Tiêu đề 2. STRUCTURE OF ATOM.pdf ✅

Chapter - 3 ATOMIC STRUCTURE 3.1 Introduction Matter is constituted by very small particles which can not exist in free state in most of the cases called atoms. These atoms are constituted further by some fundamental particles called electrons; protons and neutrons. Present chapter deals with the structure of atom in terms of these fundamental particles and influence of electronic arrangement on the properties of the molecules. Nature of cathode rays (i) They travel in straight lines away from the cathode with very high velocities ranging from 107 – 109 cm per second. (ii) They produce a green glow when strike the wall beyond anode. (iii) They produce heat energy when they collide with the matter. It shows that cathode rays possess kinetic energy. (iv) They are deflected by the electric and magnetic fields. When the rays are passed between two electrically charged plates, these are deflected towards the positively charged plate. (v) When a pin wheel is placed in their path, the blades of the wheel are set in motion this indicates cathode rays consists of material particles which have mass and velocity. (vi) When they fall on material having high atomic mass, new type of penetrating rays of very small wavelengths are emitted which are called X-rays. (vii)They affect the photographic plate. (viii) They can penetrate through thin foils of solid materials and cause ionisation in gases through which they pass. (ix) The nature of the cathode rays is independent of: (a) The nature of the cathode and. (b) The gas in the discharge tube. (x) The negatively charged particles which constitute cathode rays are called electrons. Rest mass of electron: The rest mass of electron is found to be 9.1096 × 10–31 kg. Mass of moving electron : The mass of moving electron is given by the relation. Mass of moving electron = 2 c v 1 rest mass of electron        Where v is the velocity of the electron and c is the velocity of light. When v becomes equal to c mass of moving electron becomes infinity.
Atomic Structure Charge on electron : The charge on electron is found to be –1.6022 × 10–19 coulombs. Since the electron has the smallest charge known, it is designated as unit negative charge. e/m of electron : Charge to mass ratio of the electron is found to be –1.7588 × 108 C per g. Nature of Anode Rays (i) These rays travel in straight lines and cast shadow of the object placed in their path. (ii) Like cathode rays, they also rotate the pin wheel placed in their path and also have heating effect. Thus, the rays possess kinetic energy. (iii) The rays produce fluorescence on zinc sulphide screen. (iv) The rays are deflected by electric and magnetic field in the direction opposite to that of cathode rays. These rays are attracted towards the negatively charged plate showing that they carry positive charge. (v) They can pass through thin metal foils. (vi) They can produce ionisation in gases. (vii)They are capable of producing physical and chemical changes. (viii) Positive particles in these rays have e/m values much smaller than that of the electron. (ix) e/m value is dependent on the nature of the gas taken in the discharge tube. Particles of highest e/m are obtained when hydrogen is taken in the discharge take. These particles of highest e/m are called proton. Mass of proton: Mass of proton is found to be 1.672 × 10–27 kg or 1.0072 amu. (amu = atomic mass unit) Charge on proton : Charge on proton is same as charge on electron (i.e., +1.6022 × 10–19 coulombs) in magnitude but opposite in sign. e/m of proton: Charge to mass ratio of proton is found to be +9.579 × 104 coulombs per g. Which is very small as compared to electron. Rutherford’s -scattering experiment Rutherford carried out a series of experiments using -particles. A beam of -particles was directed against a thin foil of gold, platinum, silver or copper. The foil was surrounded by a circular fluorescent zinc sulphide screen. Whenever an -particle struck the screen, it produced a flash of light.
Atomic Structure The following observations were made: (i) Most of the -particles went straight without any deflection. (ii) A few of them got deflected through small angles. (iii) A very few of them (about one in 20,000) returned back towards its source. Following conclusions were drawn from the above observations: (i) Since most of the particles went straight through the metal foil undeflected, it indicates most of space of an atom is empty. (ii) A few of the -particles were deflected from their original paths through obtuse angles, it was concluded that whole of the positive charge is concentrated and the space called nucleus. It is supposed to be present in the centre of the atom. (iii) A very few of the -particles suffered strong deflections or even rebound on their path due to maximum repulsion and minimum Impact parameter. Mosley’s Experiment and the concept of Atomic Number : Moseley studied the x-ray spectra of 38 different elements, starting from aluminium to gold. He measured the frequency of principal lines of a particular series (the -lines in the k-series) of the spectra. He observed that the frequency of the particular spectral line was related with the serial number of the element in the periodic table which he termed as atomic number (Z). He suggested the following relationship. v  a(Z  b) 20 15 10 5 20 40 60 80 100 Relative Atomic Mass (A) (a) v/10 s 8 –1 20 15 10 5 10 20 30 40 Atomic Number (Z) (b) v/10 s 8 –1 Where v = frequency of X-rays, Z = atomic number, a and b are constants. When the values of square root of the frequency were plotted against atomic numbers of the elements producing X-rays, a straight line was obtained. Chadwick’s Experiment and discovery of Neutron : Chadwick bombarded beryllium with a stream of -particles. He observed that penetrating radiations were produced which were not affected by electric and magnetic fields. These radiations consisted neutral particles, which were called neutrons. neutron 1 0 carbon 12 6 α-particle 4 2 beryllium 9 4Be  He  C  n