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CHEMISTRY Chapter 02 CONTENT * Introduction * Fundamental Particles * Thomson’s Atomic Model * Rutherford’s Atomic Model ° Electromagnetic Waves Waves) Theory or Energy ¢ Planck’s Quantum Theory ¢ Photoelectric Effect ° Bohr’s Atomic Model * Spectrum (EM Radiant ¢ Hydrogen Line Spectrum or Hydrogen Spectrum * Wave Mechanical Model of an Atom * Quantum Number ¢ Shape of Atomic Orbitals ¢ Electronic Configuration ¢ Extra Stability of Half Filled and Completely Filled Orbitals * Probability Distribution Curve S Atomic Structure 1. INTRODUCTION (a) The word atom was first introduced by Dalton (1803 - 1807) in scientific world. (b) According to him matter is ultimately made up of extremely small indivisible particles called atoms. (c) It takes part in chemical reactions. (d) Atom is neither created nor destroyed 2. FUNDAMENTAL PARTICLES Atoms are made up of three fundamental particles. The charge and mass of these fundamental particles are as follows: Electron Proton Neutron Symbol e or :e° or e~ porip* nor on* Mass (in kg) 9.109534 x 1072 | 1.6726485 x | 1.6749543 1027 x 1027 Mass in amu 5.4858026 x 10% | 1.007276471 | 1.008665 012 Relative mass oe 1 1 1837 Charge (Actual | 1.6021892 x 10° | 1.6021892 x | 0 (in )) a9 1079 Relative charge | -1 +1 0 Discovered by J.J. Thomson Goldstein Chadwick One unit charge = 4.80298 x 107° esu = 1.60210 x 10°? coulombs One amu = = x mass of ¢C’? atom DISCOVERY OF FUNDAMENTAL PARTICLES: > — Cathode rays (a) The electron was discovered as a result of the studies of the passage of electricity through gases at extremely low pressures known as discharge tube experiments. When a high voltage of the order of 10,000 volts or more was impressed across the electrodes, some sort of invisible rays moved from the negative electrode to the positive electrodes these rays are called as cathode rays (b) Properties of Cathode rays To vacuum pump Anode ® Gas at low pressure Cathode rays Cathode ° HAE High voltage (i) Path of travelling is straight from the cathode with a very high velocity As it produces shadow of an object placed in its path (ii) Cathode rays produce mechanical effects. If a small pedal wheel is placed between the electrodes, it rotates. This indicates that the cathode rays consist of material part. Page | 43 ATOMIC STRUCTUR; al Magnetic fields are applied to the cathode rays in the discharge tube, the rays ;, eS al te that they consist of charged particles. ‘i Pa ys produce X-rays when they strike against hard metals like tungsten, copper etc. en the cathode rays are allowed to strike a thin metal foil, it gets heated up. Thus the cathode ; Possess heating effect, They produce glow On striking ZnS screen coated on the glass He Veht is emi & when they strike the zinc sulphide screen. (vii) Cathode rays penetrate through thin sheets of aluminium and other metals. (viii) They affect the photographic plates (ix) The ratio of charge to mass i.e. charge/mass is same for all the cathode rays irrespective of th. used in the tube. ty SPOT LIGHT (a) The mass of electron in motion is expressed as m’= (ay (iii) (vi) where m’ = mass of the electron in motion m = rest mass, v = velocity of the electron, ¢ = velocity of light (b) In 1897, JJ. Thomson determined the e/m value (charge/mass) of the electron t studying the deflections of cathode rays in electric and magnetic fields. The value of e has been found to be -1.7588 x 10® coulomb /g (c) The first precise measurement of the charge on the electron was made by Robert A Millikan. in 1909 by oil drop experiment. Its value was found to be - 1.6022 x 1 coulomb. (d) The mass of electron can be calculated from the value of e/m and the value of e wt Ms is 9.1096 x 107°" Kg. > Positive Rays-Discovery of Proton (a) The existence of positively charged particles in an atom paises ie Positive was shown by E. Goldstein in 1886 (b) He repeated the same discharge tube experiments by using a perforated cathode. \ (c)_ It was observed that when a high potential difference was applied between the electrodes, not only cathode rays were produced but also a new type of rays were produced simultaneously from anode moving towards cathode and High-voltage source passed through the holes or canal of the cathode. These were termed as canal ray or anode r > Properties of Anode Rays (i) These rays travel in straight lines and cast shadow of the object placed in their path (ii) The anode rays are deflected by the magnetic and electric fields like cathode rays but dir different that mean these rays are positively charged. i i (iii) These rays have kinetic energy and produces heating effect also. (iv) The e/m ratio for these rays is smalle t i (v) Unlike cathode rays, their e/m value i (vi) These rays produce flashes of light on ZnS screen (vii) These rays can pass through thin metal foil (viii) They are capable to produce ionisati (ix) They can produce physical and chemica hanges. Page | 44 —— ATOMIC STRUCTUR; i ene —_—— » APPLICATIONS OF RUTHERFORD MODEL (i) Anatom consists of a heavy positively charge d nucleus where all the ns, Almost wi Protons & neutrons are collectively referred to as nucleo! protons and neutrons are prese; hole of the mass of the ator (ii) RADIUS OF NUCLEUS (iii) (iv) v contributed by these nucleons. The magnitude of the +ve charge on the nucleus is different for differ. atoms. (— Proton Rutherford's Model Of Atoms oe NEUTRON ELECTRON : ORBITS oO ELECTRONS Proton L ey, The volume of the nucleus is very small and is only a minute fraction of the total volume of the atom. Nuc! has a diameter of the order of 10° to 40°? cm and the atom has a diameter of the order of 10 cm heat Ie Dy peer atom _ ue = 10°, Dy=10° Dn D, Diameterofthenucleus 10 Thus diameter (size) of the atom is 10° times the diameter of the nucleus. The radius of a nucleus is proportional to the cube root of the number of nucleons within it. Ro AY? => R= RoAY? cm Where Ro = 1,33 x 10*(a constant) and A = mass number (p +n) and R = radius of the nucleus. R=1.33x 107 A”? cm ATOM TO NUCLEUS VOLUME RATIO volume of the atom (10°). volume of the nucleus (10)? Electrons revolve around the nucleus in closed orbits with high speeds. The centrifugal force acting on t revolving eis being counter balanced by the force of attraction between the electrons and the nucleu This model was similar to the solar system, the nucleus representing the sun and revolving electron planets. E Drawbacks of Rutherford model :- (1) (2) This theory could not explain stability of atom. According to Maxwell electron loses it energy continuously in the form of electromagnetic radiations. As a result of this, the e° should loss energy at every turn and move closer and closer to the cubs following a spiral path. The ultimate r i i i esult will be that it will fall into th making the atom unstable. eae : xy if the electrons loose ene: gy continuously, the observed spectrum should be continuous but the act ut the actu obsel i bserved spectrum consists of well-defined lines of definite frequencies. Hence electron is not continuous in an atom. the loss of energy P Page | 46 —— CHEMISTRY Some Important Definitions : - Mass Number : A = (number of protons + number of Neutrons) Atomic Number : Z = Number of proton = Number of electron (for an atom). For charged atom : Number of e- = Z— (charge on atom), Z= number of protons only Example. i7Cl°—> n= 18, p=17,e=17 Two different elements cannot have the same Atomic Number Number of Neutrons = Mass number — Atomic number =A-Z=(p+n)—p=n Representation of element —> 2X* (where X—> symbol of element) Isotopes: Given by Soddy, are the atoms of a given element which have the same atomic number but different mass number i.e. They have same Nuclear charge but different number of Neutrons. Example : Example : vc yl?” Aee 6c? Gai n=18 n=20 e=6 e=6 e=6 e=17 e=17 p=6 p=6 p=6 p=17 p=17 n=6 n=7 n=8 Example : (Protium aH* e=1 p=1 n=0 Deuterium Tritium) H? aH? e=1 e=1 p=1 p=1 nad n=2 > Isotopes have same chemical property but different physical property. > Isotopes do not have the same value of e/m. Isobars : > Given by Alfred Walter Stewart, > Isobars are the atoms of different element which have the same mass number but Different Atomic number i.e. They have different number of Electron, Protons & Neutrons But sum of number of neutrons & Protons remains same. Example : aH? p=1 e=1 n=2 p+n=3 2He? Example: p=2 e=2 n=1 ptn=3 39 K? p=19 n=21 e=19 n+p=40 va Isobars do not have the same chemical & physical property. 20 Ca“ p=20 n=20 e=20 n+p=40 lsodiapheres : » They are the atoms of different element which have the same difference of the number of Neutrons & protons. Example : 5BY 6c? —_ Example: 7N8 oF 19 p=> p=6 p=7 p=9 n=6 n=7 n=8 n=10 e=5 e=6 e=7 e=9 n-p=1 n-p=1 n-p=1 n-p=1 Isotones/ Isoneutronic species: They are the atoms of different element which have the same number of neutrons, Example : aH? 2He* Example : ik? 20Ca*? p=1 pez e=19 e=20 n=2 n=2 p=19 p=20 e=1 e=2 n=20 n=20 weiner SS _ Page | 47 PTR AVES (EM WAVES) THEORY OR RADIANT ENERGY ee De oe transmitted from one body to another in the form of waves and these Ghradlont ener i speed as light (3 x 10% m/s) and these waves are known as Electro Mas incre ee : radiant Energy do not need any medium for propagation. tes s S, infra red rays, visible rays, ultraviolet rays, x-rays, gamma rays. a ve electric and magnetic fields and travel at right angle to these fields. The upper @ wave is called crest and the lower most portion is called trough. Some of the terms dealing with the waves are described below. 8) crs ey £) UY WW adireation _ i Laas of propogation Trough It is defined as the distance between two nearest crest or trough. It is measured in terms of A (Angstrom), pm (Picometre), nm (nanometer), cm(centimeter), m (meter) 1A=10°°m, 1pm=10%2m,1inm=10°m, 1cm=102m Frequency of a wave is defined as the number of waves which pass through a : | point in 1 second. It is measured in terms of Hertz (Hz), second, or cycle per second (cps) (1 Hertz= 1 second*) Time taken by a wave to pass through one point. T Le second ws Velocity of a wave is defined as distance covered by a wave in 1 second c=A/T=Av. orv=c/Aor c=v(s*)xA(m) or c= vA (ms7) Since cis constant i.e. frequency is inversely proportional to % It is the reciprocal of the wave length that is number of waves present in 1cm v=5 It is measured in terms of cm™, m™ etc. The amplitude of a wave is defined as the height of crust or depth of trough. fig ah +) vers (7-3 i i Example :2 How long would it take a radio wave Calculate v in cm i ald pol yeloe of frequency 6 x 10*sec to travel from mars ave wavelength of 5800 A to the earth, a distance of 8 x 10’ km ? Answer: 2.66 x 10° sec. Solution: Distance to be travelled from mars to earth =8x 107 km=8x10"m (: Ag 10° +: Velocity of EM waves = 3x 10° m/sec rime = Distance __8x10%m Velocity 3x10°m/sec* cm sec? x 1.7 x 10° cm = 2.66 x 10? 104= 5,1 x 10" sec’ Page | 49 ler of decreasing nstant) then ther CHEMISTRY > Intensity v/s Kinetic Energy Average K.E. and K.Emax remains Constant with change in photo intensity t K. Emax [—_— v>Vo, ————> 'p or | (keeping constant) > Stopping Potential or Retarding Potential (V;) Itis the minimum potential require to stop the fastest moving electron completely or it is the minimum potential at which photo current becomes zero. t = slope = h/e > MV2 = eVs, Vs = Stopping potential Vs 8 ee : e = Charge on electron Moor = { ‘ eVs = hv — w Vs = hy — hvo a ee It can be commented that stopping potential increase with increase in frequency however if photo intensity is changed there is no effect on stopping potential. 8. BOHR’S ATOMIC MODEL » Bohr developed a model for hydrogen and hydrogen like atoms one-electron species (hydrogenic species). He applied quantum theory in considering the energy of an electron bond to the nucleus. Important postulates of Bohr’s Model (1) Electrons revolve around the nucleus along certain circular paths known as “ORBIT” or “SHELLS” L\M @ Niels Bohr (1885-1962) .... are shells K — 1st orbit L— 2nd orbit M — 3rd orbit (2) Electron is associated with a fixed energy in a particular orbit. The change in electronic energy is possible only when; electron changes its orbit number. Page | 53 culate the radi i culate the radius of 1, 24, 3°4, 4th Bohr's Orbit of hydrogen. ius of Bohr's orbit r= 0.529 x nf Radius of 1* orbit : 2 r= 0,529 x = =0.529A (b) — Radius of 2 orbit : 2 r=0.529x = =0.529x4=2.116A a (c) Radius of 3" orbit : i ‘ : a i (d) Radius of 4" orbit : r=0. — Z: 2 0.529 x += 0.529 x9 = 4.761 A r= 0.529 x = = 0.52916 =8.464A —— ea. of the radius of two Bohr's orbit of Li’? is 1:9. what would be their nomenclature. (2)L&M (3)K&M (4)K&N Answer : (3) ne ksi 0.529 x— solution: = *=~=———* a Ro _, Roi Kshel fo 509x" n V9 n, 3. MShell Example: 7 Calculate the energy for 2" excited state of Li"? ion. Answer : - 13.6 eV/atom 7? Solution : E=-13.6 fa +: Z=3 and e exist in 2™ excited state, means e~ present in 3 shell i.e. n= 3 3) E=-13.6 sok =- 13.6 eV/atom (3) Example: 8 If the potential energy of an electron in hydrogen atom is -6.8 eV, then find kinetic energy and total energy of electron in this orbit. Also find radius of this orbit. Answer : (K.E. = 3.4 eV, T.E. =- 3.4 eV, r == 2.16 A) Solution : 1 P.E.=-2K.E.>-6.8=-2KE => K.E.=3.4 eV 2: T.E. =—K.E.=—3.4eV Ze 2 3: Eb =—13.6x — 2. -3.4=-13.6x > n n n= pEeeG ie.n=2 3.4 2 £ 2 4. r=0529x TA = r-0sa9x 2) A = 0.529 x 4A =2.16A > _ Energy Definitions Bae (a) Ground state (G. S.) (b) Excited state (E. S.) (c) Excitation energy: Energy required (Highest e© occupied shell) The energy states above the to excite an electron from its The lowest energy state of an ground state are referred to as ground state to any excited state is atom or ion or molecule excited states called excitation energy. G.S.forH-atom n=1 n=2 ist E. S. E,— E:= first excitation energy Het ion n=1 n=3 2nd E. S. £3 — E: = second excitation energy n=4 3rd E.S. E4—1= third excitation energy Total £.S.=(n-1) 1st E. P. = E2.-E1 =-3.4-(-13.6) = 10.2 eV 2nd E. P, E3— Ex =-1,.51 —(-13.6) = 12.09 eV Page | 57 is being exposed to UV-light of A = 360 A? (1) 49.68 x 107°) (2) 4.9 68x 1079) (8 (2) 16 (3) 27.29 x (3) 32 10°") (4) 4.9x 10794 ag the total number of atomic orbitals in fourth energy level of an atom is: — (4) 4 The minimum Bee red to overcome the attractive forces between an electron and the surface of ‘Ag metal is 5.52 x 10°™ J. What will be the maximum kinetic energy of electron ejected out from AB which 10 According to the Bohr Theory, which of the following transitions in the hydrogen atom will give rise to the least energetic photon? (i)n=Ston=3 (2)n=6ton=1 Q.i1 Which of the following is wrong for Bohr model (1) It establishes stability of atom (3)n=5 to (2) Itis contradicted with Heisenberg uncertainty principle (3) It explain the concept of spectral lines n=4 (4)n=6ton=5 (4) e~ behaves as particle & wave COCOOOOOOOOOO ‘Spectrum : When a radiation is passed through a spectroscope (prism) for the dispersion of the radiation, the pattern (photograph) obtained on the screen (photographic plate) is called as spectrum of the given radiation Classification of Spectrum I (1) Emission (2) Absorption £ y + ([a)continuous (byline) — ((c)band } ( (a) line } ((b) band ) Spectrum (1) Emissions spectrum : When the radiation emitted from incandescence source (e.g. from the Candle, sun, tube light, burner, bulb, or by-passing electric discharge through a gas at low pressure, by heating some substance at high temperature) is passed directly through the prism and the pattern obtained on the screen is known as emission spectrum (a) Emission continuous spectrum or continuous spectrum : When a narrow beam of white light is passed through a prism, it is dispersed into 7 colours from violet to Red. White light Beam HUH ANN Prism Photographic Plate <-9@ Calculation of number of spectral lines (a) Total number of spectral lines = 1 + 2+..... (n2— Mm) = 2 if n: = 1(ground state) 5 (n,-1)n, _ n(n-1) Total number of spectral Une amine a = ee (b) | Number of spectral lines which falls in a particular series (n2—n1) where n2= higher energy series, n1 = lower energy series > Limitation of the Bohr's model : Bohr's theory does not explain the spectrum of multi electron atom. 2 Why the Angular momentum of the revolving electron is equal to il has not been explained by Bohr's 20 theory. eh Bohr interrelate quantum theory of radiation and classical law of physics without any theoretical explanation. 4. Bohr's theory does not explain the fine structure of the spectral lines. Fine structure of the spectral line is obtained when spectrum is viewed by spectroscope of higher resolution Power Si Bohr theory does not explain the splitting of spectral lines in the Presence of m. ic fi u or electric field (Stark's effect) Senetic field (tea aa Example: 9 Calculate the wavelength of a photon emitted when anel i ectron in H-atom maki iti € a transition from n=2ton=1. 4 Answer: A=— 3R Solution: Rr? ple ly xr (eT 1 Heat “== R(1) | —=-— a IE: Z| 1_3R 4 == — orA=— rn 4 3R Sg | 62 ee yemist® nd |; ¢ 10 calculate wavelength for 2"° line of Balmer series of He*ion parle * a N answer! 3R a Ee i = 3 otution* ny n uA me2 m=4 135-0, 1 2\| Se eee ol -2 4 1_3R)_ ee! 3R le: 11 Ina hydrogen spectrum if electron moves from 6" to 3° orbit by transition in multi steps then gamle * find out the following steps: (a) Total number of lines in spectrum (b) Total number of lines in U.V. region (c) Total number of lines in visible region (d) Total number of lines IR region q (a) 6 (b) Zero (c) Zero (d) 6 Answel eae ‘ution: (a) total number of lines: olution: : _(e.-m)f(m—n)42]_(6-)[(6=3)+4]_3x4_, 2 2 Gatos 11. (b) number lines present in U.V. region e- moves from 6" to 3 orbit in multi steps. For U.V. region, e- should be comes into 1* shell. So the number of lines in U.V. region will be zero (c) total number of lines in visible region. For visible region, e- should be comes into 2™ shell, so the number of lines in visible region is zero. (d) total number of lines in I.R. region. In |.R. region, Paschen, Bracket and Pfund series are present. Number of lines in Paschen series = Dzaso = 6-3 = 2) Number of lines in Bracket series = n-4 = 6-4 = 2 Number of lines in P fund series = m—-5 = G25 = 1 So total number of lines = 3+2+1=6 WAVE MECHANICAL MODEL OF AN ATOM This model consists three Basis. 1. de-Broglie concept (Dual nature of Matter) 2. Heisenberg's Uncertainty principle. 3, Schrodinger Equation. The Dual nature of matter (The wave nature of electron) li) In 1924, a French physicist, Louis de-Broglie suggested that if the nature of light is both that ofa particle __ and of a wave, then this dual behavior should be true for the matter also. (il) The wave nature of light rays and X-rays is proved on the basis of their interference and diffraction and many facts related to radiations can only be explained when the beam of light rays is regarded as composed of energy corpuscles or photons whose velocity is 3 x 10*° cm/s. Page 
