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Chapter4 STRUCTURE OF THE ATOM © NCERTnot to be …

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.

within an atom. For explaining this, many scientists proposed various atomic models. J.J. Thomson was the first one to propose a model for the structure of an atom. 4.2.1 THOMSON’S MODEL OF AN ATOM Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons, in a sphere of positive charge,

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Transcription of Chapter4 STRUCTURE OF THE ATOM © NCERTnot to be …

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 MatterCharged Particles in MatterCharged Particles in MatterCharged Particles in MatterCharged 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?

3 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 wasnot a simple, indivisible particle but containedat least one sub-atomic particle the electronidentified by Thomson. Even before theelectron was identified, E. Goldstein in 1886discovered the presence of new radiations ina gas discharge and called them canal rays were positively charged radiationswhich ultimately led to the discovery ofanother sub-atomic particle.

4 This sub-atomicparticle had a charge, equal in magnitude butopposite in sign to that of the electron. Itsmass was approximately 2000 times as thatof the electron. It was given the name ofproton. In general, an electron is representedas e and a proton as p+ . The mass of a protonis taken as one unit and its charge as plusone. The mass of an electron is considered tobe negligible and its charge is minus seemed highly likely that an atom wascomposed of protons and electrons, mutuallybalancing their charges. It also appeared thatthe protons were in the interior of the atom,for whereas electrons could easily be peeledoff but not protons.

5 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 ATOMTOMTOMTOMTOMC hapter NCERTnot to be republisheduestions1. What are canal rays?2. If an atom contains one electronand one proton, will it carry anycharge or not? STRUCTURE of an AtomThe STRUCTURE of an AtomThe STRUCTURE of an AtomThe STRUCTURE of an AtomThe STRUCTURE of an AtomWe have learnt Dalton s atomic theory inChapter 3, which suggested that the atomwas indivisible and indestructible.

6 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 SSSSS MODELMODELMODELMODELMODEL OFOFOFOFOF ANANANANAN ATOMATOMATOMATOMATOMT 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.

7 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 SSSSS MODELMODELMODELMODELMODEL OFOFOFOFOF ANANANANAN ATOMATOMATOMATOMATOME rnest Rutherford was interested in knowinghow the electrons are arranged within anatom.

8 Rutherford designed an experiment forthis. 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.

9 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 ATOM47 NCERTnot to be republishedSCIENCE48 Fig. : 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.

10 (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 thewall. If he repeats this ten times, he will hearthe sound ten times. But if a blind-foldedchild were to throw stones at a barbed-wirefence, most of the stones would not hit thefencing and no sound would be heard. Thisis because there are lots of gaps in the fencewhich allow the 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.


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