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30 chemistry 2.1 Discovery of Su b-atomic Particles An insight into the structure of atom was obtained from the experiments on electrical discharge through gases. Before we discuss these results we need to keep in mind a basic rule regarding the behaviour of charged particles : “Like charges repel each other and unlike charges attract each other”. Fig. 2.1(a) A cathode ray discharge tube 2.1.1 Discovery of Electron In 1830, Michael Faraday showed that if electricity is passed through a solution of an electrolyte, chemical reactions occurred at the electrodes, which resulted in the liberation and deposition of matter at the electrodes. He formulated certain laws which you will study in Class XII. These results suggested the particulate nature of electricity. Fig. 2.1(b) A cathode ray discharge tube with In mid 1850s many scientists mainly perforated anode Faraday began to study electrical discharge in partially evacuated tubes, known as The results of these experiments are summarised below. cathode ray discharge tubes. It is depicted in Fig. 2.1. A cathode ray tube is made of (i) The cathode rays start from cathode and glass containing two thin pieces of metal, move towards the anode. (ii) These rays themselves are not visible called electrodes, sealed in it. The electrical but their behaviour can be observed discharge through the gases could be observed only at very low pressures and at with the help of certain kind of materials (fluorescentor phosphorescent) which very high voltages. The pressure of different glow when hit by them. Television gases could be adjusted by evacuation of the picture tubes are cathode ray tubes glass tubes. When sufficiently high voltage and television pictures result due to is applied across the electrodes, current fluorescence on the television screen starts flowing through a stream of particles coated with certain fluorescent or moving in the tube from the negative phosphorescent materials. (iii) In the absence of electrical or magnetic electrode (cathode) to the positive electrode (anode). These were called cathode rays or field, these rays travel in straight lines (Fig. 2.2). cathode ray particles. The flow of current (iv) In the presence of electrical or magnetic from cathode to anode was further checked by making a hole in the anode and coating field, the behaviour of cathode rays are similar to that expected from negatively the tube behind anode with phosphorescent charged particles, suggesting that material zinc sulphide. When these rays, the cathode rays consist of negatively after passing through anode, strike the zinc charged particles, called electrons. sulphide coating, a bright spot is developed (v) The characteristics of cathode rays on the coating [Fig. 2.1(b)]. (electrons) do not depend upon the Reprint 2026-27