In 1879, wi thee 7 William C made of glass it discharge trong gs sing vo thin pees of metal clled electrodes, sae oks studied the conduction of electricity through gases at low pressure. A calor TY iL, The electrical h il ‘When sume ne® through the gases could be observed only at very low pressures and at very high voltages partie high voliageis applied across the electrodes, current stars lowing through a steam of were alee in the tube from the negative electrode (cathode) to the positive electrode (anode). These led cathode rays or cathode ray particles. Tovacuum pump cathode Properties of cathode rays: () The cathode rays start from cathode and move towards the anode. Gi) These rays themselves are not visible but they produce fluorescence on ZnS screen. iii) In the absence of electrical or magnetic field, these rays travel in straight lines Civ) In the presence of electrical or magnetic field, the behavior of cathode rays are similar to that expected from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles, called electrons. (¥) The characteristics of cathode rays(electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube. Charge to Mass Ratio of Electron TJ. Thomson measured the ratio of electrical charge (e) to the mass of electron (me ) by using cathode ray tube and applying electrical and magnetic field perpendicular to each other as well as to the path of electrons . e/me = 1.758820 x 10" C kg? where me = Mass of the electron in kg e= magnitude of charge on the electron in coulomb (C). iscoverv of proton anode ray In 1886, Goldstein modified the discharge tube by using a perforated cathode. On reducing the pressure, he observed a new type of huminous rays passing through the holes or perforations of the cathode and moving in a direction opposite to the cathode rays. These rays were named as positive rays or anode rays or as canal rays. Anode rays are not emitted from the anode but from a space between anode and cathode. Properties of Anode Rays (@ The value of positive charge on the particles constituting anode rays depends upon the nature of the gas in the discharge tube. ‘The charge to mass ratio of the particles is found to depend on the gas from which these originate. ‘Some of the positively charged particles carry a multiple of the fundamental unit of electrical charge. (vyThe behavior of these particles in the magnetic or electric field is opposite to that observed for electron or cathode rays. Proton: The smallest and lightest positive ion was obtained from hydrogen and was called proton. Mass of proton = 1.676 x 10°” kg Charge on a proton = (+) 1.602 x 10° C ‘Neutron: It is a neutral particle. It was discovered by Chadwick (1932). By the bombardment of thin sheets of beryllium with fast moving a-particles he observed + that highly penetrating rays consist of neutral particles which were named neutrons. 16sess 1 pal gp (WI EOE 6 wt Lheeaasins Maes of biose: 0 tigen prison ¢ ot Tw eecseon we CALI Mls 1 3 esas 4 ly Girinted Tine decom ate gee (hee punitive oarge x wenly Aivirtntes, The (1 M08 stole eheeensatic eran peent An impress feasare of th the stom, o lice gol, sve, pi (ipiatlescent zie sulphide sereenerousd Whenever apart ‘iit was prc eh thas ing, pha yatng Usd late A. Rutherford's scattering experiment wortant observations are: (1) \Aoss ofthe a-panicles passed through the foil without undergoing any deflection, (i) A few scpartcles underwent deflection trough small andae 1) Very fer mere deflected back ie, through an angle of nearly 1802, Conclusions: (Since most ofthe a-partcles passed through the foil without undergoing any deflection, there mst be suficient empty space within the atom, (1) & smal faction of acpaicls was defected by small angles. The positive charge as to be Caneetirve in very small volume that repelled and deflected afew positively charged a-pantcles, This very smal portion ofthe atom was called nucleus, (01) The volume of nucleus i very small as compared to total volume of atom, erford’s Nuclear Model of an Atom (1) The positive charye and most ofthe mass of the atom was densely concentrated in an extremely small seqion. This very small portion ofthe atom was called nucleus by Rutherford. (i) The nucleus fs surounded by elesrons that move sround the nucleus with ery igh speed in cular paths called orbits. ; ; nt Jectrons and nucleus are held together by electrostatic forces of attraction v7SE Drawbacks of Rutherford Model sis described on the (@ Rutherford’ mo * explain the stability of atom if the motio 's model a d cannot explai 7 basis of classical mechanics and elect romagnel eon. : an etic theory. Gi) Rutherford’ } smo their energies. del does not give any idea about distribution of electrons aroun Atomic Numb. Atomic Number: The number of Seder Ca eee ace ee Of proons presi inde nucleus sequal othe ‘atomic number (2). In Gtomie number, 2) ty, the number of electrons in an atom is equal to the number of protons tomic Number (z) = Ni 7 om 2) = Number of protons in the nucleus of an atom.= Number of electrons in a neutral lass Number ‘Number Mass Number ‘Number of protons and netons present in he nucleus are collectively known as nucleons. mas oual member of nucleons i termed as mass number (A) of the atom. Isotopes :. lumber of protons (p) + Number of neutrons (n). Isotopes :Atoms with identical atomi Tot done eaten docte ber but iffeent atomic mass number are known as Isotopes. Teobere sis , deuterium and tritium. Agobars i obars ‘are the atoms with same mass number but different atomic number for Example eC and Nn. Developments Leading to the Bol 0 1's Model of Atom -Two di | Feemntgion oi Behe ator sion These vee -wo developments played a major role in the @ Dual character of the electromagnetic radiation which means that radiations possess both wave like and | particle like properties. (ii) Experimental results regarding atomic spect electronic energy levels in atoms. Nature of Electromagnetic Radiation (Electromagnetic Wave Theory) Tis theory was pul forward by James Clark Maxwell in 1864. The main points of this theory are as follows: (i) The energy is emitted from any source (ike the heated rod or the Filament of a bulb through which Clectrie current is passed) continuously in the form of radiations and is called the radiant energy. Gi) The radiations consist of electric and magnetic fields oscillating perpendicular to each other and both perpendicular to the direction of propagation of the radiation vel with the velocity of light 3x 10° m/sec. Gii) The radiations possess wave character and tr {iv) These waves do not require any material medium for propagation. For example, rays from the sun reach us through space which is a non-material medium. \d the nucleus and about tra which can be explained only by assuming quantized sive crests or troughs. It is Characteristics of a Wave— the distance between any two consecut (Wavelength: It is defined as represented by X and its SL. unit is metre | lofined as the number of waves passing through a point in one GiyFrequency: Frequency of a wave is b copa IL is represented by v (au) and is expressed in Hertz (Hz). cycle/sec. Velocity: i) Velocity of a wave is del .fined as the linear distance travelled by the wave in one second. tis represented by cand is expressed in om/see OF misec. {ix)Amplitude: Amplitude of a wave is (he height of the crest or by V and is expressed in the units of length. (oWave Number: Itis defined as ihe ‘number of the depth of the through. It is represented waves present in 1 metre length. Evidently it will be 1 v= x represented by bar v (Fead.asmu bat) ; wre arranged in order of thei increasing al of the wavelength. It is tained is. called electromagnetic ‘m: When electromagnetic radiations frequencies, the complete spectrum ol equal to the reciproc Electromas netic SI ecru ‘wavelengths or decreasing sec ations of Elect tic Wave Theo! eer tions of Electromagnetic Wave Theory a ; Line art sucess in explaining propertis of right suchas interference, 18 Electromagnetic ‘wave theo!Cifaction ete; but it could not explain the following: () The phenomenon of black ody radiation. Gi) The photoelectric effect : aie (i) The variation of heat capacity of sods as a function of temperature, Ai) The line spectra of atoms with reference to hydtogen. Black Body R: and the radiation ‘The ideal body, which emits and absorbs all frequencies is called a black oa ee recat mitted by such a body is called black body radiation, The exact frequency Tadiation from a black body depends only on its temperature, : hea Ata given temperature, intensity of radiation emited increases with decrease of wavelength, reac ef ivelength as ‘aximum value ata given wavelength and then starts decreasing with further decrease of wavelengt shown in Fig Planck’s Quantum Theory The main points of this theory was as follows: ¥ Atoms and molecules could emit or absorb) energy only in discrete quantities and not in a continuous manner, iE Tee smallest quantity of energy that canbe emited or absorbed in the form of electromagnetic radiation is called quantum, Tieenetey ) ofa quantum of radiation is proportional to its frequency (v ) and is expressed by equation (2. E=nhy ‘known as Planck’s constant = 6.626x10" Js, Photoelectric effect tis the phenomenon of e Y certain metals (like potassium, rubidium, caesium Sh en light of suitable frequency incident on them. The elestcee jected are called photoelectrons. ‘This phenomenon was frst observed by H.Hertz, Theimportant characteristics of photoelectric effect are: ly Photons of ight of certzin minimum frequency called threshold frequency (ve) can cause the Pholoclectic effect. The value of vs is different for different metals 2: the kinetic energy ofthe electrons which are emitted is directly Proportional to the frequency of the Striking photons and is quite independent of ther intensity, 3. The number of electrons that are ejected per second from the the striking photons or radiations and not upon their frequen ge jection of electrons by ie metal surface depends upon the intensity of cy. Deg oo ion of Photoelectric Effect Einstein in (1905) was able to give an ex Planck’s quantum theory as under: () Photoelectrons are ected only when the incident light has a certain minimum frequency (threshold frequency vo) planation of the different point ofthe photoelectric effect using 9TC ————— (ii) If the frequency of the incide f t 7 (av — hyo) is imparted to the ‘leet () is more than the threshold frequency (Wo), the excess &* ) kinetic ene ii) On increasin; ihe rey. ct “ g the intensity of light, more electrons are ejected but the energies ofthe electrons are not ery re of conservation of energy principle, the kinetic energy of the ejected electron 1 given by hy=hyp + 12m." Dual Behaviors of Electromagnetic Radiation. Light possesses both particle and wave-like properties, i., light has dual behaviour. ‘The particle nature of light could be explained the black body radiation and photoelectric effect. The wave behavior of light could be explained by the phenomena of interference and diffraction. Atomic spectrum ‘When a ray of white light is passed through a prism, we get a series of colored bands called spectrum. This spectrum is called continuous spectrum, because here ‘violet merges into blue, blue into green and so on. Emission and Absorption Spectra Emission Spectra Emission Spectra is noticed when the radiations ‘emitted from a source are passed through a prism and then received on the photographic plate, Radiations can be emitted in a number of ways such as: (i) from sun or glowing electric bulb Gi) by passing electric discharge through a gas at low pressure. Gi) by heating a substance to high temperature, ‘The emission spectra of atoms in the gas phase do not form a continuous spectrum. Absorption Spectra -When white light is passed through the vapours of a substance and the transmitted. light is then allowed to strike a prism, dark lines appear in the otherwise continuous spectrum. The dark lines indicate thatthe radiations corresponding to them were absorbed by the substance from the white light. This spectrum is called absorption spectrum. Dark lines appear exactly at the same positions where the lines in the emission spectra appear. Each element has a unique line emission spectrum. So line emission spectra are also called finger print of atoms. Line Spectrum of Hydrogen ‘When electric discharge is passed through hydrogen gas enclosed in discharge tube under low pressure and the emitted light is analysed by a spectroscope, the spectrum consists of a large number of lines which are ‘grouped into different series, The complete spectrum is known as hydrogen spectrum, ¥ = 109,677[ 4, where my “The value 109, 677 dav ‘it called" He ip. 2.8 Aiomt spectrum of hoger On the basis of experimental observations, Johannes Rydberg noted that all series of lines in the hydrogen spectrum could be described by the following expression: where Z isthe atomic number ofthe species. Here R= constant, called Rydberg constant for hydrogen and ny , nz are integers (n2> nl) For any particular series, the value of nl is constant while that of n2.changes. For example, For Lyman series, mi= 1, m= 2,3, 4, 5 For Balmer series, m 3,4,5, 6. For Paschen series, m= 4,5, 6,7. For Brackett series,n; 5,6, 7,8... For Pfund series, ny 6,7,8,9. ‘Thus, by substituting the values of; and n in the above equation, wavelengths and wave number of different spectral lines can be calculated. When m =2, the expression given above is called Balmer's formula 20con the basis of Planck's Bohr’s Model of Atom Niels Bobr in 1913; Drapes new model of atom : antum Théory. The main points of this model areas follows: aths called orbits, Qin atom, the leston evolve sou iene n cern defit irculr as energy Gi) Each orbit is associated with definite energy and therefore these are known levels or energy shells. These are numbered as 1, 2,3, 4.... or K, L,M, Nessee---- @. which a f the electron is a @ (Gil) Only those energy orbits are permitted for the electron in which angular momentum of & whole number multiple of hi2x 5 Angular momentum of electron mit) =" (a= 1,2,3,4 etc). ‘m= mass of the electron 'V= tangential velocity of the revolving electron. t= radius of the orbit. h=Planck’s constant. nis an Integer. (iv) As long as electron is present in a particular orbit, it neither absorbs nor loses energy and its energy, therefore, remains constant, jamps to Ppatee eterey i supplied to an electron it absorbs energy only in fixed amounts as quanta and jumps higher energy state away from the nucleus known as excited state. The excited state is unstable, the () It rules out existence of definite paths or trajectories of electrons and other similar particles. Gi) The effect of Heisenberg’s uncertainty principle is significant only for microscopic objects and is negligible for macroscopic objects, Reasons for the Failure of Bohr Model” (@) The wave character of the electron is not considered in Bohr Model. Gii) According to Bohr Model an orbit is a clearly defined path and this path can completely be defined only if both the position and the velocity of the electron are known exactly at the same time. This is not possible according (o the Heisenberg’s uncertainty principle. ‘Quantum Mechanical Model of Atom Quantum mechanics: Quantum mechanics is a theoretical science that deals. with the study of the motions of the microscopic objects that have both observable wave like and particle like properties. Important Features of Quantum Mechanical Model of Atom: (@ The energy of electrons in atom is quantized i.c., can only have certain values. (ii) The existence of quantized electronic energy level is a direct result of the wave like properties of electrons. ii) Both the exact position and exact velocity of an electron in an atom cannot be determined simultaneously. . (iv) An atomic orbital has wave function @. There are many orbitals in an atom. Electron occupy an atomic orbital which has definite energy. An orbital cannot have more than two electrons. The orbitals are filled in. increasing order of energy. All the information about the electron in an atom is stored in orbital wave function 9. (v) The probability of finding electron at a point within an atom is proportional to square of orbital wave funetion ie., [plat that point. It is known as probability density and is always positive From the value of 9 at different points within atom, it is possible to predict the region around the nucleus where electron most probably will be found, Quantum Numbers ‘Atomic orbitals can be specified by giving their corresponding energies and angular momentums which are quantized (i.e,, they have specific values). The quantized values can be expressed in terms of quantum number. These are used to get complete information about electron i.e.,its location, energy, spin etc. The principal quantum number gives us the following information: The following information’s are obtained from n. 1. It gives the size the orbit. 2. It gives the energy of electron in an or! 3. It gives the shell in which the electron is found. 4. Italso gives the average distance between the electron and the nucleus. As the value of n increases, the distance between the electron and the nucleus also increases. The possible values of n are 1, 2, 3, 4, 5 etc. Ifn= the electron is in K shell n=2 the electron is in L shell n=3 the electron is in M shell and so on. : 5, The maximum number of electrons in the shell with principal quantum number n is equal to 2m”, Azimuthal or Subsidiary or Orbital Angular Quantum Number (1) 1. It gives the shape of the orbital. 2. It gives the sub shell or sub level in which the electron is located 3. It also gives the orbital angular momentum of the electron, 4, the values of |= 0 to (n-1) ‘The number of sub shells in a principal shell is equal to the value of n. For example, When n= 1, = 0. ie. K shell contains only one sub shell - s sub shell and1. i.e. L shell contains two sub shells - s and p sub shells ), | and 2. i.e. M shell contains three sub shells —s, p and d sub shells 22Whenn= 4,12 : lls 4. the number tec, 243. i.e. N shel contain foursub shells, psd and rio 22HD. TE1=0 for ggeetttons that can be accommodated in sub-energy level is equ for TEI=1 for sorbital then= 2¢2*14+1)29 electrons 11=2 for Tonal et 22°241)=6 eleetons if|= ther : =] HT=3 for gorbita ne 22%341)=10 electrons 2*3+1)=14 electrons M: ; Agnetic Orbital Quantum Number (vor mi 1. The ma, i trons resent in rn mane ‘wantum number determines the number of preferred orientations of the elect 2. The magnetic I : quantum numb fl, it can have all the values Fanging from~ 110 + includ euoted by later m and fora given vale of it ws, for energy value of I, m hae 21 | values, rie 2 (sub-shell), m can have only one value ie, m=0, Cans that s-sub-shell has only one orientation space, In other words, s-subshell has only one orbital called s-orbital, Spin Quantum Number (§ or mg Hs quantum number helps to explain the magnetic properties of the substances, A spinning electron density along all The plots of probability density (42) against distance from the nucleus (r) for 1s and 2s atomic orbitals are as follows: 5000, 300 4000 20 31000. S10 2000 S 0 1000. ” ° ° erm 0" 04” os (nm) (nm) ‘Tee Probability of 1s electron is found tobe maximum near the nucleus and decreases with the increase in the distance from the nucleus. In 2s electron, the probability is also maximum near the nucleus sed decreases t0 zero probability. The spherical empty shell for 2s electron i called nodal surface ot simply node. Shapes of ls Prorbitals are present in the p-subshell for which I= 1 and m, can have three possible orientations — 1,0, + 1 ns Thus, there are three orbitals in the p-subshell which are designated as p., Py and p, orbitals depending upon the axis along which they are directed. The general shape of p-orbital is dumb-bell consisting of two portions known as lobes. Moreover, there is a plane passing through the nucleus along which finding of the electron density is almost nil. This is known as nodal plane as shown in the fig. 2B>P } x x |“ TNedal plane z Goneral shape of port ; a », Shapes of 2p, 2p and dumb-bell pictures, itis quite obvious met 2 ae influences the shone rt!® Ovi that unite i here au th bila, a p-orbital i d Number of angular nodes ~ Total number of node + Shapes of d-orbital For d-orbitals, 1= 2 and ml =-2, 2,-1,0,41 and 42,16 there are Stypes of d-orbitals. They are da ee there are five possible aed dees sible orientations for d orbitals, So orbitals are double dumb-bell and that ofthe hues et id Teche the sour plane. The five d-orbitals have equivalent energies. Foi having a citcular collar in the XY- total number of nodes is n-2. Bound: tuumber of radial nodes is 2 and the Is are as follows, Fd orbitals thy lary surface diagrams for deorbital For Forbitals, 1= 3 and m =-3, -2,-1.0.+1,+2 and +3. ie,, there are seven possible orientations for f orbitals.so there are 7 types of f-orbitals, They have diffused shapes, Electronic Configuration of Atoms The distribution of electrons into orbitals of an atom is called its electronic configuration, The electronic configurationis explained based on Aufbau principle, Hund’s rule, Pauli’s exclusion principe and stability concept. 1 Aufbau principle: ‘The principle states: In the ground state of the atoms, the orbitals are fille gies. In other words, electrons first occupy the lowest energy orbital available to them and enter into higher energy orbitals only after the lower energy orbitals are filled. This rule has two sub rules: ani various orbitals are filled in the increasing order > itwoobins have the same (ntl) values, The obi wih the Tower n values filled frs-Theinreasng order of orbitals is as follows. order of their increasing 24Pauli Exclusion Principle ‘According to this principle, no two electrons in an atom can have the a 7 othe ate umber, Only two electrons may exist inthe same orbital and these electrons must have OPPost spins 2 eecton, have same values for n, Land m, they should have different values for s. ie. if's=+1/2. forthe first electron, it should be -1/2. for the second electron. 3. Hund’s rule of maximum multiplicity It states that: pairing of electrons in the orbitals belonging to the same sub-shell (p, d or D) does not take place until each orbital belonging to that sub-shell has got one electron each with parallelspin. For example the electronic configuration of N is 1s* 2s” 2p,'py'pz' and not Is” 2s° 2ps'Py - nL ](N) (474 Tt o [NS] A) CRTTTt Stability of Completely Filled and Half Filled Subshells For atoms having half filled or completely filled electronic configurations have extra stability compared to other atoms. This is due to their symmetrical distribution of electrons and greater exchange energy, For example, the electronic configuration of Cr is [Ar] 34°4s! and not 3d‘4s”, This is because d° represents a half filled configuration and has extra stability. Similarly for Cu the electronic configuration is [Ar] 3d!°4s! and not 3474s", Cr(24) = [Ar] 30 4s! not [Ar] 3d! 4s? Similarly Cu(29) = [Ar] 3d" 4s! not [Ar] 3a°4s? cg 1.Which of the following options does not represent ground state electronic configuration of an atom? (a) 1s? 2s? 2p® 3s? 3p® 38 4s? (b) 1s? 2s? 2p® 3s? 3p° 34? 4s? (©) Is? 2s? 2p 3s? 3p6 3d! 4s! (a) 1s? 2s? 2p® 3s? 3p 30° as! Ans .(b) 1s” 2s? 2p* 3s? 3p 3a? 4s? 2.Number of angular nodes for 4d orbital is @4 03 (2 @I Ans: (c) 2 Angular nodes =‘ =2 for d-orit. 3. The number of radial nodes for 3p orbital is @3 (b)4 ©2 @1 Ans: d) 1, Number of radial nods = n-1-1=3-1-1=1 4. g subshell is characterised by: @n=5 )m=3 ()l=4 @l=5 Ans: (¢) 5. Which expression represents de Brogile relationship? @ Wmu=p — @)A=h/imv CA=hmp (A=ulp Ans: (b) 6.Which of the following is responsible to rule out the existence of definite paths or trajectories of electrons? (a) Pauli’s exclusion principle. (b) Heisenberg’s uncertainty principle. (c) Hund’s rule of ‘maximum multiplicity. (d) Aufbau principle ‘Ans: (b) Heisenberg’s uncertainty principle. 7. For which af the following sets of quantum numbers, an electron will have the highest energy? (a) 3,2,+1, #12 (b) 4, 2-1#1/2 (©) 4,1,0,-1/2 (d) 5,0,0,+1/2 Ans: (b) : 8, Which of the following atoms or atonvion have identical ground state configuration ? (a) Lit and He (6)CI'and Ar (c)NaandK 94)F*and Ne Ans: (b) 25