Sir Joseph John Thomson was born on 18th December 1856 near Manchester, England. His father was a book seller. He completed his Engineering course with the help of his friends as his father was dead. In 1881 he wrote a scientific paper which was a fore runner of the Einstein's theory.
In 1897, he was crowned the father of electron. Thomson was a highly gifted teacher. Not less than eight of his students distinguished themselves as Nobel Prize winners. His son won Nobel Prize for Physics. He got the Nobel Prize in 1906.
He left a legacy of text books on M.P.C. Mathematics, Physics and Chemistry, the brilliant trio of basis sciences which makes its students brilliant.
| KEY WORDS | |
| * Wave | * Sub - shell |
| * Electromagnetic spectrum | * Shapes of Orbitals |
| * Intensity | * Electron spin |
| * Discrete energy | * Electronic configuration |
| * Line spectrum | * The Pauli's exclusion principle |
| * Orbital | * Electron spin |
| * Quantum Numbers | * Aufbau Principle |
| * Shell | * Planck's constant |
| * Spectrum | |
KEY WORDS EXPLANATION
Wave: A periodic disturbance in a medium or in space is called a wave. This is vibration of a medium that moves from the one point to another carrying the energy but not the medium itself, with it.
Electromagnetic spectrum: The entire range of electromagnetic wave frequencies is known as the electromagnetic spectrum.
Intensity: The amount of energy per unit volume is called intensity.
Discrete energy: Definite amount of energy is called discrete energy.
Line spectrum: Each element emits its own characteristic colour. These colours correspond to certain discrete wave lengths of light and are called line spectra.
Orbital: The region of space around the nucleus where the probality of finding the electron is maximum is called an orbital.
Quantum numbers: Each electron in an atom is described by a set of three numbers n, l and ml. These numbers are called quantum numbers. These numbers indicate the probability of finding the electron in the space around the nucleus.
Shell: Electrons in an atom occupy 'stationary' orbits (states) of fixed energy at different distances from the nucleus. These stationary orbits are called shells.
Spectrum: Spectrum is a group of wavelengths.
Sub - shell: Sommerfeld modified Bohr's atomic model by adding elliptical orbits. These elliptical orbits are called sub-shells.
Shapes of orbitals: s - orbital is spherical in shape, p - orbital is dumbell shaped and d - orbitals are double dumbell shaped.
Electron spin: Electrons revolve round the nucleus and during this process they revolve around themselves. This is called electron spin. The spinning of the electron may be clock - wise or anti - clock wise.
Electronic configuration: The distribution of electrons in shells, sub - shells, and orbitals in an atom is known as Electronic configuration.
The Pauli’s exclusion Principle: No two electrons of the same atom can have all four quantum numbers the same.
Aufbau Principle: Orbitals are filled in the order of increasing energy. This is Aufbau principle.
Hund's rule: According to this rule electron pairing in orbitals starts only when all available empty orbitals of the same energy (degenerate orbitals) are singly occupied.
Planck's constant: Planck's constant h = 6.626 . 10-34 Js.
SYNOPSIS
* There are seven colours namely violet, indigo, blue, green, yellow, orange and red (VIBGYOR) in a rainbow. Electromagnetic waves are produced when an electric charge vibrates.
* A vibrating electric charge creates a change in the electric field. The changing electric field creates a changing magnetic field.
* Visible light is an electromagnetic wave and the speed of light is c = 3 × 108 ms-1
* c = υλ where υ is the frequency of a wave and λ is its wave length.
* Electromagnetic waves can have a wide variety of frequencies. The entire range of electromagnetic wave frequencies is known as the electromagnetic spectrum.
* The familiar example of the visible spectrum in nature is the formation of a rainbow.
* Max Planck broke with the 'continuous energy' tradition of electromagnetic energy by assuming that the energy is always emitted in multiples of hυ.
* The energy E for a certain frequency can be represented by the equation E = hυ where 'h' is Planck's constant which has the value 6.626 × 10-34 Js and υ is the frequency of radiation emitted or absorbed.
* The significance of Planck's proposal is that, electromagnetic energy can be gained or lost in discrete values and not in a continuous manner.
* Emission or absorption of light spectrum is a collection of a group of wavelengths.
* Sodium vapours produce yellow light in street lamps.
* Scientists found that each element emits its own characteristic colour. These colours correspond to certain discrete wavelengths of light and are called line spectra.
* The lines in atomic spectra can be used to identify unknown atoms, just like finger prints are used to identify people.
* Niels Bohr proposed a model for the atom. It is successful model as far as line spectra of hydrogen atom is concerned.
* Bohr's model failed to account for splitting of line spectra.
* To account for the structure (Splitting) of line spectra known as fine spectra, Sommerfeld modified Bohr's model of atom by adding elliptical orbits.
* Bohr - Sommmerfeld model, though successful in accounting for the fine line structure of hydrogen atomic spectra, does not provide a satisfactory picture of the structure of atom in general.
* This model failed to account for the atomic spectra of atoms of more than one electron.
* As the electrons are very small, light of very short wavelength is required to find the position and velocity of the electron.
* This short wavelength of light interacts with the electron and disturbs the motion of the electron.
* Hence, simultaneously the position and velocity of electron cannot be measured accurately.
* It is not possible to pin point an electron in an atom.
* Under these circumstances, to understand the properties of electrons in an atom, a quantum mechanical model of atom was developed by Erwin Schrodinger.
* The region of space around the nucleus where the probability of finding the electron is maximum is called an orbital.
* In a given space around the nucleus, only certain orbitals can exist.
* Each orbital of stable energy state for the electron is described by a particular set of quantum numbers.
* Each electron in an atom is described by a set of three numbers n, l, and ml. These numbers are called quantum numbers.
* These quantum numbers indicate the probability of finding the electron in the space around the nucleus.
* The principal quantum number (n) is related to the size and energy of the main shell.
* The angular - momentum quantum number (l) has integer values from 0 to (n - 1) for each value of n.
* Each 'l' value represents one sub - shell.
* The value of 'l' for a particular sub - shell is generally designated by the letters s, p, d, f etc.
* The magnetic quantum number (ml) has integer values from -l and l including zero. These values describe the orientation of the orbital in space relative to the other orbitals in the atom.
* s - orbital is spherical in shape, p - orbital is dumbell - shaped and d - orbitals are double dumbell - shaped.
* Each sub - shell holds a maximum of twice as many electrons as the number of orbitals in the sub - shell.
* To account for closely spaced doublet of yellow light, and to explain for such behaviour of the electron an additional quantum number spin quantum number is introduced. It is denoted by ms.
* The spin quantum number (ms) refers to the two possible orientations of the spin of an electron, one clock wise (+ 
) and the other anticlock wise (- ) spin. 1 2
* The distribution of electrons in shells, sub - shells and orbital in an atom is known as electronic configuration.
* According to Pauli exclusion principle no two electrons of the same atom can have all four quantum numbers the same.
* The maximum number of electrons in any shell is '2n2' where n is the principal quantum number.
* The lowest - energy oribitals are filled with electrons first. This is Aufbau principle.
Hund's rule: The orbitals of equal energy (degenerate) are occupied with one electron each before pairing of electron starts.
A brief introduction of Scientists
1. Sir C.V.Raman
Our beloved scientist and Noble prize winner, Sir C.V. Raman explained the phenomenon of light scattering in gases and liquids. He found experimentally that the frequency of scattered light by the liquids is greater than the frequency of incident light. This is called Raman Effect. By using this effect scientists determine the shapes of molecules.
2. Niels Bohr
Niels Henrik David Bohr was a Danish psysicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. Bohr was also a philosopher and a promoter.
3. Max Planck
Max Karl Ernest Ludwig Planck was a German theoretical physicist who originated quantum theory, which won him the Nobel Prize in Physics in 1918. Planck made many contributions to theoretical physics, but his fame rests primarily on his role as originator of the quantum theory. This theory revolutionized human understanding of atomic and subatomic processes.
Writer: C.V. Sarveswara Sarma
