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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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Chain Termination Step

The combination of two radicals, which removes the reactive species that propagate the change reaction.

PseudobinaryIonic Compounds

Compounds that contain more than two elements but are named like binary compounds.


A two-armed barometer.

Standard Entropy

The absolute entropy of a substance in its standard state at 298 K.

Hund's Rule

All orbitals of a given sublevel must be occupied by single electrons before pairing begins.


The absorption of heat by a system as the process occurs.

First Law of Thermodynamics

The total amount of energy in the universe is constant (also known as the Law of Conservation of Energy) energy is neither created nor destroyed in ordinary chemical reactions and physical changes.

Covalent Bond

Chemical bond formed by the sharing of one or more electron pairs between two atoms.

Weak Field Ligand

A Ligand that exerts a weak crystal or ligand field and ge- nerally forms high spin complexes with metals.

Bond Order

Half the numbers of electrons in bonding orbitals minus half the number of electrons in antibonding orbitals. Bond order gives an indication to the stability of a bond. Also defined as the difference between the number of bonding electrons and antibonding electrons divided by two.