CONCEPT OF ATOMIC MODEL'S By-N.K.D. Sir Please -Rate review and share this lesson
Thomson's Model Putting together all the facts known at that time, Thomson assumed that an atom is a sphere of positive charges uniformly distributed, with the electrons scattered as points throughout the sphere. This was known as plum-pudding model at that time. However this idea was dropped due to the success of -particle scattering experiments studied by Rutherford and Mardson. Rutherford's Model a-particle emitted by radioactive substance were shown to be dipositive Helium ions (He") having a mass of 4 units and 2 units of positive charge.
Rutherford allowed a narrow beam of -particles to fall on a very thin gold foil of thickness of the order of 4x10 cm and determined the subsequent path of these particles with the help of a zinc sulphide fluorescent screen. The zinc sulphide screen gives of a visible flash of light when struck by an a-particle, as ZnS has the remarkable property of converting kinetic energy of particle into visible light. [For this experiment, Rutherford specifically used a-particles because they are relatively heavy resulting in high momentum]. Observation i) Majority of the a-particles pass straight through the gold strip with little or no deflection ii) Some -particles are deflected from their path and diverge
iii) Very few particles are deflected backwards through angles greater than 90 . iv) Some were even scattered in the opposite direction at an angle of 180 [Rutherford was very much surprised by it and remarked that "It was as incredible as if you fired a 15 inch shell at a piece of tissue paper and it came back and hit you"] Conclusions I. The fact that most of the particles passed straight through the metal foil indicates the most part of the atom is empty 2. The fact that few particles are deflected at large angles indicates the presence of a heavy positively charged body i.e., for such large deflections to occur -particles must have come closer to or collided with a massive positively charged body, and he named it nucleus 3. The fact that one in 20,000 have deflected at 180 backwards indicates that volume occupied by this heavy positively charged body is very small in comparison to total volume of the atom
On the basis of the above observation, and having realized that the rebounding particles had met something even more massive than themselves inside the gold atom, Rutherford proposed an atomic model as follows. All the protons (+ve charge) and the neutrons (neutral charge) i.e. nearly the total mass of an atom is present in a very small region at the centre of the atom. The atom's central core is called nucleus i) ii The size of the nucleus is very small in comparison to the size of the atom. Diameter of the nucleus is about 10-13 while the atom has a diameter of the order 10 of cm. So, the size of atom is 10 times more than that of nucleus iii) Most of the space outside the nucleus is empty. iv) The electrons, equal in number to the net nuclear positive charge, revolve around the nucleus with high speed in various circular orbits The centrifugal force arising due to the high speed of an electron balances the columbic force of attraction of the nucleus and the electron remains stable in its path. Thus according to him atom consists of two parts (a) nucleus and (b) extra nuclear part. v)
Defects of Rutherford's atomic model Position of electrons: The exact positions of the electrons from the nucleus are not mentioned 1. Stability of the atom: Neils Bohr pointed out that Rutherford's atom should be highly unstable. According to the law of electro-dynamics, the electron should therefore, continuously emit radiation and lose energy. As a result of this a moving electron will come closer and closer to the nucleus and after passing through a spiral path, it should ultimately fall into the nucleus 2. It was calculated that the electron should fall into the nucleus in less than 10* sec. But it is known that electrons keep moving outsided the nucleus. To solve this problem Neils Bohr proposed an improved form of Rutherford's atomic model. Before going into the details of Neils Bohr model we would like to introduce you some important atomic terms.
ATOMIC SPECTRUM If the atom gains energy the electron passes from a lower energy level to a higher energy level, energy is absorbed that means a specific wave length is absorbed. Consequently, a dark line will appear in the spectrum. This dark line constitutes the absorption spectrumm Hydrogen Atom If an electric discharge is passed through hydrogen gas taken in a discharge tube under low pressure, and the emitted radiation is analysed with the help of spectrograph, it is found to consist of a series of sharp lines in the UV, visible and IR regions. This series of lines is known as line or atomic spectrum of hydrogen. The lines in the visible region can be directly seen on the photographic film. Each line of the spectrum corresponds to a light of definite wavelength. The entire spectrum consists of six series of lines each series, known after their discoverer as the Balmer, Paschen, Lyman, Brackett, Pfund and Humphrey series
The wavelength of all these series can be expressed by a single formula. ni n v= wave number 2wave length R Rydberg constant (109678 cm1') n, and n, have integral values as follows Main spectral lines Ultra-vio Visible Infra-red Infra-red Infra-red Series Lyman Balmer Paschen Brackett Pfund 2, 3, 4, etc 3, 4, 5 etc 4, 5, 6 etc 5, 6, 7 etc 6, 7, 8, etc 4 Note: All lines in the visible region are of Balmer series but reverse is not true, i.e., all Balmer lines will not fall in visible region
The pattern of lines in atomic spectrum is characteristic of hydrogen Types of emission spectra i) Continuous spectra: When white light from any source such as sun or bulb is analysed by passing through a prism, it splits up into seven different wide bands of colour from violet to red (like rainbow). These colour also continuous that each of them merges into the next. Hence the spectrum is called as continuous spectrum. ii Line spectra: When an electric discharge is passed through a gas at low pressure light is emitted. If this light is resolved by a spectroscope, it is found that some isolated coloured lines are obtained on a photographic plate separated from each other by dark spaces. This spectrum is called line spectrum. Each line in the spectrum corresponds to a particular wavelength. Each element gives its own characteristic spectrum.
Niranjay Kumar Dwivedi
(N.K.D.Sir) is a Complete Chemistry faculty for IIT~JEE, and NEET examination. with more then 8 year of Teaching experience