Title THEORY.pdf ✅

 Digital [1] Various models for structure of atom Dalton's Theory Every material is composed of minute particles known as atom. Atom is indivisible i.e. it cannot be subdivided. It can neither be created nor be destroyed. All atoms of same element are identical physically as well as chemically, whereas atoms of different elements are different in properties. The atoms of different elements are made up of hydrogen atoms. (The radius of the heaviest atom is about 10 times that of hydrogen atom and its mass is about 250 times that of hydrogen). The atom is stable and electrically neutral. Dalton's Atomic Theory Part - 01 TG: @Chalnaayaaar
 Digital [1] Thomson's Atom Model The atom as a whole is electrically neutral because the positive charge present on the atom (sphere) is equal to the negative charge of electrons present in the sphere. Atom is a positively charged sphere of radius 10–10 m in which electrons are embedded in between. The positive charge and the whole mass of the atom is uniformly distributed throughout the sphere. Shortcomings of Thomson's model (i) The spectrum of atoms cannot be explained with the help of this model (ii) Scattering of –particles cannot be explained with the help of this model Electron Positively charged matter Thomson's Atomic Model Part - 02 TG: @Chalnaayaaar
 Digital [1] Rutherford experiments on scattering of  – particles by thin gold foil Ernst Rutherford, H. Geiger and E. Marsden Performed this experiment. In this experiment a beam of a-Particles (Helium nucleus) of energy 5.5 MeV, emitted by a 214 83 Bi radioactive source was directed at a thin metal foil made of gold. They studied the -particles scattered at various angles. The experimental arrangement is shown in figure. –particles are emitted by some radioactive material (polonium), kept inside a thick lead box. A very fine beam of –particles passes through a small hole in the lead screen. This well collimated beam is then allowed to fall on a thin gold foil. While passing through the gold foil, –particles are scattered through different angles. A zinc sulphide screen was placed out the other side of the gold foil. This screen was movable, so as to receive the –particles, scattered from the gold foil at angles varying from 0 to 180°. When an –particle strikes the screen, it produces a flash of light and it is observed by the microscope. It was found that : • Most of the  – particles went straight through the gold foil and produced flashes on the screen as if there were nothing inside gold foil. Thus the atom is hollow. • Few particles collided with the atoms of the foil which have scattered or deflected through considerable large angles. Few particles even turned back towards source itself. • The entire positive charge and almost whole mass of the atom is concentrated in small centre called a nucleus. • The electrons could not deflected the path of a  – particles i.e. electrons are very light. • Electrons revolve round the nucleus in circular orbits. So, Rutherford 1911, proposed a new type of model of the atom. According to this model, the positive charge of the atom, instead of being uniformly distributed throughout a sphere of atomic dimension is concentrated in a very small volume (Less than 10–13m is diameter) at it centre. This central core, now called nucleus, is surrounded by clouds of electron makes. lead box source of -particle lead screen vacuum ZnS screen most -pass through  some are deviated through large angle  about 1 in 8000 is repelled back beam of -particle gold foil 107m N() cosec4 θ 2 90 180  N() Rutherford Atomic Model Part - 03 TG: @Chalnaayaaar
Atoms Part-03  Digital [2] The entire atom electrically neutral. According to Rutherford scattering formula, the number of  – particle scattered at an angle  by a target are given by 2 2 0 2 2 2 2 4 0 0 2 N nt(2Ze ) 1 N 16(4 ) r (mv ) sin   =   Where N0 = number of  -particles that strike the unit area of the scatter n = Number of target atom per m3 t = Thickness of target Ze = Charge on the target nucleus 2e = Charge on  – particle r = Distance of the screen from target v0 = Velocity of  – particles at nearer distance of approach the size of a nucleus or the distance of nearer approach is given by. 2 2 0 0 0 K 2 0 1 (2Ze) 1 (2Ze) r 4 4 E 1 mv 2 =  =         where EK = K.E. of  - Particle Failure of Rutherford's Atomic model :- (i) It couldn't explain the stability of atom. (ii) It couldn't explain discrete nature of hydrogen spectra. target nucleus  b area = b 2 -particle r0 Ze nucleus TG: @Chalnaayaaar