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Crhapter4, , In Chapter 3, we have learnt that atoms and, molecules are the fundamental building, blocks of matter. The existence of different, kinds of matter is due to different atoms, constituting them. Now the questions arise:, (i) What makes the atom of one element, different from the atom of another element?, and (ii) Are atoms really indivisible, as, proposed by Dalton, or are there smaller, constituents inside the atom? We shall find, out the answers to these questions in this, chapter, We will learn about sub-atomic, particles and the various models that have, been proposed to explain how these particles, are arranged within the atom., , A major challenge before the scientists at, the end of the 19th century was to reveal the, structure of the atom as well as to explain its, important properties. The elucidation of the, structure of atoms is based on a series of, experiments., , One of the first indications that atoms are, not indivisible, comes from studying static, electricity and the condition under which, electricity is conducted by different, substances., , 4.1 Charged Particles in Matter, , For understanding the nature of charged, particles in matter, let us carry out the, following activities:, , Activity, , A, , 4.1, , Comb dry hair. Does the comb then, attract small picces of paper?, , Rub a glass rod with a silk cloth and, bring the rod near an inflated balloon., Observe what happens., , B., , , , escent, , STRUCTURE OF THE ATOM, , From these activities, can we conclude, that on rubbing two objects together, they, become electrically charged? Where does this, charge come from’? This question can be, answered by knowing that an atom is divisible, and consists of charged particles,, , Many scientists contributed in revealing, the presence of charged particles in an atom,, , It was known by 1900 that the atom was, indivisible particle but contained at least one, sub-atomic particle - the electron identified by, J.J. Thomson. Even before the electron was, identified, E. Goldstein in 1886 discovered the, presence of new radiations in a gas discharge, and called them canal rays. These rays were, positively charged radiations which ultimately, led to the discovery of another sub-atomic, particle. This sub-atomic particle had a charge,, equal in magnitude but opposite in sign to that, of the electron. Its mass was approximately, 2000 times as that of the electron. It was given, the name of proton. In general, an electron is, represented as ‘e” and a proton as ‘p". The, mass of a proton is taken as one unit and its, charge as plus one. The mass of an electron is, considered to be negligible and its charge is, minus one., , It seemed that an atom was composed of, protons and electrons, mutually balancing, their charges. It also appeared that the protons, were in the Interior of the atom, for whereas, electrons could easily be removed off but, not protons. Now the big question was:, what sort of structure did these particles of, the atom form? We will find the answer to, this question below., , 2021-22
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uestions, , 1, What are canal rays?, 2. If an atom contains one electron, , and one proton, will it carry any, charge or not?, , 4.2 The Structure of an Atom, , We have learnt Dalton’s atomic theory in, Chapter 3, which suggested that the atom, was indivisible and indestructible. But the, discovery of two fundamental particles, {electrons and protons) inside the atom, led, to the failure of this aspect of Dalton’s atomic, theory. It was then considered necessary to, know how electrons and protons are arranged, 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,, were like currants (dry frutts) in a spherical, Christmas pudding. We can also think of a, watermelon, the positive charge in the atom, is spread all over like the red edible part of, the watermelon, while the electrons are, , studded in the positively charged sphere, like, the seeds in the watermelon (Fig. 4.1)., , Positive «phere, , , , Fig.4.1: Thomson's model of an atom, , Stauctune or mx Aros, , 2021-22, , , , Thomson proposed that:, (i) An atom consists of a positively, charged sphere and the electrons are, embedded tn tt., , (ii) The negative and positive charges are, equal in magnitude, So, the atom as a, whole is electrically neutral., , Although Thomson's model explained that, atoms are electrically neutral, the results of, experiments carried out by other scientists, could not be explained by this model, as we, will see below,, , 4.2.2 RUTHERFORD’S MODEL OF AN ATOM, , Ernest Rutherford was interested in knowing, , how the electrons are arranged within an, , atom. Rutherford designed an experiment for, this. In this experiment, fast moving alpha, , (a)-particles were made to fall on a thin, , gold foil., , * He selected a gold foil because he wanted, as thin a layer as possible. This gold foil, was about 1000 atoms thick., , * q-particles are doubly-charged helium, ions. Since they have a mass of 4 u, the, fast-moving a-particles have a, considerable amount of energy., , * Itwas expected that a-particles would be, deflected by the sub-atomic particles in, the gold atoms. Since the a-particles were, much heavier than the protons, he did, not expect to see large deflections,, , 47
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Gold aroma, , u-pariictes, , , , Pig. 4.2: Scattering of a-particles by a gold fotl, , But, the a@-particle scattering experiment, gave totally unexpected results (Fig. 4.2). The, following observations were made:, , fi) Most of the fast moving a-particles, passed straight through the gold foil., (ii) Some of the a-particles were deflected, by the foil by small angles., (iii) Surprisingly one out of every 12000, particles appeared to rebound,, , In the words of Rutherford, “This result, was almost as incredible as (f you fire a, 15-inch shell at a piece of tissue paper and it, comes back and hits you., , , , Let us think of an activity in an open field, to understand the implications of this, experiment. Let a child stand in front of a, wall with his eyes closed. Let him throw, stones at the wall from a distance. He will, , hear a sound when each stone strikes the wall., If he repeats this ten times, he will hear the, sound ten times. But if a blind-folded child, were to throw stones at a barbed-wire fence,, most of the stones would not hit the fencing, and no sound would be heard. This is because, there are lots of gaps in the fence which allow, the stone to pass through them., , Following a similar reasoning, Rutherford, concluded from the a-particle scattering, experiment that, ()} Most of the space inside the atom ts, empty because most of the a-particles, passed through the gold foil without, getting deflected,, , (tt) Very few particles were deflected from, their path, indicating that the positive, charge of the atom occupies very little, space., , (iii) A very small fraction of a-particles, were deflected by 180°, indicating that, all the positive charge and mass of the, gold atom were concentrated in a very, small volume within the atom., , From the data he also calculated that the, radius of the nucleus is about 10° times less, than the radius of the atom., , On the basis of his experiment., Rutherford put forward the nuclear model of, an atom, which had the following features:, , @) There is a positively charged centre in, an atom called the nucleus. Nearly all, the mass of an atom resides in the, nucleus,, , (t) The electrons revolve around the, nucleus in circular paths., , (iii) The size of the nucleus is very small, as compared to the size of the atom., , Drawbacks of Rutherford's model of, the atom, , ‘The revolution of the electron in a circular orbit, is not expected to be stable. Any particle in a, circular orbit would undergo acceleration., During acceleration, charged particles would, radiate energy. Thus, the revolving electron, would lose energy and finally fall into the, nucleus. If this were so, the atom should be, highly unstable and hence matter would not, exist in the form that we know. We know that, atoms are quite stable., , 2021-22
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4.2.3 BOHR’s MODEL OF ATOM, , In order to overcome the objections raised, against Rutherford’s model of the atom,, Neils Bohr put forward the following, postulates about the model of an atom:, , (i) Only certain special orbits known as, discrete orbits of electrons, are allowed, inside the atom., , (ii) While revolving in discrete orbits the, electrons do not. radiate energy., , , , ‘These orbits or shells are called energy, levels. Energy levels in an atam are shawn in, Fig. 4.3., , N shel (r-4), M shell (n<7), Lshell (12), K shell (r-1), , Nucleas, , Fig. 4.3: A few energy levels in an atam, These orbits or shells are represented by, the letters K,L.M.N.... or the numbers,, n=1,2.3,4....., , Sravcrore or me Aros, , 2021-22, , uestions, , 1. Onthe basis of Thorison’s model, of an atom, explain how the atom., is neutral as a whole,, , 2. On the basis af Ruftherford's, model of an atom, which subatomic particle is present in the, nucleus of an atom?, , 3. Draw a skeich of Bohr’s model, of an atom with three shells., , 4. What do you think would be the, observation tf the a-particle, scattering experiment ts carried, out using a foil of a metal other, than gold?, , 4.2.4 NEUTRONS, , In 1932, J. Chadwick discovered another subatomic particle which had no charge and a, mass nearly equal de that ofa proton, Twas, eventually named as neutron, Neutrons are, present in the nucleus of all atoms, except, hydrogen. In general, a neutron is, represented as ‘n’. The mass of an atom is, theretore given by the sum of the masses of, protons and neutrons present in the nacleus., , uestions, 1. Name the three sub-atomic, particies of an atom., , 2. Heliumatom has an atomic mass, of 4 and twa protons in its, nucleus. How many meutrons, does it have?, , 4.3 How are Electrons Distributed, in Different Orbits (Shells)?, , ‘The distribution of electrons into different, orbits ol an alom was suggested by Bohr and, Bury., , The following rules are followed for writing, the number of clectrons in different encrgy, levels or shells:, , (@) The maximum mumber of electrons, , present in a shell is given by the, , 49
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formula 2n*, where ‘n’ is the orbit, number or energy level index, 1,2,3,...., Hence the maximum number of, electrons tn different shells are as, follows:, first orbit or K-shell will be = 2x 1? = 2., second orbit or L-shell will be = 2x 2#=8,, third orbit or M-shell will be = 2 x 3*= 18,, fourth orbit or N-shell will be = 2 « 4?, = 32, and so on., ‘The maximum number of electrons that, can be accommodated in the outermost, orbit is 8., Electrons are not accommodated ina, given shell, unless the inner shells are, filled. That is, the shells are filled in a, step-wise manner., , Atomic structure of the first eighteen, elements is shown schematically in Fig. 4.4., , ©, , H, , 20oe, , (ii), , * The composition of atoms of the first, , eighteen clements is given in Table 4.1., , uestions, 1, Write the distribution of electrons, in carbon and sodium atoms,, 2. ifK and L shells of an atom are, Jull, then what would be the total, number of electroms in the atom?, , 4.4 Valency, , We have learnt how the electrons in an atom, are arranged in different shells/orbits, The, electrons present in the outermost shell of, an atom are known as the valence electrons., , From the Bohr-Bury scheme, we also know, that the outermost shell of an atom can, , ©, , He, , @O6@, , 99000000, , Fig.4.4: Schematic atomic structure of the first eighteen elements, , Activity —__ 4.2, , * Make a static atomic model displaying, electronic configuration of the first, , tighteen elements, , accommodate a maximum of 8 electrons. It, was observed that the atoms of elements,, completely filled with 8 electrons tn the, outermost shell show little chemical activity., In other words, their combining capacity or, valency is zero, Of these inert elements, the, , Seavcxe, , 2021-22
