Atomic Structure
Atom is a Greek word which means can not be cut . [Dalton]
Rutherford’s Atomic Model : On the basis of the observation made by Rutherford , he proposed that a complete model of the atom which is named as “Rutherford’s Nuclear Atomic Model” .
According to this model :
I) Atom Contains a massive (heavy) and positively charged part at it’s center . This central part of the atom is called nucleus .
II) The volume occupied by the nucleus is only a minute fraction of the total volume of the atom .
III) Atom is not all solid as was earlier suggested by Dalton but it’s extraordinary hollow , since it consists of lot of empty space round the nucleus .
IV) Electrons are revolving round the nucleus is closed orbit with a fast speed and hence almost all the spaces round the nucleus is occupied by the revolving electrons .
Limitations of Rutherford’s Atomic Model :
Bohr pointed out the Rutherford’s atomic model is extremely unstable for the following reasons .
I) Newton’s law of motion and gravitation can only be applied to neutral bodies such as planets and not to charged bodies such as tiny electrons moving round a positive nucleus . The analogy does not hold good since the electron in an atom repel one another , where as plants attract each other because of gravitational forces .
II) According to Maxwell’s theory any charged body such as electrons rotating in an orbit must radiate energy continuously thereby losing kinetic energy . Hence the electron must gradually spiral in towards the nucleus , the radius of the electron will gradually decrease and it will ultimately fall into the nucleus .
III) Since the process of radiating energy would go on continuously the atomic spectra should also be continuous and should not give sharp and well defined lines .
Planck’s quantum theory of radiation :
I) A hot body emits radiant energy not continuously but discontinuously .
[Quantum : A quantum is the smallest unit of waves in the form of which a hot body emits radiant energy and which can exist independently .]
II) The energy associated with each quantum or photon of a given radiation or light is proportional to the frequency of the emitted radiation or light .
E ∝ ν
III) The energy emitted or absorbed by a body can be either equal to one quantum of energy or any whole number . Say , ‘n’ multiply of this unit . [ E = nhν ]
Bohr Atom Model
Bohr’s atom model retains the two essential features of ‘Rutherford Atomic Model’.
1. The atom has a very small positively charged nucleus at its center. All the protons and neutrons are contained in the nucleus. Thus most of the mass of the atom is concentrated in the nucleus.
2. Negatively charged electrons are revolving round the nucleus in the same way as planets are revolving round the sun.
Bohr made the following postulates.
1. Fixed Circular Orbit : Bohr assumed that an electron is a material particle which is revolving round the nucleus in concentric circular orbits situated at definite distance from the nucleus and with a definite velocity .
2. Stationary Energy Levels : As long as electron remains in a particular orbit , it neither emits nor absorbs energy. Thus in a particular orbit , the energy of a revolving electron remains constant or stationary. Hence each of the fixed orbits is associated with a definite amount of energy. The orbits are therefor also called stationary energy levels or simply energy levels or energy shell .
3. Jumping of the Electrons From One Energy Level to the Other : When the electron is excited , from a lower energy level to a higher one it absorbs energy . On the other hand when it comes back from higher energy level to a lower energy level , it emits energy . The quantity of energy emitted or absorbed is equal to the differences between the energies the atom passed before and after the electron changed energy levels .
If E1 and E2 are the energies of the electron in the energy level 1 and 2 . then the amount of energy absorbed by the electron incoming from energy level 1 to energy level 2 is given by the relation ,
E2 − E1 = hv
4. Principal of Quantisation of Angular Momentum of the Moving Electron : Bohr postulated that, an electron can not move in all the orbits. It can move only in that orbit in which the angular momentum of the electron moving round the nucleus is integral whole number multiple of n .
Characteristics of Light De Broglie’s Equation
According to Max Plunks Quantum Theory of Radiation some phenomena like photo electric effects and black body radiation can be explained if light radiation is supposed to behave as a material particle. Which is localized at a particular point in space. While according to Maxwell’s Wave Theory of Radiation certain other phenomena like detraction , interference and polarization can be explained if light radiation is supposed to behave as a wave which is spread out over space. Thus we find that light shows dwell character i.e. (that is/ id est) behaves both as a material particle or as a wave.
In 1924 a French physicist Louis de Broglie extended the idea of duel character of light to matter particles and suggested that ,“All matter particles in motion have a duel character.” I.e. all matter particles possess characteristics of both a material particle and a wave. The wave associated with matter particles is called De Broglie’s Matter Wave.
Since an electron is also kind of matter particle , it also has a dual character.
