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Atomic Structure

The document discusses Dalton's Atomic Theory, which posits that all matter is made of indivisible particles called atoms, and outlines its postulates and drawbacks. It details the discovery of subatomic particles, including electrons, protons, and neutrons, and describes various experiments that led to these discoveries. Additionally, it covers Thomson's and Rutherford's atomic models, highlighting their contributions and limitations in explaining atomic structure.

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24 views50 pages

Atomic Structure

The document discusses Dalton's Atomic Theory, which posits that all matter is made of indivisible particles called atoms, and outlines its postulates and drawbacks. It details the discovery of subatomic particles, including electrons, protons, and neutrons, and describes various experiments that led to these discoveries. Additionally, it covers Thomson's and Rutherford's atomic models, highlighting their contributions and limitations in explaining atomic structure.

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keshwaditya121
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Atomic Structure U Chapter ys 1. DALTON’s ATOMIC THEORY Al the objects around us, this book, your pen or pencil and things of nature such as racks, water, plants ete. are all different forms of mater, Mauer is any substance which lias ‘mass and occupies space. John Dalton, in 1808, proposed that: All elements are made up of extremely small, indivisible particles called atoms. 1.1 Postulates of Dalton’s Atomic Theory © Elements are made up of sill indivisible particles called atoms which are invisible to the naked eve. Atoms can neither be created nor destroyed. This means, that a chemical react ‘atoms and the sume number of alos must be present before and after the reaction, ‘Atom is the smallest particle of an element which takes, part in a chemical reaction ‘Atoms of the same element are identical in all respects size, shape and mass. ° is simply rearrangement of Atoms of different elements have different mass, shape and size, 1.2. Drawbacks of Dalton’s Atomic Theory ‘The discovery of isotopes and isobars showed that ‘atom of same element may have different atomic mass (Gsotopes) and atom of different kinds may have same atomic masses (isobars). ‘Atoms can be split into more fundamental particles electrons, protons and neutrons. DISCOVERY OF SUB-ATOMIC PARTICLES Dalton’s atomic theory was able to explain the law ot ‘conservation of mass, law of constant composition and law ‘of multiple proportions very successfully. However, it filed to explain the results of many experiments. Many different kinds of sub-atomic particles were discovered over the ‘twentieth century. However, in this chapter we will tak about ‘only the three fundamental particles, namely electron, proton and newton. ° 2.1 Discovery of Electron Michacl Faraday showed that chemical changos occur ‘when electricity is passed though an electrolyte. He stated that slectricity ie made up of particles called atoms of eleoticity. However, in today’s world it is known that this <0 called “atom of eletrcity” is nothing but electron. The credit forthe discovery of electrons goes to J.J. Thomson, who did an experiment involving cathode ray tubes, was able to establish that all atoms contain negatively charged particles. A cathode ray tube is made of glass containing two thin pieces of meta, called electrodes. sealed init. When a high potential difference is applied (10,000V) across the two electrodes at a very low pressure, an electrical discharge through the gases could be observed. A stream of particles is observed to move in the tube from the negative electrode (cathode) to the positive electrode (anode). These were called cathode rays. Cathode rays Gas atlow \__ pressure =~ Vaccum pump Cathode Anovde High voltage ot ‘The results of this experiment are summarised below. ( The cathode rays start from cathode and move towards the anode. (i) Cathode rays consist of negatively charged particles ‘hich in today’s date are called electrons (ii) Cathode rays themselves are not visible but they ccause fluorescence 1. a fiuorescent material like ZnS glows when these particles hit a surface coated ‘with ZnS, (jv) In the presence of electric or magnetic field, the cathode rays get deflected from their straight line path. 24 Chemistry (0) Cathode rays produce heating effect. When these rays are made to strike on a metal foil, the foil gets heated. (i) Cathode rays. produce X-rays when they strike on surface of hard metals such as tungsten, copper molybdenum ete (ii) Cathode rays have high penetration power and can ass through thin foils of metals ike aluminum. (ii) Cathode rays ionize the gas through which they pass. | (© Thomson performed a series of experiments in which he was able to determine the charge/mass (2) ratio ofthe pactile that malo up the othod rye" (©. The vale ofthe charge to mass ratio for eathode rays (GE) was fund be 176 10! Colon perk (©. Thomson performed the came experiments using. different metals for the cathode and anode, as well as different vases inside the tube. The e/m ratio for cathode rays was found to be same irrespective of the nature of ‘cathode and nature of gas taken in discharge tube. This shows that the electrons are universal constituent of all at 2.2 Quantization of Charge and Mass of Electron 2.2.4 Charge on an Electron “Milliken devised a method known ao oil drop exporimont to determine the charge on the electrons. Mulliken estimated the value af charge on the oil droplets and observed that the ‘charge on any cil droplet is always a multiple of the same value (Suppose. a). ‘Thus, he concluded that the fundamental value of the ‘magnitude of electrical charge is q as the charge on any oi droplet is a multiple ofthis “q He estimated the value of “q° as —1.6 =10 " € which Is the charge present on a single electron 2.2.2 Mass of an Electron With the ($) ratio and the charge on an electron known, it is possible to calculate the mass (m) of the electron as follows, e_ (60x10 "CO em ~ 76x10) 10x10 *g 2.3 Discovery of Protons (Goldstein) We know that an atom is electrically neutral. If it ‘contains negatively charged elecitons it must also contain some positively charged particles. This was confirmed by Goldstein in his discharge tube experiment with perforated cathode. Un passing nigh voltage between the electrodes ot Mass uf election (4) 1 1UX10 kg 4 discharge tube it was found that some rays were coming from the side of the anode which passed through the holes in the cathode. These anode rays (canal rays) consisted of positively charged particles formed by ionization of gas patticles by dhe catiode rays. The charge 0 us rativ (e/a value) of these positively charge particles was found to be ‘maximum when the discharge tube was filled with hydrogen ‘gas as it is the lightest gas.

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