Chemistry Atomic Structure and Radioactivity Part-2
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Cathode rays (Discovery of electron)

`=>` During the time when atomic theory was put forward, the factor that carries electric current was still unidentified.

`=>` Attempts were done to determine what cathode rays were.

`=>` Many scientists began to study electrical discharge in evacuated discharge tube.

`=>` When a high voltage of 1 0,000 volts or more was impressed across the electrodes, some sort of invisible rays moved from the negative electrodes to the positive electrode. Since the negative electrode is referred to as cathode. These rays were called as cathode
rays.

`=>` In simple words, cathode rays are nothing but the beam of electrons. These are also called as electron beam or `e^(-)` beam.

Properties of Cathode Rays

`=>` Cathode rays always travel in straight line path away from the cathode and cast shadows of metallic objects placed in their path.( this is due to impact of the particles of cathode rays on the blades of paddle wheel. Cathode rays particle strike the blades of the paddle wheel and set into the motion).
`=>` Possess kinetic energy and must be material particles.
`=>` Deflected by an electric and as well as magnetic field.
`=>` Deflected towards the positively charged plate, hence the particles in the cathode rays carry negative charge.
`=>` Penetrate through thin layers of matter.
`=>` They ionize the gas through which they pass.
`=>` Cathode rays upon striking glass or certain other material cause them to glow.
`=>` Produce X-rays when they strike a metallic target with high melting point like tungsten.
`=>` The nature of cathode rays is independent of (i) the nature of the cathode and (ii) the gas in the discharge tube.
`=>` They have very high velocity ranging from `10^7` to `10^9` m/sec.
`=>` They cause green fluoroescence on glass surface.












Charge/Mass Ratio [e/m]

J.J. Thomson determined the ratio of the charge (`e^-`) of the electron to its mass (`m`).

`=>` It was discovered by J.J. Thomson in 1897.

`=>` The value of e/m was found to `-1.7588xx10^(11) C kg^(-1)` for electrons.

`=>` The charge on the electron was measured by R.A. Millikan.

`=>` When an electric charge at high voltage is passed through a gas at low pressure, (`10^(-6)` atm) a stream of rays is emitted from the cathode surface. These are called as cathode rays.

`=>` Magnitude of charge on electron `= -1.6xx10^(-19) C`.

`=>` Mass of electron ` = 9.108 xx 10^(-28) g`

 
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