The ability of an element to give away its outermost electrons to form positive ions is manifested in the amount of energy supplied to its atoms sufficiently enough to take away the electrons out of them. This energy is known as Ionisation Energy. Simply speaking, the Ionisation Energy is the energy supplied to an isolated atom or molecule to knock out its most loosely bound valence shell electron to form a positive ion. Its unit is electron-volt eV or kJ/mol and is measured in an electric discharge tube in which a fast-moving electron collides with a gaseous element to eject one of its electrons. The lesser Ionisation Energy (IE), the better the ability to form cations.
This can be explained with the Bohr model of an atom, in that it considers a hydrogen-like atom in which an electron revolves around a positively charged nucleus due to the columbic force of attraction and the electron can only have fixed or quantized energy levels. The energy of a Bohr model electron is quantized and given as below :
Where Z is the atomic number and n is the principal quantum number where n is an integer. For a hydrogen atom, Ionisation energy is 13.6 eV.
The Ionisation Energy (eV) is the energy required to take the electron from n = 1 (ground state or most stable state) to infinity. Hence taking 0 (eV) reference at infinity, the Ionisation Energy can be written as :
The concept of Ionisation Energy supports the evidence of Bohr model of atom that the electron can revolve around the nucleus in fixed or discrete energy levels or shells represented by the principal quantum number ‘n’. As the first electron goes away from the vicinity of the positive nucleus, then greater energy is required to remove the next loosely bound electron as the electrostatic force of attraction increases, i.e., the second Ionisation Energy is greater than the first one.
For example, the first ionization energy of Sodium (Na) is given as :
And where its second Ionisation Energy is
Hence, IE2 > IE1 (eV). This is also true if there are K number of ionisations, then IE1 < IE2 < IE3……….< IEk
Metals have low Ionisation Energy. Low Ionisation Energy implies better conductivity of the element. For example, the conductivity of Silver (Ag, atomic number Z = 47) is 6.30 × (10^7) s/m and its Ionisation Energy is 7.575 eV and for Copper (Cu, Z = 29) is 5.76 × (10^7) s/m and its Ionisation Energy is 7.726 eV. In conductors the low Ionisation Energy causes the electrons to move throughout the positively charged lattice, forming an electron cloud.
Factors Affecting Ionisation Energy
In the periodic table, the general trend is that the Ionisation Energy increases from left to right and decreases from top to bottom. So the factors affecting ionization energy can be summarised below:
Size of the Atom: The Ionisation Energy decreases with the size of the atom because as the atomic radius increases the columbic force of attraction between the nucleus and outermost electron decreases and vice-versa.
Shielding Effect:
The presence of inner shell electrons shield or weaken the columbic force of attraction between the nucleus and the valence shell electrons. Hence ionization energy decreases. The number of inner electrons means more shielding. However, in the case of gold, the Ionisation Energy is greater than silver even if the size of gold is more than silver. This is due to the weak shielding offered by the inner d and f orbitals in case of gold.
Electronic Configuration:
The more stable the electronic configuration of the atom, the more difficult is to withdraw an electron hence more Ionisation Energy.
Nuclear Charge:
The more the nuclear charge, the more it will be difficult to ionize the atom due to more attraction force between nucleus and electrons.
What is Electrical Energy?
Before explaining what electrical energy is, let us try to review the potential difference between two points in an electric field.
Suppose potential difference between point A and point B in an electric is v volts. As per the definition of potential difference we can say, if one positive unit electrical charge that is a body containing one-coulomb positive charge travels from point A to point B, it will do v joules work.
Now instead of one-coulomb charge if q coulomb charge moves from point A to B, it will do vq joules work. If the time taken by the q coulomb charge to travel from point A to B is t second, then we can write the rate of work done as
Again, we define the work done per second as power. In that case, the term
would be electrical power. In differential form, we can write, electric power where the Watt is the unit of power.
Now, if we place a conductor in between A and B, and through which the quantity of electric charge q coulomb is passing. The charge passing through a cross-section of the conductor per unit time (second) is
It is nothing but the electric current i, through the conductor. Now, we can write,
If this current flows through the conductor for a time t, we can say the total work done by the charge is
We define this as electrical energy. So, we can say,
Electrical Energy Definition
Electrical energy is the work done by electric charge. If current i ampere flows through a conductor or through any other conductive element of potential difference v volts across it, for time t second, the electric energy is,
Electrical Energy Formula
The expression of electric power is
The electrical energy is
The unit of Electrical Energy is joule.
This equals to one watt X one second. Commercially, we also use other units of electrical energy, such as watt-hours, kilo watt hours, megawatt hours etc
If one watt power is being consumed for 1 hour time, the energy consumed is one watt-hour.
BOT Unit or Board of Trade Unit or Kwh
The practical, as well as a commercial unit of electrical energy, is kilowatt hour. The fundamental commercial unit is watt-hour and one kilowatt hour implies 1000 watt hours. The electrical supply companies take electric energy charges from their consumer per kilowatt hour unit basis. This kilowatt hour is board of trade unit that is BOT unit.
Electron volt or eV
The concept of electron volt is very simple. Let us start from very basic. We know the unit of power is watt.
W = VI, where V is the voltage and I is the current.
Now as I is current, it is nothing but the rate of charge transfer. Therefore, the instantaneous impression of power would be
Where q(t) is the amount of charge transferred in time t. Hence now the energy is expressed as
Where q is the charge in Coulomb crosses a voltage V volts. From the expression of energy, we can write the energy required or work to be done for crossing an electric field of total voltage V by a charge Q coulomb is QV coulomb – volt or joules. Now we know the charge of an electron is – 1.6 × 10-19 coulomb and consider it has crossed an electric field of total voltage 1 V. Then the total work to be done is a charge of electron × 1 V.
This amount of energy is considered as a micro-unit of energy called electron-volt.
Definition of Electron – volt
One electron – volt is the unit of energy in joules which equals to the amount of work to be done for bringing one electron against an electric field of potential difference 1 volt.
This is very tiny or micro unit of energy mainly used for different calculation in atomic and electronic levels. The concept of energy levels in the materials is dealt with this micro unit of energy that is electron volt. Not only the energy of electrons, this unit is also used for all types of energy like thermal, light etc.
What is Drifting and Definition of Drift Velocity
If a particle moves in space in such a manner that it randomly changes its directions and velocities, the resultant of these random motions as a whole is called drift velocity.
The definition of drift velocity can be understood by imagining the random motion of free electrons in a conductor. The free electrons move in a conductor with random velocities and random directions. When we apply an electric field across the conductor, the randomly moving electrons experience an electrical force in the direction of the field.
Due to this field, the electrons do not give up their randomness of motion, but they will shift towards higher potential with their random motion. That means the electrons will drift towards higher potential along with their random motions. Thus, every electron will have a net velocity towards the higher potential end of the conductor, and we refer to this net velocity as the drift velocity of electrons. Hopping, you understand the definition of drift velocity. The current due to this drift movement of electrons inside an electrically stressed conductor, is known as drift current. It is needless to say that every electric current is “drift current”.
Drift Velocity and Mobility
There are always some free electrons inside any metal at room temperature. More scientifically, at any temperature above the absolute zero, there must be at least some free electrons if the substance is conductive such as metal. These free electrons inside the conductor move randomly and frequently collide with heavier atoms and change their direction of motion every time. When a steady electric field is applied to the conductor, the electrons start moving towards the positive terminal of the applied electrical potential difference. But this movement of electrons does not happen in a straight way.
During travelling towards the positive potential, the electrons continuously collide with the atoms and bounced back randomly. During the collision the electrons lose some of their kinetic energy and again due to the presence of electric field, they are re-accelerated towards the positive potential and regain their kinetic energy. Again, during the further collisions, the electrons partly lose their kinetic energy in the same manner. Thus the applied electric field cannot stop the random motion of the electrons inside a conductor. Although in the presence of the applied electric field, the motions of the electrons are still random, there will be a resultant overall movement of electrons towards positive terminals.
In other words, the applied electric field makes the electrons to drift towards positive terminal. That means the electrons get an average drift velocity. If electric field intensity gets increased the electrons are accelerated more rapidly towards positive potential after each collision. Consequently, the electrons gain more average drift velocity towards positive potential, i.e. in the direction opposite to the applied electric field.
If ν is the drift velocity and E is the applied electric field where μe is referred to as electron mobility.
Animation of Drift Velocity Drift Current and Electron Mobility
The current caused by the steady flow of electrons due to drift velocity is called drift current.
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