of free electrons in a material
Remember that when every atom emits an electron, a positive charge appears on it. These types of atoms are known as Positive Ions. On the contrary, when an atom gains an electron from some other atom, a negative charge appears on it. Such atoms are known as Negative Ions.
Such materials, e.g., copper, aluminum, etc., have free electrons in their atoms, even if the slightest of energy is exerted on them (i.e., if the slightest of a potential difference is supplied on them), the free electrons found in them, start their motion in a particular direction. This type of rate of flow of such electrons is called electric current. This has been represented in figure 1.4 (c).
Figure 1.4 (c). Electrons flow from negative to positive when a voltage is applied across a conductive or semiconductive material.
Energy Bands
As has been described earlier, all electrons existing in an atom, rotate in a particular shell or orbit, and every electron has a specific energy level, which is related to the radius of an electron orbit. When a significant number of atoms are mutually brought closer, then all energy levels are divided into sub-energy levels and overlapped. Thus, they assume the shape of a band. The energy band represents a large number of energy levels, which are mutually very closer, however, they remain isolated from each other. Remember that the energy of a free electron keeps changing continuously.
For example, there are 4 valence electrons in a silicon atom, which rotate in 4 different orbits. As such, these electrons have 4 distinct energy levels. In figure 1.5 9a), all these four energy levels have been illustrated. The r1, r2, r3, and r4 are the specific radius of orbits of all four electrons. In this figure, the energy levels of internal electrons have not been shown, because they are not so important.
All valence electrons of a silicon atom remain present in their respective orbits at 0°C. As a result of the orbits of all these electrons, an energy band forms up, which is known as a valence band. At 0°C, the valence electrons of this silicon atom remain present in this particular band. If the temperature is raised from 0°C, then the energy of valence electrons at 25°C increases to such an extent that these electrons are segregated from the atom. Now they are known as free electrons. This process can be described like this; these electrons emit from the valence band and drift into the conduction band. In order to move from the valence band to the conduction band, these electrons require definite energy. The quantity of this energy is different for different materials. In figure 1.5 (b), the formation of an energy band has been elaborated.
In figure 1.5 (b), a forbidden energy gap has also been illustrated between the conduction band and valence band, which has been represented by “Eg”. As no electron can stay within this gap, therefore, in order to move from a valence band to the conduction band, an electron requires an external energy equivalent to forbidden energy gap or even higher than this. As such, three types of energy bands are found in a solid, which have been explained below;
(i). Filled Band
As the name suggests, it is a type of band, which is very close to the nucleus and it always remains completely filled. No free electrons are found in a completely filled band.
(ii). Conduction Band
If a free electron possesses that much quantity of energy, that it can easily escape from the surface of an atom, it is called conduction, and this type of energy level or band is known as the conduction band. All electrons, existing in the conduction band are called free electrons.
(iii). Valence Band
There tends to be another band beneath a conduction band, which is known as a valence band. The band available for the energies of valence electrons is known as a valence band. The number of electrons found in the last shell or orbit is known as valence electrons. It means that a valence band contains such electrons, the energy level of which is very high, and this band is either completely or partially filled.
Conductor, Insulator, and Semi-Conductor
Metal tends to be neutral from an electricity perspective, however, it can be charged positively or negatively. That’s electrons can either be escaped from it or intruded into it. If electrons are escaped from a metal, a positive charge appears on it, and if electrons are included in it, then a negative charge appears on the metal surface. According to the atomic theory, the electric characteristics of different elements and metals depending on the nature of the band being formed alongside the nucleus of this metal’s electron. The solid or metal substances have been bifurcated as insulators, semi-conductor, and conductors according to the energy band.
(i). Insulator
A material having very few free electrons, and through which current cannot easily transmit (i.e., the flow of electrons does not occur easily) is called an insulator. For example, glass, mica, plastic, rubber, paper, dry wood, etc. Insulators are those materials that have very few free electrons and require a large applied voltage to establish a measurable current level. In figure 1.6 (a), the energy band diagram of an insulator has been illustrated. It is obvious from the figure that a huge energy gap exists between the valence band and conduction band. As a result, free electrons require a huge quantity of energy to move from the valence band to the conduction band. Therefore, valence electrons cannot jump and enter the conduction band easily at room temperature. Remember that the conductivity (ability to pass current) of an insulator tends to be extremely low. That’s the reason, the flow of electrons does not occur so easily in them.
(ii). Semi-Conductor
A material in which the number of free electrons is far less than a conductor, however quite higher as compared to an insulator, is known as a semiconductor. In other words, semiconductors are such materials, that neither completely have characteristics of a conductor nor that of an insulator (that’s they are neither good conductors, nor good insulators) rather their characteristics lie between a conductor and an insulator. In other words, there is another material between a conductor and an insulator, which is known as a semiconductor. Germanium, silicon, and carbon are examples of the best semiconductor materials.
Semiconductors are a specific group of elements that exhibit characteristics that lie between those insulators and conductors. Semiconductor materials have four electrons in the outermost orbit.
In figure 1.6 9b), it is evident that a small energy gap exists between a valence band and a conduction band, which means that less energy is required for shifting valence electrons to the conduction band. Some of the valence electrons jump to the conduction band by getting a reasonable amount of energy at room temperature and converting it into free electrons. Thus, holes are produced as a result of the emission of electrons from the valence band. It has also to be remembered that the electrical conductivity of a conductor declines as a result of an increase in temperature, whereas the conductivity of a semiconductor increases owing to an increase in temperature. The conductivity of a semiconductor depends on electron-hole pairs, which tend to increase with an increase in the temperature of a semiconductor.
(iii). Conductor
A material in which free electrons are found in a very large number is known as a conductor. Normally, metals are good conductors, e.g., copper, gold, aluminum, etc. In other words, materials having electrons very weakly associated with their nucleus, and wherefrom current can be transmitted quite easily, are called conductors. In figure 1.6, the energy band diagram of a conductor has been illustrated, in which the conduction band and valence band can be seen overlapping each other. As a result, electrons are at liberty to move from the valence band to the conduction band without the consumption of any kind of energy. That’s electrons can jump and enter the conduction band easily from the valence band without any kind of obstacles. As the conductivity (ability to pass current) of a conductor tends to be extremely high, therefore a very negligible energy loss can occur while passing a current through a conductor.
Conductors are those materials that permit a generous flow of electrons with very little external force (voltage) applied. Good conductors have only one electron in the valence ring.
The conductivities of different materials have been given in the table below;
| Metal | Conductivity |
| Silver
Copper Gold Aluminum Tungsten Nickel Iron Nichrome |
105
100 70.5 61 31.2 22.1 14 1.73 |
Electric Charge
The deficiency and excess of electrons in any item is known as a charge of that item. If there is an excess of electrons, that particular substance is called negatively charged, and if there is a deficiency, it is called positively charged.
The unit of charge is known as a Coulomb, and it is denoted by “C”, whereas a charge is denoted by Q or q. A Coulomb can be defined as follows;
A charge equivalent to 6.29 x 1018 electrons, is called coulomb.
A charge that an ampere current can transfer in one second from one place to another, is known as one Coulomb.
A Coulomb is a charge which when placed in an electric field (having a value of one volt per meter) a one-newton force tends to exert on it. Remember that the value of charge on an electron tends to be 1.60 x 10-19.
Resistance
That characteristic of any material, by means of which it causes a hurdle in the transmission of an electric current through it, is known as resistance. The property of a substance that opposes the flow of electricity through it is called resistance or resistance is the ability of a circuit to oppose current. Glass, Mica, air, wood, etc., are substances that cause extreme hindrances in the flow of current through them. The unit of resistance is called Ohm and it is represented by “Ω”, whereas resistance is denoted by R. Ohm can be defined as follows; If the difference of potential between two ends of any conductor is one volt, and one ampere current flows through it, then the resistance of such a conductor will be one Ohm.
A resistance, in which a heat of one joule per second (or one watt) produces if a one-ampere current is transmitted, is known as one Ohm resistance. It is that quantity of resistance, which produces one-joule heat per second upon passing one-ampere current.
Conductance
The inverse of resistance is called conductance, i.e., the reciprocal of resistance is called conductance. Conductance is a property of any material that demonstrates how easily this material can let the current pass through it. Conductance refers the capability of a circuit to permit a current to flow it or the current carrying capacity of a material is known as a conductance. Current can flow through silver, copper, aluminum, etc., quite easily. Therefore, such materials are called good conductors. Because their resistance tends to be very low. Conductance is denoted by G. As it is a reciprocal of resistance, therefore;
G = 1 / R or R = 1 / G
The unit of conductance is Mho or Siemens. Mho is denoted by “℧”. Remember that if any circuit has a capacity of high resistance, its conductance tends to be very low.
Electric Current
The rate of flow of electrons or flow of charge in any conductor is called electric current. See figure 1.7 (a). Remember that electric current is always produced as a result of the potential difference between two charges. The rate of flow of electrons is called current or current is the rate of flow of charge.
Figure 1.17 (a) – The drift of the electrons through a conductor is a current
If a Q Coulomb charge flows in “t” second through any point of a conductor, then the rate of flow of electric current can be described as Coulomb per second. i.e.,
I = Q / t or Current = Charge / Time
The unit of current is known as an ampere, and it is denoted by A. An ampere can be defined as follows;
If a one Coulomb charge flows through any point of a conductor in one second, then the current passing through that point tends to be one ampere. (see figure 1.7 (b).
Figure 1.7 (b) – illustration of one ampere of current in a material (1. C/s)
Note: Current flows in conductors as a result of a flow of electrons, however in semi-conductors, apart from the flow of electrons, current also occurs as a consequence of holes, which are usually positively charged. If the current is taken in the direction of a positive charge, it is known as conventional current. And if the current is taken in the direction of the flow of electrons, it is known as electronic current.
Difference Between Conventional Current and electron Current
Suppose a copper wire has been connected between two terminals of a battery. As has been illustrated in figure 1.8 (a). As soon as copper wire is touched to the positive and negative terminals of the battery, free copper wire electrons start their motion toward the positive terminal of the battery, whereas positive ions will keep oscillating in a simple manner on a mean fixed position. When electrons of a copper wire drift towards the positive terminal, the supply of electrons from the negative terminal keep continuing. The positive terminal keeps absorbing electrons under a chemical reaction of the battery, whereas the negative terminal keeps supplying electrons continuously.
Figure 1.8 (a)
Two dimensions of the flow of charge have been illustrated in the figure. The flow of electrons from a battery’s negative terminal (via a copper wire) to the positive terminal of the battery is known as electron flow or electron current. Whereas, the flow of positive charge from a positive terminal to the negative terminal of a battery, is called charge flow or conventional current (I), though, no positive charge flows through this circuit (flow of conventional current is a hypothetical thing)
Figure 1.8 (b) -Conventional current direction diagram
Remember that most of the books are written keeping in view the conventional flow. The reason is that this theory is profusely used in educational institutions as well as industrial concerns. This concept is also used in designing the symbols of all electronic gadgets or components. And the selection of this hypothesis for all computer software packages is also quite popular. The reciprocally type two flow theory was devised at a time when electricity was discovered. The foundation of this theory was laid on the assumption that positive charge was considered a movable particle in metal conductors. Remember that the selection of a conventional flow does not cause any sort of difficulty in understanding and reading. Electronics engineers mostly prefer electron flow, whereas electrical engineers generally keep a positive charge flow (i.e., a conventional flow) before them.
Potential and Potential Difference
If a charge Q1 is placed alongside charge Q, then according to Coulomb force, charge Q will either attract the second charge Q1 towards it, or it will repel it. It means that charge Q will do some work on other charge Q1. The working ability of this charge is known as its charge potential. The energy being capable of performing some type of work is called potential. The quantity of work done for moving a charge from one place to another through attraction or repulsion is known as potential difference. In other words, a potential difference is a type of electrical pressure existing between two points, and it keeps existing due to the presence of different quantities of charge on these points. That’s if two charges are placed on two different points, then a potential difference will exist between these two points.
To measure potential and a protentional difference, a volt unit is used, which is denoted by V. A volt may be defined as follows;
(i). The potential difference between two points will be one volt if one Coulomb charge uses one Joule energy for drifting from one point to another. A potential difference of 1 volt (V) exists between two points if 1 Joule of energy is exchanged in moving 1 Coulomb (C) of charge between the two points.
Volt = 1 Joule / Coulomb
The following equation can be derived from the aforementioned definition;
W = VQ
Here, W means work in Joule, V means volt (or P.d) and Q means charge (Coulomb).
(ii). If a one-ampere current flows through a resistor, and its own value is 1 ohm, so the difference of its parallel potential will be one volt.
It must be remembered that a potential difference is required for generating a flow of charge, which can normally be attained through a battery etc.
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