SSSTRUCTURE OFOFOF THETHETHE A A ATOMTOMTOM

1 In chapter 3, we have learnt that atoms andmolecules are the fundamental buildingblocks of matter. The existence of differentkinds of matter is due to different atomsconstituting them. Now the questions arise:(i) What makes the atom of one elementdifferent from the atom of another element?and (ii) Are atoms really indivisible, asproposed by Dalton, or are there smallerconstituents inside the atom? We shall findout the answers to these questions in thischapter. We will learn about sub-atomicparticles and the various models that havebeen proposed to explain how these particlesare arranged within the major challenge before the scientists atthe end of the 19th century was to reveal thestructure of the atom as well as to explain itsimportant properties.

2 The elucidation of thestructure of atoms is based on a series of the first indications that atoms arenot indivisible, comes from studying staticelectricity and the condition under whichelectricity is conducted by Particles in MatterFor understanding the nature of chargedparticles in matter, let us carry out thefollowing dry hair. Does the comb thenattract small pieces of paper? a glass rod with a silk cloth andbring the rod near an inflated what these activities, can we concludethat on rubbing two objects together, theybecome electrically charged? Where does thischarge come from? This question can beanswered by knowing that an atom is divisibleand consists of charged scientists contributed in revealingthe presence of charged particles in an was known by 1900 that the atom wasindivisible particle but contained at least onesub- atomic particle the electron identified Thomson.

3 Even before the electron wasidentified, E. Goldstein in 1886 discovered thepresence of new radiations in a gas dischargeand called them canal rays. These rays werepositively charged radiations which ultimatelyled to the discovery of another sub-atomicparticle. This sub- atomic particle had a charge,equal in magnitude but opposite in sign to thatof the electron. Its mass was approximately2000 times as that of the electron. It was giventhe name of proton. In general, an electron isrepresented as e and a proton as p+ . Themass of a proton is taken as one unit and itscharge as plus one. The mass of an electron isconsidered to be negligible and its charge isminus seemed that an atom was composed ofprotons and electrons, mutually balancingtheir charges. It also appeared that the protonswere in the interior of the atom, for whereaselectrons could easily be removed off butnot protons.

4 Now the big question was:what sort of structure did these particles ofthe atom form? We will find the answer tothis question OFOFOFOFOF THETHETHETHETHE A A A A are canal rays? an atom contains one electronand one proton, will it carry anycharge or not? structure of an AtomWe have learnt Dalton s atomic theory inChapter 3, which suggested that the atomwas indivisible and indestructible. But thediscovery of two fundamental particles(electrons and protons) inside the atom, ledto the failure of this aspect of Dalton s atomictheory. It was then considered necessary toknow how electrons and protons are arrangedwithin an atom. For explaining this, manyscientists proposed various atomic Thomson was the first one to propose amodel for the structure of an S MODEL OF AN ATOMT homson proposed the model of an atom tobe similar to that of a Christmas electrons, in a sphere of positive charge,were like currants (dry fruits) in a sphericalChristmas pudding.

5 We can also think of awatermelon, the positive charge in the atomis spread all over like the red edible part ofthe watermelon, while the electrons arestudded in the positively charged sphere, likethe seeds in the watermelon (Fig. ).Thomson proposed that:(i)An atom consists of a positivelycharged sphere and the electrons areembedded in it.(ii)The negative and positive charges areequal in magnitude. So, the atom as awhole is electrically Thomson s model explained thatatoms are electrically neutral, the results ofexperiments carried out by other scientistscould not be explained by this model, as wewill see S MODEL OF AN ATOME rnest Rutherford was interested in knowinghow the electrons are arranged within anatom. Rutherford designed an experiment forthis.

6 In this experiment, fast moving alpha( )-particles were made to fall on a thingold foil. He selected a gold foil because he wantedas thin a layer as possible. This gold foilwas about 1000 atoms thick. -particles are doubly-charged heliumions. Since they have a mass of 4 u, thefast-moving -particles have aconsiderable amount of energy. It was expected that -particles would bedeflected by the sub- atomic particles inthe gold atoms. Since the -particles weremuch heavier than the protons, he didnot expect to see large : Thomson s model of an Thomson (1856-1940), a Britishphysicist, was born inCheetham Hill, a suburbof Manchester, on18 December 1856. Hewas awarded the Nobelprize in Physics in 1906for his work on thediscovery of directed the Cavendish Laboratory atCambridge for 35 years and seven of hisresearch assistants subsequently wonNobel OF THE ATOM472021-22 SCIENCE48 Fig.

7 : Scattering of -particles by a gold foilBut, the -particle scattering experimentgave totally unexpected results (Fig. ). Thefollowing observations were made:(i)Most of the fast moving -particlespassed straight through the gold foil.(ii)Some of the -particles were deflectedby the foil by small angles.(iii)Surprisingly one out of every 12000particles appeared to the words of Rutherford, This resultwas almost as incredible as if you fire a15-inch shell at a piece of tissue paper and itcomes back and hits you .hear a sound when each stone strikes the he repeats this ten times, he will hear thesound ten times. But if a blind-folded childwere to throw stones at a barbed-wire fence,most of the stones would not hit the fencingand no sound would be heard.

8 This is becausethere are lots of gaps in the fence which allowthe stone to pass through a similar reasoning, Rutherfordconcluded from the -particle scatteringexperiment that (i)Most of the space inside the atom isempty because most of the -particlespassed through the gold foil withoutgetting deflected.(ii)Very few particles were deflected fromtheir path, indicating that the positivecharge of the atom occupies very littlespace.(iii)A very small fraction of -particleswere deflected by 1800, indicating thatall the positive charge and mass of thegold atom were concentrated in a verysmall volume within the the data he also calculated that theradius of the nucleus is about 105 times lessthan the radius of the the basis of his experiment,Rutherford put forward the nuclear model ofan atom, which had the following features:(i)There is a positively charged centre inan atom called the nucleus.

9 Nearly allthe mass of an atom resides in thenucleus.(ii)The electrons revolve around thenucleus in circular paths.(iii)The size of the nucleus is very smallas compared to the size of the of Rutherford s model ofthe atomThe revolution of the electron in a circular orbitis not expected to be stable. Any particle in acircular orbit would undergo acceleration, charged particles wouldradiate energy. Thus, the revolving electronwould lose energy and finally fall into thenucleus. If this were so, the atom should behighly unstable and hence matter would notexist in the form that we know. We know thatatoms are quite Rutherford (1871-1937)was born at Spring Groveon 30 August 1871. He wasknown as the Father ofnuclear physics. He isfamous for his work onradioactivity and thediscovery of the nucleus of an atom withthe gold foil experiment.

10 He got the Nobelprize in chemistry in us think of an activity in an open fieldto understand the implications of thisexperiment. Let a child stand in front of awall with his eyes closed. Let him throwstones at the wall from a distance. He will2021-22 structure OF THE S MODEL OF ATOMIn order to overcome the objections raisedagainst Rutherford s model of the atom,Neils Bohr put forward the followingpostulates about the model of an atom:(i)Only certain special orbits known asdiscrete orbits of electrons, are allowedinside the atom.(ii)While revolving in discrete orbits theelectrons do not radiate the basis of Thomson s modelof an atom, explain how the atomis neutral as a the basis of Rutherford smodel of an atom, which sub- atomic particle is present in thenucleus of an atom?