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ATOMIC STRUCTURE Notes - National Institute of Open …

3 5 MODULE - 2 ATOMIC StructureNotesAtomic STRUCTURE andChemical Bondinghemistry has been defined as the study of matter in terms of its STRUCTURE , compositionand the properties. As you are aware, matter is made up of atoms, and therefore anunderstanding of the STRUCTURE of atom is very important. You have studied in your earlierclasses that the earliest concept of atom ( smallest indivisible part of matter ) was givenby ancient (600-400 BC) Indian and Greek philosophers. At that time there were noexperimental evidence. The origin of the concept of atom was based on their thoughts on What would happen if we continuously keep dividing matter.

The model proposed by Rutherford explained the observation in the -ray scattering experiments as shown below in Fig 3.4. Fig 3.4 : Explanation of the results of α- ray Fig. 3.5 : Failure of Rutherford’s scattering experiment. model However, there was a problem with the Rutherford’s model. According to the Maxwell’s

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Transcription of ATOMIC STRUCTURE Notes - National Institute of Open …

1 3 5 MODULE - 2 ATOMIC StructureNotesAtomic STRUCTURE andChemical Bondinghemistry has been defined as the study of matter in terms of its STRUCTURE , compositionand the properties. As you are aware, matter is made up of atoms, and therefore anunderstanding of the STRUCTURE of atom is very important. You have studied in your earlierclasses that the earliest concept of atom ( smallest indivisible part of matter ) was givenby ancient (600-400 BC) Indian and Greek philosophers. At that time there were noexperimental evidence. The origin of the concept of atom was based on their thoughts on What would happen if we continuously keep dividing matter.

2 John Dalton revived theconcept of atom in the beginning of nineteenth century in terms of hisatomic theorywhich successfully explained the laws of chemical combination. Later experiments showedthat the atom is not indivisible but has an internal this lesson you will learn about the internal STRUCTURE of an atom which will help you tounderstand the correlations between its STRUCTURE and properties. You would learn aboutthese in the later reading this lesson you will be able to : recognize the fundamental particles of atom; describe rutherford s experiment and explain its results; define electromagnetic radiation; list and define the characteristic parameters of electromagnetic radiation; discuss line spectrum of hydrogen; explain Bohr s postulates and discuss his model; draw energy level diagram of hydrogen atom showing different series of lines in itsspectrum; explain wave particle duality of matter and radiation; formulate Heisenberg s uncertainty principle.

3 3 ATOMIC STRUCTUREC3 6 MODULE - 2 ChemistryNotesAtomic STRUCTURE andChemical Bonding explain the need for quantum mechanical model; draw probability pictures of an electron in an atom; list quantum numbers and discuss their significance; draw the shapes of s, p and d orbitals; recognize nodal plane; explain Pauli s exclusion principle; define Aufbau principle and explain Hund s rule of maximum Particles ofAtomIn 1897 Thomson discoveredelectron as a constituent of atom. He determined thatan electron had a negative charge and had very little mass as compared to that of theatom.

4 Since an atom was found to be electrically neutral it was inferred that some sourceof positive charge must be present in the atom. This soon led to the experimental discoveryof the proton, which is a positively charged subatomic particle. Proton was foundapproximately 1840 times heavier than an electron. Further experiments revealed thatthe ATOMIC masses were more than that expected from the presence of just protons andelectrons in the atom. For example, the mass of helium atom was expected to be doublethat of hydrogen atom but was actually found to be almost four times the mass of hydrogenatom.

5 This suggested the presence of neutral particles with mass comparable to that ofprotons in the atom. Sir James Chadwick discovered this neutral particle and calleditneutron subsequently in 1932. Thus we may conclude that atoms are not indivisible butare made up of three fundamental particles whose characteristics are given in Table Fundamental particles of atom and their characteristicsParticleSymbolMass/ kgActual Charge / CRelative 389 10 31 177 10 19 623 10 177 10 19+ 928 10 2700 Since atoms are made up of still smaller particles, they must have an internal STRUCTURE .

6 Inthe next section we shall take up some of the earlier ideas about the internal STRUCTURE Question the mass of an electron with that of the is a fundamental particle?..3 7 MODULE - 2 ATOMIC StructureNotesAtomic STRUCTURE andChemical is the name given to neutral particles in the atom?.. ModelsOnce it was established that the atom is not indivisible, the scientists made attempts tounderstand the STRUCTURE of the atom. A number of models have been proposed for theinternal STRUCTURE of the atom. The first attempt to describe the STRUCTURE of atom in termsof a model was made by Thomson s ModelOn the basis of his experiments on discharge tubes, Thomson proposed that atoms can beconsidered as a large positively charged body with a number of small negatively chargedelectrons scattered throughout it.

7 This model ( ) was called as plum pudding modelof the :A pictorial representation of Thomson s plum-pudding model The electrons represent the plums in the pudding made of positive charge. It is sometimesalso called aswatermelonmodel. In this, the juicy pulp of the watermelon represents thepositive charge and the seeds represent the rutherford (1856-1940)(1871-1937)Won Nobel prize in Physics in 1906 Won Nobel prize in Chemistry in s ExperimentErnest rutherford performed an experiment called Gold Foil Experiment or - rayscattering experiment to test the STRUCTURE of an atom as proposed by Thomson.

8 In thisexperiment a beam of fast moving alpha particles (positively charged helium ions) waspassed through a very thin foil of gold. He expected that the alpha particles would just passstraight through the gold foil and could be detected by a photographic plate. But, the actualresults of the experiment (Fig. ) were quite surprising. It was observed that most of the -particles did pass straight through the foil but a number of particles were deflectedfrom their path. Some of these deflected slightly while a few deflected through largeangles and about1 in 10,000 - particles suffered a rebound3 8 MODULE - 2 ChemistryNotesAtomic STRUCTURE andChemical BondingFig :Schematic representation of rutherford sFig : Schematic representation -ray scattering experiment.

9 Of rutherford s modelThese results led rutherford to conclude that : the atom contained some dense and positively charged region located at the centerof the atom that he called asnucleus. all the positive charge of the atom and most of its mass was contained in the nucleus. the rest of the atom must be empty space which contains the much smaller andnegatively charged electrons (Fig. ).The model proposed by rutherford explained the observation in the -ray scatteringexperiments as shown below in Fig :Explanation of the results of - rayFig. :Failure of rutherford s scattering , there wasa problem with the rutherford s model.

10 According to the Maxwell stheory of electromagnetic radiation, a charged particle undergoing acceleration wouldcontinuously emit radiation and lose energy. Since the electron in the atom is also a chargedparticle and is under acceleration, it is expected to continuously lose energy. As aconsequence, the electron moving around the nucleus would approach the nucleus by aspiral path (Fig. ) and the atom would collapse. However, since it does not happen wecan say that the rutherford s model failed to explain the stability of the next attempt to suggest a model for atom was made by Neils Bohr- a student ofRutherford.


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