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| Summary |
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- When certain metals are heated to a high temperature, they emit thermions (electrons) and the phenomenon is called thermionic
emission
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- Cathode rays are a stream of negatively charged particles emitted from the cathode of an evacuated bulb, when a voltage of nearly 3000 volts given across the terminals.
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- Certain heavy elements when they become unstable, they disintegrate giving out certain radiations -
 particles and  radiations.
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- All living things contain carbon and the amount of carbon 14 in them is enough to make them slightly radioactive.
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- Exposure to large amounts of radiation is harmful to health.
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- The nucleus of a radioactive substance spontaneously decays, giving off various kinds of radiations and getting transformed into the nucleus of another element.
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- Uranium and radium are two important naturally radioactive substances.
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- When a nucleus emits radiation it is said to decay.
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- Rays from radioactive isotopes are dangerous to living things but are also useful in variety of ways.
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- Exposure to large amounts of radiation is harmful to health.
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- All radioactive materials should always be handled with care and kept inside lead boxes.
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- An alpha decay causes the mass number to decrease by 4 and atomic number by 2.
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- Beta decay causes no change in the mass number, but the atomic number increases by 1.
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- Gamma emission has no effect on either the mass or atomic number.
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- Nuclear fission is the splitting of a heavy atom such as uranium into two lighter parts, accompanied with release of energy.
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| Thermionic Emission, Cathode Rays, Radioactivity, Alpha
Particles, Beta Particles, Gamma Rays, Carbon Dating, Background Radiation. |
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