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1 1 PHYSICAL CHEMISTRY Dalton (1805) Tomson (1896) - Positive and negative carges Ruterford (1909) - Te Nucleus Bor (1913) - Energy levels Atomic Model : Timeline CATHODE RAYS THE DISCOVERY OF ELECTRON Scrödinger (196) - Electron cloud model Te knowledge about te electron was derived as a result of te study of te electric discarge in te discarge tube (J.J. Tomson, 1896). Te discarge tube consists of a glass tube wit metal electrodes fused in te walls (Fig. 1.1). Troug a glass side-arm air can be drawn wit a pump. Te electrodes are connected to a source of ig voltage (10,000 Volts) and te air partially evacuated. Te electric discarge passes between te electrodes and te residual gas in te tube begins to glow. If virtually all te gas is evacuated from witin te tube, te glow is replaced by faintly luminous rays wic produce fluorescence on te glass at te end far from te catode. Te rays wic proceed from te catode and move away from it at rigt angles in straigt lines are called Catode Rays. Page of 4 Catode rays To vacuum pump Fluorescence Catode Anode Hig voltage Figure 1.1 Production of catode rays.

2 STRUCTURE OF ATOM CLASSICAL MECHANICS 15 SOLVED PROBLEM. Te wavelengt of a violet ligt is 400 nm. Calculate its frequency and wave number. SOLUTION. We know tat frequency, v = c λ Here c = m sec 1 ; λ = 400 nm = m Also, wave number ν = = 8 1 c m sec = λ m 3 10 sec = 10 sec 400 = sec ν = 1 λ 1 ν = m = m 1 SOLVED PROBLEM. Te frequency of strong yellow line in te spectrum of sodium is sec 1. Calculate te wavelengt of te ligt in nanometers. SOLUTION. We know tat wavelengt, λ = c ν Here c = m sec 1 Page 15 of 4 ν = sec 1 (given) m sec Wavelengt λ = sec = 10 m 5.09 = m = 589 nm [ 1 nm = 10 9 m] SPECTRA A spectrum is an array of waves or particles spread out according to te increasing or decreasing of some property. An increase in frequency or a decrease in wavelengt represent an increase in energy. THE ELECTROMAGNETIC SPECTRUM Electromagnetic radiations include a range of wavelengts and tis array of wavelengts is referred to as te Electromagnetic radiation spectrum or simply Electromagnetic spectrum. Te electromagnetic spectrum wit marked wavelengts is sown in Fig

3 16 1 PHYSICAL CHEMISTRY Figure 1.14 Electromagnetic spectrum. Wavelengt boundaries of eac region are approximate. CONTINUOUS SPECTRUM Wite ligt is radiant energy coming from te sun or from incandescent lamps. It is composed of ligt waves in te range Å. Eac wave as a caracteristic colour. Wen a beam of wite ligt is passed troug a prism, different wavelengts are refracted (or bent) troug different angles. Wen received on a screen, tese form a continuous series of colour bands : violet, indigo, blue, green, yellow, orange and red (VIBGYOR). Tis series of bands tat form a continuous rainbow of colours, is called a Continuous Spectrum. Page 16 of 4 Figure 1.15 Te continuous spectrum of wite ligt.

4 Name STRUCTURE OF ATOM CLASSICAL MECHANICS Region were located (1) Lyman Series Ultraviolet () Balmer Series Visible (3) Pascen Series Infrared (4) Brackett Series Infrared (5) Pfund Series Infrared Balmer equation ad no teoretical basis at all. Nobody ad any idea ow it worked so accurately in finding te wavelengts of te spectral lines of ydrogen atom. However, in 1913 Bor put forward is teory wic immediately explained te observed ydrogen atom spectrum. Before we can understand Bor teory of te atomic structure, it is necessary to acquaint ourselves wit te quantum teory of energy. QUANTUM THEORY OF RADIATION Te wave teory of transmission of radiant energy appeared to imply tat energy was emitted (or absorbed) in continuous waves. In 1900 Max Planck studied te spectral lines obtained from ot-body radiations at different temperatures. According to im, ligt radiation was produced discontinuously by te molecules of te ot body, eac of wic was vibrating wit a specific frequency wic increased wit temperature. Tus Planck proposed a new teory tat a ot body radiates energy not in continuous waves but in small units of waves. Te unit wave or pulse of energy is called Quantum (plural, quanta). In 1905 Albert Einstein sowed tat ligt radiations emitted by excited atoms or molecules were also transmitted as particles or quanta of energy. Tese ligt quanta are called potons. Te general Quantum Teory of Electromagnetic Radiation in its present form may be stated as : (1) Wen atoms or molecules absorb or emit radiant energy, tey do so in separate units of waves called quanta or potons. Tus ligt radiations obtained from energised or excited atoms consist of a stream of potons and not continuous waves. Page 19 of 4 Continuous wave 19 Potons or Quanta Individual poton Figure 1.0 A continuous wave and potons. () Te energy, E, of a quantum or poton is given by te relation E = ν...(1) were ν is te frequency of te emitted radiation, and te Planck s Constant. Te value of = erg sec. or J sec. We know tat c, te velocity of radiation, is given by te equation c = λν...() Substituting te value of ν from () in (1), we can write

5 8 1 PHYSICAL CHEMISTRY Significance of Negative Value of Energy Te energy of an electron at infinity is arbitrarily assumed to be zero. Tis state is called zero-energy state. Wen an electron moves and comes under te influence of nucleus, it does some work and spends its energy in tis process. Tus te energy of te electron decreases and it becomes less tan zero i.e. it acquires a negative value. Bor s Explanation of Hydrogen Spectrum Te solitary electron in ydrogen atom at ordinary temperature resides in te first orbit (n = 1) and is in te lowest energy state (ground state). Wen energy is supplied to ydrogen gas in te discarge tube, te electron moves to iger energy levels viz.,, 3, 4, 5, 6, 7, etc., depending on te quantity of energy absorbed. From tese ig energy levels, te electron returns by jumps to one or oter lower energy level. In doing so te electron emits te excess energy as a poton. Tis gives an excellent explanation of te various spectral series of ydrogen. Lyman series is obtained wen te electron returns to te ground state i.e., n = 1 from iger energy levels (n =, 3, 4, 5, etc.). Similarly, Balmer, Pascen, Brackett and Pfund series are produced wen te electron returns to te second, tird, fourt and fift energy levels respectively as sown in Fig Page 8 of 4 Figure 1.8 Hydrogen spectral series on a Bor atom energy diagram. TABLE 1.3. SPECTRAL SERIES OF HYDROGEN Series n 1 n Region Wavelengt λ (Å) Lyman 1, 3, 4, 5, etc. ultraviolet Balmer 3, 4, 5, 6, etc. visible Pascen 3 4, 5, 6, 7, etc. infrared Brackett 4 5, 6, 7 infrared Pfund 5 6, 7 infrared

6 38 1 PHYSICAL CHEMISTRY 9. In ydrogen atom te energy of te electron in first Bor's orbit is J mol 1. Wat is te energy required for te excitation of second Bor's orbit? (Burdwan BSc, 005) Answer J mol Calculate te wavelengt in Å of te poton tat is emitted wen an electron in Bor orbit n = returns to te orbit n = 1 in te ydrogen atom. Te ionisation potential in te ground state of ydrogen atom is erg per atom. (Kalayani BSc, 005) Answer. 10 Å 31. A line at 434 nm in Balmer series of spectrum corresponds to a transition of an electron from te nt to nd Bor orbit. Wat is te value of n? (Gulbarga BSc, 006) Answer. n = 5 3. Te energy transition in ydrogen atom occurs from n = 3 to n = energy level. (R = m 1 ). (i) Calculate te wavelengt of te emitted electron (ii) Will tis electron be visible? (iii) Wic spectrum series does tis poton belong to? (Vikram BSc, 006) Answer Å ; Yes ; Balmer series 33. Te energy of te electron in te second and tird Bor orbits of te ydrogen atom is erg and erg respectively. Calculate te wavelengt of te emitted radiation wen te electron drops from tird to second orbit. (Calicut BSc, 006) Answer Å MULTIPLE CHOICE QUESTIONS 1. Catode rays are deflected by (a) electric field only (b) magnetic field only (c) electric and magnetic field (d) none of tese. Te e/m value for te particles constituting catode rays is te same regardless of (a) te gas present in catode rays tube (b) te metal of wic catode was made (c) bot of tese (d) none of tese 3. Te carge to mass ratio (e/m) of positive particles (a) varies wit te nature of gas in discarge tube (b) is independent of te gas in discarge tube (c) is constant (d) none of te above 4. A sub atomic particle wic as one unit mass and one unit positive carge is known as (a) ydrogen atom (b) neutron (c) electron (d) proton Answer. (d) 5. Atomic number of an element is equal to te number of in te nucleus of te atom. (a) neutrons (b) protons (c) bot te neutrons and protons (d) electrons 6. Te mass number of an atom is equal to te number of in te nucleus of an atom (a) protons (b) neutrons Page 38 of 4

7 40 1 PHYSICAL CHEMISTRY 17. In potoelectric effect, te kinetic energy of te potoelectrons increases linearly wit te (a) wavelengt of te incident ligt (b) frequency of te incident ligt (c) velocity of te incident ligt (d) none of tese 18. Te kinetic energy of te potoelectrons emitted from te metal surface is given by te relation (v o is te tresold frequency and v is te frequency of incident ligt) (a) ½ m v = v v o (b) ½ mv = v + v o (c) ½ mv = v (d) ½ mv = v o 19. In Bor s model of atom, te angular momentum of an electron orbiting around te nucleus is given by te relation (a) n m ν r = π (b) m ν r = π (c) n n m ν r = (d) m ν r = 4 π 4 π 0. Te radius of first orbit in ydrogen atom according to Bor s Model is given by te relation (a) r = (b) r = (c) r = (d) r = 4 1. Te radius of first orbit in ydrogen atom is 0.59 Å. Te radius of second orbit is given by (a) ½ 0.59 Å (b) 0.59 Å (c) Å (d) Å. Te energy of an electron in te first orbit in ydrogen atom is 313.6/n kcal mol 1. Te energy of te electron in 3rd orbit is given by te relation (a) E 3 = kcal mol (b) E 3 3 = kcal mol (c) E 3 = kcal mol 9 (d) 1 E 3 = kcal mol 3. Lyman series is obtained wen te electrons from iger energy levels return to (a) 1st orbit (b) nd orbit (c) 3rd orbit (d) 4t orbit 4. A line in Pfund series is obtained wen an electron from iger energy levels returns to (a) 1st orbit (b) 3rd orbit (c) 5t orbit (d) 6t orbit 5. Te energy of an electron in Bor s atom as we move away from te nucleus (a) remains te same (b) decreases (c) increases (d) sometimes increases, sometimes decreases 6. Wen an electron drops from a iger energy level to a lower energy level, ten (a) te energy is absorbed (b) te energy is released (c) te nuclear carge increases (d) te nuclear carge decreases Page 40 of 4

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