STRUCTURE OF ATOM Objectives

1 29 STRUCTURE OF ATOMThe rich diversity of chemical behaviour of different elementscan be traced to the differences in the internal STRUCTURE ofatoms of these 2 STRUCTURE OF ATOMA fter studying this unit you will beable to know about the discovery ofelectron, proton and neutron andtheir characteristics; describe Thomson, Rutherfordand Bohr atomic models; understand the importantfeatures of the quantummechanical model of atom; understand nature ofelectromagnetic radiation andPlanck s quantum theory; explain the photoelectric effectand describe features of atomicspectra; state the de Broglie relation andHeisenberg uncertainty principle; define an atomic orbital in termsof quantum numbers; state aufbau principle, Pauliexclusion principle and Hund srule of maximum multiplicity; and write the electronic configurationsof existence of atoms has been proposed since the timeof early Indian and Greek philosophers (400 ) whowere of the view that atoms are the fundamental buildingblocks of matter.

2 According to them, the continuedsubdivisions of matter would ultimately yield atoms whichwould not be further divisible. The word atom has beenderived from the Greek word a-tomio which means uncut-able or non-divisible . These earlier ideas weremere speculations and there was no way to test themexperimentally. These ideas remained dormant for a verylong time and were revived again by scientists in thenineteenth atomic theory of matter was first proposed on afirm scientific basis by John Dalton, a British schoolteacher in 1808. His theory, called Dalton s atomictheory, regarded the atom as the ultimate particle ofmatter (Unit 1). Dalton s atomic theory was able to explainthe law of conservation of mass, law of constantcomposition and law of multiple proportion verysuccessfully. However, it failed to explain the results ofmany experiments, for example, it was known thatsubstances like glass or ebonite when rubbed with silkor fur get electrically this unit we start with the experimentalobservations made by scientists towards the end ofnineteenth and beginning of twentieth century.

3 Theseestablished that atoms are made of sub-atomic particles, , electrons, protons and neutrons a concept verydifferent from that of OF SUB-ATOMICPARTICLESAn insight into the STRUCTURE of atom wasobtained from the experiments on electricaldischarge through gases. Before we discussthese results we need to keep in mind a basicrule regarding the behaviour of chargedparticles : Like charges repel each other andunlike charges attract each other . Discovery of ElectronIn 1830, Michael Faraday showed that ifelectricity is passed through a solution of anelectrolyte, chemical reactions occurred atthe electrodes, which resulted in theliberation and deposition of matter at theelectrodes. He formulated certain laws whichyou will study in class XII. These resultssuggested the particulate nature mid 1850s many scientists mainlyFaraday began to study electrical dischargein partially evacuated tubes, known ascathode ray discharge tubes.

4 It is depictedin Fig. A cathode ray tube is made ofglass containing two thin pieces of metal,called electrodes, sealed in it. The electricaldischarge through the gases could beobserved only at very low pressures and atvery high voltages. The pressure of differentgases could be adjusted by evacuation of theglass tubes. When sufficiently high voltageis applied across the electrodes, currentstarts flowing through a stream of particlesmoving in the tube from the negative electrode (cathode) to the positive electrode (anode).These were called cathode rays or cathoderay particles. The flow of current fromcathode to anode was further checked bymaking a hole in the anode and coating thetube behind anode with phosphorescentmaterial zinc sulphide. When these rays, afterpassing through anode, strike the zincsulphide coating, a bright spot is developedon the coating [Fig. (b)].Fig. (a) A cathode ray discharge tubeFig.

5 (b)A cathode ray discharge tube withperforated anodeThe results of these experiments aresummarised below.(i)The cathode rays start from cathode andmove towards the anode.(ii)These rays themselves are not visible buttheir behaviour can be observed with thehelp of certain kind of materials(fluorescent or phosphorescent) whichglow when hit by them. Television picturetubes are cathode ray tubes andtelevision pictures result due tofluorescence on the television screencoated with certain fluorescent orphosphorescent materials.(iii)In the absence of electrical or magneticfield, these rays travel in straight lines(Fig. ).(iv)In the presence of electrical or magneticfield, the behaviour of cathode rays aresimilar to that expected from negativelycharged particles, suggesting that thecathode rays consist of negativelycharged particles, called electrons.(v)The characteristics of cathode rays(electrons) do not depend upon the2021-2231 STRUCTURE OF ATOM material of electrodes and the nature ofthe gas present in the cathode ray , we can conclude that electrons arebasic constituent of all the to Mass Ratio of ElectronIn 1897, British physicist Thomsonmeasured the ratio of electrical charge (e) tothe mass of electron (me ) by using cathode raytube and applying electrical and magnetic fieldperpendicular to each other as well as to thepath of electrons (Fig.)

6 When only electricfield is applied, the electrons deviate from theirpath and hit the cathode ray tube at point A(Fig. ). Similarly when only magnetic fieldis applied, electron strikes the cathode ray tubeat point C. By carefully balancing the electricaland magnetic field strength, it is possible tobring back the electron to the path which isfollowed in the absence of electric or magneticfield and they hit the screen at point argued that the amount of deviationof the particles from their path in the presenceof electrical or magnetic field depends upon:(i)the magnitude of the negative charge onthe particle, greater the magnitude of thecharge on the particle, greater is theinteraction with the electric or magneticfield and thus greater is the deflection.(ii)the mass of the particle lighter theparticle, greater the deflection.(iii)the strength of the electrical or magneticfield the deflection of electrons from itsoriginal path increases with the increasein the voltage across the electrodes, or thestrength of the magnetic carrying out accurate measurements onthe amount of deflections observed by theelectrons on the electric field strength ormagnetic field strength, Thomson was able todetermine the value of e/me as:eem = 1011 C kg 1( )Where me is the mass of the electron in kg ande is the magnitude of the charge on the electronin coulomb (C).

7 Since electrons are negativelycharged, the charge on electron is Charge on the Millikan (1868-1953) devised a methodknown as oil drop experiment (1906-14), todetermine the charge on the electrons. He foundthe charge on the electron to be 10 19 C. The present accepted value ofelectrical charge is 10 19 C. Themass of the electron (me) was determined bycombining these results with Thomson s valueof e/me ratio. = 10 31 kg ( )Fig. The apparatus to determine the charge to the mass ratio of Discovery of Protons and NeutronsElectrical discharge carried out in the modifiedcathode ray tube led to the discovery of canalrays carrying positively charged particles. Thecharacteristics of these positively chargedparticles are listed below.(i)Unlike cathode rays, mass of positivelycharged particles depends upon thenature of gas present in the cathode raytube. These are simply the positivelycharged gaseous ions.

8 (ii)The charge to mass ratio of the particlesdepends on the gas from which theseoriginate.(iii)Some of the positively charged particlescarry a multiple of the fundamental unitof electrical charge.(iv)The behaviour of these particles in themagnetic or electrical field is opposite tothat observed for electron or smallest and lightest positive ion wasobtained from hydrogen and was calledproton. This positively charged particle wascharacterised in 1919. Later, a need was feltfor the presence of electrically neutral particleas one of the constituent of atom. Theseparticles were discovered by Chadwick (1932)by bombarding a thin sheet of beryllium by -particles. When electrically neutral particleshaving a mass slightly greater than that ofprotons were emitted. He named theseparticles as neutrons. The importantproperties of all these fundamental particlesare given in Table MODELSO bservations obtained from the experimentsmentioned in the previous sections havesuggested that Dalton s indivisible atom iscomposed of sub-atomic particles carryingpositive and negative charges.

9 The majorproblems before the scientists after thediscovery of sub-atomic particles were: to account for the stability of atom, to compare the behaviour of elements interms of both physical and chemicalproperties,Millikan s Oil Drop MethodIn this method, oil droplets in the form ofmist, produced by the atomiser, were allowedto enter through a tiny hole in the upper plateof electrical condenser. The downward motionof these droplets was viewed through thetelescope, equipped with a micrometer eyepiece. By measuring the rate of fall of thesedroplets, Millikan was able to measure themass of oil droplets. The air inside thechamber was ionized by passing a beam ofX-rays through it. The electrical charge onthese oil droplets was acquired by collisionswith gaseous ions. The fall of these chargedoil droplets can be retarded, accelerated ormade stationary depending upon the chargeon the droplets and the polarity and strengthof the voltage applied to the plate.

10 By carefullymeasuring the effects of electrical fieldstrength on the motion of oil droplets,Millikan concluded that the magnitude ofelectrical charge, q, on the droplets is alwaysan integral multiple of the electrical charge,e, that is, q = n e, where n = 1, 2, .Fig. The Millikan oil drop apparatus formeasuring charge e . In chamber, theforces acting on oil drop are:gravitational, electrostatic due toelectrical field and a viscous drag forcewhen the oil drop is moving. to explain the formation of different kindsof molecules by the combination of differentatoms and, to understand the origin and nature of thecharacteristics of electromagnetic radiationabsorbed or emitted by OF ATOMT able Properties of Fundamental ParticlesDifferent atomic models were proposed toexplain the distributions of these chargedparticles in an atom. Although some of thesemodels were not able to explain the stability ofatoms, two of these models, one proposed Thomson and the other proposed byErnest Rutherford are discussed Thomson Model of AtomJ.