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Discovered : by Sir William Ramsay in London, and independently by P.T. Cleve and N.A. Langlet in Uppsala, Sweden in 1895.
Origin : The name is derived from the Greek ‘helios’,sun.
Description :A colourless, odourless gas that is totally unreactive. It is extracted from natural gas wells, some of which contain gas that is 7% helium. It is used in deep sea diving for balloons and, as liquid helium, for low temperature research. The Earth’s atmosphere contains 5 parts per million by volume, totalling 400 million tons, but it is not worth extracting it from this source at present.
Atomic No:2 MAss No:4

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Absorption of high energy radiation by a substance and subsequent emission of visible light.

Nuclear Reactor

A system in which controlled nuclear fisson reactions generate heat energy on a large scale, which is subsequently converted into electrical energy.

Amine

Derivatives of ammonia in which one or more hydrogen atoms have been replaced by organic groups.

Specific Rate Constant

An experimentally determined (proportionality) constant, which is different for different reactions and which changes only with temperature, k in the rate-law expression: Rate = k [A] x [B]v.

Diagonal Similarities

Refers to chemical similarities in the Periodic Table of elements of Period 2 to elements of Period 3 one group to the right, especially evident toward the left of the periodic table.

End Point

The point at which an indicator changes colour and a titration is stopped.

Mass

A measure of the amount of matter in an object. Mass is usually measured in grams or kilograms.

Photoelectric Effect

Emission of an electron from the surface of a metal caused by impinging electromagnetic radiation of certain minimum energy, current increases with increasing intensity of radiation.

 

Lead Storage Battery

Secondary voltaic cell used in most automobiles.

Nuclear Reaction

Involves a change in the composition of a nucleus and can evolve or absorb an extraordinarily large amount of energy.