A phacdend oleprticle- scattering Cap) rah £27 capers pe shi al, uras comes put | Me Hort, CYS kp ‘ D9 ee oe Saf om meer woth fe ae NGS, < oe echt \ Z pol oruuem oC \ in ‘ be Boor ota-pomha : ss mv Os tee oa s a3 - ee ed ho a (axiom) ou rie ‘ Oy Comer Hho ver of a gern aig oer” aT ex porbicled See eee ath Spores g pberrmsael « con & Srv ror Hex govbicles scottemed CAI) Bgeirk ory le. 0 of thet Seal? @) Nw oe ce o ciaacea toi Os ree lS posse Homes 04 Fok wutheuk 277 apy reciable oleh Gis aoa aetvaeed thes potha meals: 4 es kn ke se me eo mh@ A AA hh A CBSE Class-12 Physics Quick Revision Notes Chapter-12: Atoms Thomson's Model of an Atom: An atom consists of positively charged matter in which the negatively charged electrons are uniformly embedded like plums in a pudding, This model could not éxplain scattering of alpha-particles through thin foils and hence discarded Rutherford’s Model of an Atom: (1411) Ge and Marsden in their experiment on scattering of alpha-particles found that mos of the alpha-particles passed undeviated through thin foils but some of them were scattered through very large angles he following model of ar From the results of these experiments, Rutherford proposed atom, ae ay a tom Consists of a small and massive central cc ye which the entire positive charge and almost the whole mass of the atom are coucentrated. This Gore is caller the nucle <14m) % 10 : , pe fosedeits tatupies avery sali Spoce asCompared to the size of the stung 10. ™ ¢) The atom is surrounded by a suitable number of clectros so that their total negative charge is equal to the total positive charge on the nucleus and the atom as a whole electrically neutral a) The electrons volve around the nucleus in various orbits jist as planets revolv« around the sun ¢) The centripetal force required for their revolution is provided by the elvetrostatio attraction between the electrons and the nucleus Draw-back of Rutherford Modi This model could not expiain lability of the atom because according to classical electromagnetic theory the electron revolving around the nucleus must continuo oly radiate energy revolving around the nucleus must continuously radiate energy in the form of electromagnetic radiate energy in the form of ¢lectromagnetic radiation ane hence it should f. Distance of Closest Approach: When an alpha-particle of mass m and velocily Vmoves directly towards a nuch atomic number Z, its initial energy E, which is just the kinctic energy K gets complete! converted into potential energy U at stopping point. This stopping point happens te be ab a distance of closest approach from the nucleus. Lip | Ze I 97e 2 Aly do Afétly Alegk i into the nucleus. Hence, It. Impact Parameter: yor a) Itis defined as the perpendicular distance of the velocity’of the alpha-particle from the central of the nucleus, when it is far away from the atom b)_ The shape of the trajectory of the scattered alpha-particle depends on the impact parameter b and the nature of the potential field. , c) Rutherford deduced the following relationship between the impact parameter band the scatte 4% np 20 D Geb 275 Cal Quantisation or Discretisatior The quantization or discretisation of a physical quantity means hat it cannot Vary conunuously to have any arbitrary valu€but can change only discontinaously to take certain specitic values. Bohr’s Model for the Hane Atom: (/9/2) Basic postulates (Quontum iene es Ruthescfirdt abornte model) 4) Nuclear concepts An atom consists of a small ynassive central called nucleus around which Fe=Fe planetary clectrons revolve. The centripetal force required fot their rotationie tut. p Se provided by the electrostatic attraction between the electrons and the queleug b) Quantum condition: , Of all the possible girewlar orbits allowed by the classical theory the electrons are. permitted to circ ly in such orbits in which the angular momentur electron is an integral multipleof h/2ar, hleeing Planck’s constant - tre b-62%I0 eters eDr ye led principal quantum number, tionary orbits: While revolving in the per ‘These non-radiating orbits a d) Frequency, conditions An atom Can emit or absorb radiation in the form of discrete when an electron jumps from 4 orbit Es and ts are the energies axsoctated with these pert issibie orbits, an clectron does not called stationary orbits. frequency Y of the emitted absorbed radiation is, hw = Ej - &f ©) Radius of the orbit of an cleetron in hydrogen atom is,ww ewe eee ree ere ee ee 7 TF Fy FF [i s —_ Larch. hagpem of Hal Eo= €s € «6 | a (x) 2 13% 2 0. 85eU ; eat 23 . = Coen th ponradttd Be CBSE de rar Cxoddl of 1 FO rs teh 5 A abem e _ berth Qadivs of 3 op BE (n=) Of amin ~ prnert ' 44 cableol Bob radtus (Aa) again ‘Aol Kinetic energy K ony tic potential energy U of the electron in hydrogen atom E= K+U BP Total enemy E of the electron it hydrogen atom - 2 _ -2' ne mer _ En |. ks Boksve-ge, = Gane (eatwe) 9|-greemt "PS aye on h) Speed of an eletyon in th rast murs hse OF tind A Where a 1s (ine structure constont ot y ch i) Energy ofan electron in ath orbit tf € En = EE for BEA Ey Rye - Be + Failure of Bohr’s Model: (heowe Ry = AO called Ting Cette 8) This mode! is applicahiconly tolyerager-tke Sams and folaincaee other = 10% F a ‘atoms. b) Itcould not explain the tine structure of the spectral Fines in the spectrum of hydrogen atom he ehergies of the different stationary’states of an atem are izontal lines drawn according to Some suita ble energy seals. Atom: epresented by paralle! | + Spectral Series of Hydrog Whenever an cle in hydrogen atom makes a transition from a higher ene piled ni to lower « vel vy. the difference of energy appears iti the form of a phoror ot frequency Visgwenby— Ej Eg=hO > yr EI-Ef whee ee va deneel als +: CeVA gy ca gh Ee ae % ae Different Spectral Series of Hydrogen a $ ehh ae These are 2s follows: a) Lymon Series. Herem=2.3.4....andryed This series ies inthe suite on 5) Bolmer Series, Here} 45 ond ¥y=2 This Senes lies inthe visible region | Paschen Series. Herenj= 45.6 andmy =3 This series lies inthe infrared regien Brockett Series Here mg= 5,67... ty =4-This sores hes in the infrored region 136 - 3.400 “i exiled flabes ee t- nee 5 ABE = ST «)te) Pfund Series. Here nye 6.7.8 ong? 8 ‘This series lies in the infrared region. Excitation Energy: Its defined as the energy required by an electron ofan atom to jump from its ground state to any one of its existed state. lonisation Energy Wis defined as the energy required to remove an electron from an atom, ie, the energy required Lo Lake an electron from its ground state to the outermost orbit (n= ©) Excitation Potential: Tis the accelerating potential which gives sullicient energy to a bombarding electron se to excite the target atom by raising one of its electrons from an inner to and outer oF Jonisation Potential; {tis the accelerating potential which gives to bombardjng elect: on the sufficient energ: to an outer orbit De Broglie’s Hypothesis: The electrons having a wavelength 2 =h/mv gave an explan: orbits by bringing in the wa ton for Boli's quantised ve particle duality. The arbits correspond to circular standing waves in which the circumference of the orbit equals a whole number of MASER: a) Maser stands for Microwaves Amplification by Stimulated Emission of Radiation b)_Itis simply a device for producing collimated beam of microwaves LASER: a) Itstand for Light Amplification by Stimulated Emission of Radiation. b) tis a device used to produce highly intense strong monochromatic coherent anct epllimated beam of hi y intense, monochromatic coherent and ESS n=6 st Leonel m4 NeB