Tiêu đề 2_Atomic structure.docx ✅

PRE-MEDICAL 28 PHYSICAL CHEMISTRY Serial No. MODULE-1 Page No. 1. Some basic concepts of Chemistry 2. Atomic structure 3. Chemical Equilibrium 4. Ionic Equilibrium 5. Thermodynamics & Chemical Energetics 6. Redox reactions 7. Behaviour of Gases 8. Hints and Solutions
PRE-MEDICAL 28 ATOMIC STRUCTURE S.No. CONTENTS Page No. 1. Introduction 2. Atomic models 3. Atomic number, mass number and important definitions 4. Electromagnetic waves 5. Planck’s quantum theory 6. Bohr’s atomic model 7. Spectrum 8. De-Broglie concept 9. Heisenberg uncertainity principle 10. Quantum numbers 11. Rules for filling of electrons 12. Exercise-I (Conceptual Questions) 13. Exercise-II (Previous Years Questions) 14. Exercise-III (Analytical Questions) 15. Exercise-IV (Assertion & Reason)
PRE-MEDICAL 28 ATOMIC STRUCTURE 2.0 INTRODUCTION Atom is a Greek work and its meaning Indivisible i.e. and ultimate particles which cannot be further subdivided. John Dalton considered that" all matter was composed of smallest indivisible particle called atom Daltons Atomic Theory : This theory is based on law of mass conservation and law of definite proportions., the salient feature’s of this theory are :- (1) Each element is composed of extremely small particles called atoms. (2) Atoms of a particular element are like but differ from atom’s of other element. (3) Atom of each element is an ultimate particle and it has a characteristic mass but is structureless (4) Atom’s are indestructible i.e. they can neither be created nor be destroyed. (5) Atom of element’s take part in chemical reaction to form molecule. GOLDEN KEY POINTS  Particles carrying negative charge were called negatrons by Thomson. The name negtron was changed to ‘electron’ by Stoney. In cathode ray experiment particles (electron) forming the rays hve same specific charge (e/,) which is independent of the nature of gas and electrode used. It points out that electrons are present in all atoms.  Mass of electron is 1 1837 times that of proton.  mass of moving e – = 2 restmassofe 1(vc)   (Where v is the velocity of the e – and c is the velocity of light.)  In anode ray experiment the particles forming rays have e/m value, dependent on the nature of ht gas taken in the discharge tube, i.e. +ve particles are different in different gases. Therefore, the mass of the proton can be calculated.  Mass of the proton = e e/m = 19 4 1.60210 9.57910    = 1.672×10 –24 g = 1.672×10 –27 kg Mass of proton in amu = 24 24 1.67210 1.6610     = 1.0072 amu. 2.1 ATOMIC MODELS (A) Thomson’s Model of Atom [1904]  Thomson was the first to propose a detailed model of the atom.  Thomson proposed that an atom consists of a uniform sphere of positive charge in which the electrons are distributed more or less uniformly.  This model of atom is known as “ Plum-Pudding model” or “Raisin Pudding Model” or “Water Melon Model”.
PRE-MEDICAL 28 Sphere of +ve change e – Drawbacks :  An important drawback of this model is that the mass of the atoms is considered to be evenly spread over that atom.  It is a static model. It does not reflect the movement of electron.  If couldn’t explain the stability of an atom. (B) Rutherford’s Scattering Experiment thin gold foil (0.00004 cm) Slit system (lead plate) ¼Ray½ Source (Ra) of & Rays = [ 2 He 4 ] +2 [doubly ionised He particle] Most of &particles strike here ZnS screen Circular Fluorescent screen Rutherford observed that- (i) Most of the - particles (nearly 99.9%) went straight without suffering any deflection. (ii) A few of them got deflected through small angles. (iii) A very few (about one in 20,000) did not pass through the foil at all but suffered large deflections (more than 90°) or even came back in the direction from which they have come i.e. a deflection of 180°. Following conclusions were drawn from the above observation- (i) Since most of the -particle went straight through the metal foil undeflected. It means that there must be very large empty space within the atom. (ii) Since few of ht -particles were deflected from their original paths through moderate angles; it was concluded that whole of the +ve charge is concentrated and the space occupied by this positive charge is very small in the atom.  When -particles come closer to this point, they suffer a force of repulsion an d deviate from their paths.  The positively charged heavy mass which occupies only a small volume in an atom is called nucleus. It is supposed to be present at the centre of the atom.