STRUCTURE OF ATOM Objectives Aller studying this unit you will be able to know about the discovery of electron, praton and nevitran and their characteristics: deseribe Thomoon, Rutherford ‘and Bohr atone model understand the important features of the quantum mechanical model of atom: understand —naturé (of electromagnetic radiation “and Planes quantum theory’ explain the photoelectric effect and describe features of atomic spectra: state the de Broglle relatiomand Heisenberg uncertainty principle: define an atomic orbatal'wi terms of quantum numbers: state aufbau. piineiple. Paull exclusion prineiple and Hund’s rraleof maxima multiphoity: and write the electro configurations of atome ‘The rich diversity of chemical bchaviour of different elements can be traced to the differences in the internal structure of atoms of these elements. ‘The existence of atoms has been proposed since the time of early Indian and Greek philosophers (400 B.C.) who were of the view that atoms are the fundamental building blocks of mattcr. According to them, the continued subdivisions of matter would ultimately yield atoms which ‘would not be further divisible. The word ‘atom’ has been derived from the Greek word ‘a-tomio' which means ‘uncut-able’ or ‘non-divisible’. These earlier ideas were mere speculations and there was no way to test them experimentally. These ideas remained dormant fora very long time and were revived again by scientists in the nineteenth century, ‘The atomic theory of matter was first proposed on a firm scientific baci by John Dalton, a British school teacher in 1808. Ills theory, called Dalton's atomic theory, regarded the atom as the ultimate particle of matter (Uinit 1). Dalton’s atomic theory was able to explain the law of conservation of mass, law of constant composition and law of multiple proportion very successfully. However. it failed to explain the results of many experiments, for example, it was known that substances like glass or ebontte when rubbed with silk or fur get electrically charged. In this unit we start with the experimental obscrvations made by scicntists towards the cnd of nineteenth and beginning of twentieth century. ‘These established that atoms are made of sub-atomic particles. i.e., electrons, protons and neutrons a concept very different from that of Dalton. 2018-1930 1 DISCOVERY OF SUB-ATOMIC PARTICLES An insight intu he suucture of atom was obtained from the experiments on electrical discharge through gases. Before we discuss these results we need to keep in mind a basic rule regarding the behaviour of charged particles : “Like charges repel each other and. unlike charges attract each other” 2.1.1 Discovery of Electron In 1830, Michael Faraday showed that if electricity is passed through a solution of an electrolyte, chemical reactions occurred at the electrodes. which resulted in the liberation and deposition of matter at the electrodes. He formulated certain laws which you will study in class XII. These results suggested the particulate nature of electricity, In mid 1850s many scientists mainly Faraday began to study electrical discharge in partially evacuated tubes, known as cathode ray discharge tubes. It is depicted in Fig 2.1 A cathode ray tbe is made of glass coulaining (wo in pieces of metal, called electrodes. sealed in it. The electrical discharge through the gases could be observed only at very low pressures and at very high voltages. The pressure of different gases could be adjusted by evacuation of the glass tubes. When sufliciently high voltage is applicd across the clectrodes, current starts flowing through a stream of particles moving in the tube from the negative electrode (cathode) to the positive electrode (anode). ‘These were called cathode rays or cathode ray particles. The flow of current from cathode to anode was further checked by making a hole in the anode and coating the tube behind anode with phosphorescent material zinc sulphide. When these rays, after passing through anode, sirike the zine sulphide coating, a bright spot is developed on the coating [Fig. 2. 1(b]] CHEMISTRY “Ta waesna puny cathode fh 2 anode a ‘ Fig. 2.1(a) A cathode ray discharge tube ‘ro rien pay Lal ghee A cathode ray discharge tube with perforated anode Fig. 2.106) ‘The results of these experiments are summarised below. (‘The cathode rays start from cathode and move towards the anode. (il) ‘These rays themselves are not visible but their behaviour can be observed with the help of certain kind of materials {iluorescent or phosphorescent) which glow when hit by them. Television picture tubes are cathode ray tubes and television pictures result due to fluorescence on the television screen coated with certain fluorescent or phosphorescent materials, (il) In the absence of electrical or magnetic field, these rays travel in straight lines (wig. 2.2). (iv) _In the presence of electrical or magnetic field, (he behaviour 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 2018-19‘STRUCTURE OF ATOM material of electrodes and the nature of the gas present in the cathode ray tube. Thus, we can conclude that etectrons are basic constituent of all the atoms. 1.2 Charge to Mass Ratio of klectron In 1897, British physicist J.J. Thomson measured the ratio of electrical charge (¢) to the mass of electron (m,1 by using cathode ray tube and applying electrical and magnetic field perpendicular to each other as well as to the path of electrons (Fig. 2.2). When only electric field is applied, the electrons deviate from their path and hit the cathode ray tube at point A (Fig. 2.2). Similarly when only magnetic field is applied, clectron strikes the cathode ray tube at point C, By carefully balaneing the electrical and magnetic field strength, it is possible to bring back the electron to the path which is followed in the absence of electric or magnetic field and they hit the screen at point B ‘Thomson argued that the amount of deviation ofthe particles from their path in the presence of electrical or magnetic field depends upon: () the magnitude of the negative charge on the particle, greater the magnitude of the charge on the particle, greater Is the interaction with the electric or magnetic field and thus greater is the detection. (i) the mass of the particle — lighter the particle, greater the deflection. Cathode Anode at (ii) the strength of the electrical or magnetic, field — the deflection of electrons from its original path increases with the increase. in the voltage across the electrodes, or the strength of the magnetic field. Ry carrying ont accurate measirements.on the amount of deflections observed by the electrons on the electric field strength or magnetic field strength, Thomson was able to determine the valuc of ¢/m, as: e Tay 7 1:758020 * 10" C ke! ig en Where m,is the mass af the electron in kg and eis the magnitude of the charge on the clectron in coulomb (C). Since electrons are negatively charged. the charge on electron is -e. 2.108 Charge on the Electron RA. Millikan (1868-1959) devised a method Known as oll drop experiment (1906-14), to determine the charge on the electrons, He fonnd the charge on the electron to be = 1.6 10" C. The present accepted value of electrical charge is ~ 1.602176 x 10°C. The mass of the electron (mn) was determined by coulbining Whese results will Towson’ value ot e/m, ratio. __&_ 1.602176 x 10""e *~e/m, — 1.758820 x 10"'C kg = 9.1094x1051 kg, 0.2) Fluorescent sereen Magnet Pig. 2.2. The apparatus to determine the charge to the mass ratio of electron 2018-1932 2.1.4 Discovery of Protons and Neutrons Flectrical discharge carried ant in the modified cathode ray tube led to the discovery of canal rays carrying positively charged particles. The characteristics of these positively charged particles are listed below. (Unlike cathode rays, mass of positively charged particles depends upon the nature of gas present in the cathode ray lube. These are simply dhe positively charged gaseous ions. (i) The charge to mass ratio of the particles depends on the ges fou which dese originate. (ii) Some of the positively charged particles carry u tmuluple of (ue fundarmenital unit of electrical charge. (iv) The behaviour of these particles in the magnetic or electrical field is opposite to that observed for electron or cathode ‘The smallest and lightest positive ion was obtained from hydrogen and was called proton. This positively charged particle was, characterised in 1919. Later, a need was fell tor the presence ot electrically neutral particle as one of the constituent of atam. These particles were discovered by Chadwick (1932) by bombarding a thin sheet of beryllium by acparticles. When electrically neutral particles, having a mass slightly greater than that of protons were emitted. He named these particles as neutrons. The important properties of all these fimdamental particles are given in Table 2.1. 2.2 ATOMIC MODELS. Observations obtained from the experiments mentioned in the previous sections have suggested (hat Dalton’s tndivisible atom ts, composed of sub-atomic particles carrying positive and negative charges. The major problems before the scicntists after the discovery of sub-atomic particles were: + toaccount for the stability of atom, © to compare the behaviour of elements in terms of both physical and chemical properties, CHEMISTRY Millikan's Oil Drop Method In this method, oll droplets tn the form of mist, pradiiced hy the atemier were alloword (enter through a tiny hole in the upper plate ofeleciical condenser. The downward motion of these droplets was viewed through the telescope, equipped with a micrometer eye piece. By measuring the rate of fll of these droplets, Millikan was able to measure the mase of oil droplets. The air inside the chamber was ionized by passing a beam of X-rays through it, The electrical charge on these oll droplets was acquired by collisions with gaseous fons. The fall of these charged fll droplets can be retarded, accelerated or ‘made stationary depending upon the charge ‘ou the droplets aud the polarity aul strenght ‘of the voltage applied to the plate. Hy carefully measuring the ellects of electrical field strength on the motion of oil droplets. Millikan conchided that the magnitude of electrical charge, q, on the droplets is alwaye ‘an Integia) uculiple of Ue eectuical cage, e, that is, q= ne, wheren = 1, 2,3... ‘ena at | oe yom theeoree sted ite ag ‘er teratan Fig. 2.3 The Millikan oll drop apparatus for ‘measuring charge ‘e’ In chamber. the forces acting on oll drop are: ‘gravitational, electrostatic due to electrical feld andl a viscous drag force ‘when the oil drop is moving. * to explain the formation of different kinds of molecules by the combination of different atoms and, ® tounderstand the origin and nature of the characteristics of electromagnetic radiation absorbed or emitted by atoms. 2018-19‘STRUCTURE OF ATOM 33 ‘Table 2.1 Propertics of Fundamental Particles Absolute charge/C Name ‘Symbol ‘Mass/u ‘Approx. mass/u Different atomic models were proposed to explain the distributions of these charged particles in an atom, Although some of these ‘models were not able to explain the stability of atoms. two of these models. one proposed by J.J. Thomson and the other proposed by Emiest Rutherford are discussed below. 2.2.1 Thomson Model of Atom J.J. Thomson. in 1898. proposed that an atom. possesses a spherical shape (radius approximately 10 !°m) tn which the postive charge is uniformly distributed. The electrons, are embedded into it in such a manner as to give te anos stable elect ustalic ai raugenent (Fig. 2.4). Many diferent names are given to this model, for example. plum pudding, raisin pudding or watermelon. This model can be Positive sphere leetron Fig.2.4 Thomson model of atom visualised as a pudding or watermelon of positive charge with plums or seeds (electrons) embedded into it. Areoryor tun feuture of duis model is that the mass of the atom is assumed to be uniformly distributed over the atom. Although this model was able to explain the overall neutrality of the atom, but was not consistent with the results of later experiments, ‘Thomson was awarded Nobel Prize for physics in. 1906, for his Uieoretical and experimental investigations on the conduction of electricity by gases. different kinds of rays were d besides those mentioned Roentgen (1845-1923) in that when electrons strike the cathode ray tub ctively when electrons strike metal anode, called targets. ‘not deflected by the electric and ic fields and have a very high jetrating power through the matter "and that is the reason that these rays are used to study the interior of the objects. ‘These rays are of very short wavelengths (-U.1 nm) and possess electro-magnetic character (Section 2.3.1). Henri Becqueral (1852-1908) observed that there are certain elements which cmit radiation on their own and named this phenomenon as radioactivity and the elements known as radioactive elements. This field was developed by Marie Curie, Piere Curie, Rutherford and Fredriele Soddy. It was ‘obscrved that threc kinds of rays i.¢.. B- and -rays are emitted. Rutherlord found that a-rays consists of high energy particles carrying wo units of positive charge and four unit of atomic mass. He concluded that @- particles are helium 2018-19nuclei as when a particles combined with two electrons yielded helium gas. B-raye, are negatively charged particles similar to electrons. The y-rays are high energy radiations like X rays, are neutral in nature and do not consist of particles. AS regards penetrating, power, c-particles are the least, followed by B-rays (100 times that of a-particles) and 7-rays (1000 times of that c-particles).. 2.2.2. Rutherford's Nuclear Model of Atom Rutherford and his students (Ilans Geiger and Emest Marsden) bombarded very thin gold toil with a-particles. Rutherford's famous a-particle scattering experiment is spe parce Lon late A. Rutherford's scattenng experment ‘Thin gold fold S Fy oeomas Ser eee —= oS : — + particle eflected «= particle B. Schematic molecular view of the gol Joil Fig. 2.5 Schematic view of Rutherford's seattering experiment. When @ beam of alpha (a) particles (3 “shot” at a thin gold foll, most of them pass itrougit withow much effect. Some, however, are deflected. CHEMISTRY represented in Fig, 2.5. A stream of high energy a-particles from a radioactive source was directed at a thin foil (thickness - 100 nm) of old metal, ‘The thin gold foil had a circular fluorescent zinc sulphide screen around it. Whenever o-particles struck the sereen, a tiny fash of light was produced at thal point. The results of scattering experiment were quite unexpected. According to Thomson model of atom, the mass of each gold atom n the foil should have been spread evenly over the entire atom, and «particles had enough energy to pass directly through such a uniform distribution of mass. It was expected that the particles would slow down and change directions only by'a small angles as they passed through the foil. It was observed that: (most of the o-particles passed through the gold foil undetlected. (ii) a small fraction of the o-particles was detlected by small angles, (ua very few a-particles (-1 in 20,000) bounced back, that is, were deflected by nearly 180" On the basis of the observations, Rutherford drew the following conclusions regarding the structure of atom: (@ Most of the space in the atom is empty as most of the o-particles passed through the foll undeflected. (i) A few positively charged a-particles were deflected. The deflection must be duc to enormous repulsive force showing that the positive charge of the atom is not spread throughout the atom as Thomson had presumed. The positive charge has tobe concentrated in a very small volume that repelled and deflected the positively charged a-particles, (ii) Calculations by Rutherford chowed that the volume occupied by the nucleus is negligibly small as compared to the total volume of the atom. The radius of the atom is about 10° m, while that of nucleus is 10m. One can appreciate this difference in size by realising that if 2018-19‘STRUCTURE OF ATOM a cricket ball represents a nucleus, (lien the radius of atom would be about 5 km, On the basts of above observations and conclusions. Rutherford proposed the nuclear model of atom. According to this mode! (The positive charge and most of the mass of the atom was densely concentrated in extremely small region. ‘This very small portion of the atom was called muelens, by Rutherford. (W) The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular paths called orbits, Thus, Rutherford'’s model of atom resembles the solar system in which the nucleus plays the role of sun and the electrons that of revolving planets. Gif) Electrons and the nucleus are held together by cl attraction, 2.2.3 Atomic Number and Mass Number The presence of positive charge on the nucleus 18 due to the protons in the nucleus, ‘As established earlier, the charge on the proton is equal but opposite to that of electron. The number of protons present mn the nucleus is equal to atomic number (Z) For example, the number of protons in the hydrogen nucleus 15 1, in sodium atom 1t 1s 11. therefore their atomic numbers are 1 and 11 respectively. In order to keep the clectrical neutrality, the number of electrons in an atom is equal to the number of protons (atomic number, Z). For example, number of electrons in hydrogen atom and sodium atom are 1 and 11 respectively. Atomfe number (Z) = number of protons in ‘the nucleus of an atom = number of electrons inanuetralatom (2.3) While the positive charge of the nucleus is due to protons, the mass of the nucleus, due (y protons aud neuuuus, As discussed earlier protons and neutrons present in the nucleus are collectively known as nucleons, 35 ‘The total number of nucleons is termed as mass number (AJ of the atom. ‘mass number (A) = number of protons (2) + number of neutrons (n) (2.4) 2.2.4 Isobars and Isotopes The composition of any atom can be represented by using the normal element symbol (X) with super-seript on the left hand side as the atomic mass number (A) and subscript (Z) on the lell hand siele as the atomic number (i.e.. 3X). Isobars are the atoms with same mass number but different atomie number for example, U and'N. On the other hand, atoms with identical atomic nnmber but different atoinic mass number are known as Isotopes. In other words faccording to equation 2.4). it {a evident that difference between the isotopes is due to the presence of dillerent number of neutrons present in the nucleus. For example, considering of hydrogen atom again, 99.985% of hydrogen atoms contain only one proton ‘This isotopc is called protium (7H). Rest of the percentage o! hydrogen atom coniains two other isotopes, the one containing | proton and 1 neutron is called deuterium (7D, 0.015%) and the ather one possessing 1 proton and 2 neuluus is called tritium (?T ), The latter {isotope is found in trace amounts on the earth. Other examples of commonly occuring isotopes are: carbon atoms containing, 7 and 8 neutrans besides 6 protons (!2C, #C, }!C): chlorine atoms contatning 18 anid 20 neutrons besides 17 protons (C1, C1) Lastly an important point to mention regarding isotopes is that chemical properties of aioms are controlled by the number of electrons, which are determined by the number of protons in the nucleus. Number of neutrons present in the nucleus have very ttle effect on the chemical properties of an element. ‘Therefore, all the Isotupes ofa given elemerL show same chemical behaviour. 2018-19Problem 2.1 Calculate the number of protons, neutrons and electrons in Br Solution In this case, Br. Z isneutral Number of protons = number of electrons =Z=35 Number of neutrons ~ 80 ~ 35 - 45, (equation 2.4) Problem 2.2 The number of electrons, protons and neutrons ina species are equal to 18, 16 and 16 respectively. Assign the proper symbol to the species. Solution The atomic number is equal to number of protons = 16. ‘rhe element is sulphur (5) Atomic mass number = number of protons + number of neutrons =16 + 16=32 Species is not neutral as the number of protons is not equal to electrons. This anion (negatively charged) with charge ‘equal to excess electrons = 18 ~ 16 = 2. Symbol is 2S" Note : Before using the notation £X, find. ‘out whether the species is a neutral atom, cation oran anion. ifitisaneutral atom, equation (2.3) is vald. i.c.. number of protons = number of electrons = atomic number, Ifthe species isatt ton, determine whether the number of protons are larger (cation, positive ion) or smaller (anion, negative ion) than the number of electrons. ‘Number of neutrons is always given by A-7, whether the species is nevitral or ion. 15, A= BO, species: 2.2.5 Drawbacks of Rutherford Model ‘As you have learnt above, Rutherford nuclear model of an atom is like a small scale solar system with the nucleus playing the role of the CHEMISTRY massive sun and the electrons being similar to the lighter planets. When classical mechanies* is applied to the solar system, it shows that the planets describe well defined orbits around the sun, The gravitational force between the planets is given by the expression ) where m, and m, are the masses, r 1s the distance of separation of the masses and Gis the gravitational constant. The theory can also calculate precisely the planetary orbits and these are in agreement with the experimental measurements, The similarity between the solar system and nuclear model suggests that electrons should move around (he nucleus in well defined orbits. further, the coulomb force (kq,9,/° where q, and q, are the charges, ris the distance of separation of the charges and k is the proportionality constant) between electron and the nucleus is mathematically similar to the gravitational force, However, when a body 1s moving in an orbit, IL undergoes acceleration even ifitis moving with a constant speed in an orbit hecanse of changing direction. So an electron in the nuclear model describing planet hike orbits 1s under acceleration. According to the electromagnetic theory of Maxwell, charged particles when accelerated should emit electromagnetic radiation (This feature does not exist for planets since they are uncharged). Therefore, au election in ain orbit will eutit radiation, the energy carried by radiation comes from electronic motion. ‘The orbit will thus continue to shrink. Calculations show nal ¢ should (ake att electron only 10* 8 to spiral into the nucleus. But this does not happen. Thus, the Rutherford model cannot explain the stability of an atom. Ifthe motion of an electron is described on the basis of the classical mechanics and electromagnetic theory, you may ask that since the motion of clectrons in orbits is leading to the instability of the atom, then why not consider electrons as stationary Glassiral mechanies (sa theoretical setence based an Nentons tains af motion. W aperthies the laws af mation af macroscopic objec. 2018-19‘STRUCTURE OF ATOM around the nucleus. If the electrons were stationary, electrostatic attraction between the dense nucleus and the electrons would pull the electrons toward the nucleus to form, a miniature version of Thomson's model of atom. Another serious drawback of te Rutherford model is that it says nothing about distribution of the electrons around the nucleus and the energies of these electrons. 2.8 DEVELOPMENTS LEADING TO THE BOHR'S MODEL OF ATOM Historically, results observed from the studies, ofinteractious of radiaious with mralter Lave provided immense information regarding the structure of atoms and molecules. Neils Bohr utilised these reaulta to improve upon the model proposed by Rutherford. Two developments played a major role in the formulation of Bohr’s model of atom. These (i) Dual character of the electromagnetic radiation which means that radiations possess both wave like and particle like properties, and QW) Experimental results regarding atomic spectra, First, we will discuss about the duel nature of electromagnetic radiations. Experimental results regarding atomie spectra will he discussed in Scction 2.4. 2.3.1 Wave Nature of Electromagnetic Radiation In the mid-nineteenth century. physicists actively siudied absorption and emission of radiation by heated objects. These are called thermal radiations, They tried to find out of what the thermal radiation is made. It is now a well-known fact that thermal radiations consist of electromagnetic waves of various frequencies or wavelengths. It is based on a number of modern concepts, which were umknown in the mid-nineteenth century. First active study of thermal radiation laws occured inthe 1850's and the theory of electromagnetic waves and the emission af sch waves hy accelerating charged particles was developed 37 in the early 1870's by James Clerk Maxwell, which was experimentally confirmed later by Heinrich Hertz. Here. we will learn some facts, about electromagnetic radiations. James Maxwell (1870) was the first to give a comprehensive explanation about the interaction between the charged bodies and the behaviour of electrical and magnetic lields. on macroscopic level. He suggested that when. electrically charged particle moves under accelaration, alternating electrical and magnetic fields are produced and transmitted. These fields are transmitted in the forms of waves called electromagnetic waves or electromagnetic radiation. Light is the form of radiation known from early days and speculation about its nature dates back to remote ancient times. In earlier days (Newton) light was supposed to be made of pur ticles (curpuscules). IU was ouly int Uae 19th century when wave nature of light was established, Maxwell was again the first to reveal that light waves are associated with oscillating electric and magnetic character (Fig. 2.6). Fig.2.6 The electric and magnetic field components ofan electromagnetic inane. These components have the same wavelength, frequency, speed and amptitude, but they vibrate mn two mutually perpendicular planes. Although clectromagnetic wave motion is complex in nature, we will consider here only a few simple properties. (‘The osctllaiingelectric and magnette felds produced by oscillating charged particles 2018-1938 are perpendicular each other and boll, are perpendicular to the direction of propagation of the wave. Simplified picture of electromagnetic wave is shown in Fig. 2.6 Unlike sound waves or waves produced 1m water, electromagnetic waves do not require medium and can move in vacuum. It is now well established that there are many types of electromagnetic radiations which differ from one another in wavelength (or frequency]. These constitute what is called electromagnetic spectrum (Fig. 2.7). Different regions of the spectrum are identified by different names. Some examples are: radio frequency region around 10*Hz, used for broadcasting; microwave region around 10" Hz used for radar; infrared region around 10! Hz used for heating: ultraviolet region around 10!Hz a component of sun’s radiation. The small portion around 10° Hz. is what is ordinarily called visible light. It is only this part which our eyes can see (or detect). Special instruments are required to detect non-visible radiation. w) (ii) fo" fo" 40" 49% 10" 10" > 0" © | yraye Xraye| Uv} 1 rT he TT 10% 10" 197 10% 40%} 49 10° - Visible spectrum 0 400 500 600 Wavelengths (nanometers) Fig. 2.7 4 smalll part of the entire spectrum 2018- CHEMISTRY (iy) Different kinds of units are used to represent electromagnetic radiation. ‘These radiations are characterised by the properties, namely, frequency (v) and wavelength (i) The SE unit for frequency (¥) is ber. (Hz, 5"), after Heinrich Hertz. It is defined as the number of waves that pass a given point, mone second, Wavelength should have the units oflengih and as you know that the SI units of length is, meter (m). Since electromagnetic radiation consists of different kinds of waves of much. smaller wavelengths, smaller units are used. Fig.2.7 shows various types of eleviro- magnetic radiations which differ from one another in wavelengths and frequencies. In vacenm alll types of electromagnetic radiations, regardless of wavelength, travel at, the same speed. i.e. 3.0 X 10° ms" (2.997925 % 10" m5", to be precise). This is called speed of light and ts given the symbol ‘c’. ‘The frequency (v), wavelength Q) and velocity of light (© aie related by thie equation (2.5). e=va B® an 10° w (He) Microwave Long radio waves dom — xten) 18 wo to 700750 fa) The spectrum of electromagnetic radiation. (b) Visible spectrum. The visible region is only 419