Atoms Class 12 Notes Physics Chapter 12  CBSE
Chapter : 12
What are Atoms ?
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Model  Important Points  Limitations 
Alpha particle Scattering experiment 
 
Rutherford’s Atomic Model 


Bohr’s Atomic Model  Bohr incorporated following postulates in his theory: (i)The electron in an atom could revolve in certain stable orbits without the emission of radiant energy. (ii)The electron revolves around the nucleus only in those orbits for which angular momentum is some integral multiple of (h/2π) , where h is Planck constant ( = 6.6 × 10^{–34} Js) L = mvr = n(h/2π) where n = 1, 2, 3 and is called principal quantum number. (iii)Emission of radiation taken place when an electron makes a transition from a higher orbit to a lower orbit. During this transition, a photon is emitted having energy equal to the energy difference between the initial and final states hv = E_{i} – E_{f} 

Topic/Term  Formula  Symbol Representation  Important Points 
Rydberg formula for Spectrum of Hydrogen Atom  =(1/λ)=[(1/n2f) (1/n2i)]  v= Wave number R = Rydberg constant = 1.097 × 10^{7} m^{–1} n_{i} = Initial state n_{F} = Final state 

Relation between Impact parameter (b) and scattering angle θ  b =(Ze^{2} cot(θ /2)/(4πε0K))  Z = atomic number of the nucleous e = charge on electron θ = scattering angle K = kinetic energy of the αparticle i.e. (1/2)mv^{2}  
Distance of closest approach (r_{0})  r_{0} =(Ze(2e)/4πε_{0} × K)  Z = atomic number e = charge on electron K = kinetic energy  At the distance of closest approach whole of the kinetic energy the alpha particle is converted into potential energy. 
Energy of hydrogen atoms  E = (e^{2}/8πε_{0}r)  e = charge of electron  r = radius 