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According to "'Rutherford-bohr" theory ,the atom is made of acentral postively charged nuceus containing positively charged particles called protons, and neutral particles called neutrons. the nucleus is surrounded by negatively charged particles called electrons.

The electrons revolve around the nucleus in fixed orbits or energy levels.while the electron moves in such a level, it possesses a definite quantity of energy and it neither emits nor absorbs energy.the electrons are arranged in the orbits so that the maximum number of electrons in the various orbits starting from the one nearest the nucleus is 2,8,18,32,18,8.the outermost orbit of electrons in different atoms (except inert gases),is incomplete and the electrons in it are known as the ""VALENCE ELECTRONS"".


The classical concept about the formation of molecule (chemical bond); is based upon the electronic structure of atoms of inert gases have either 2 or 8 electrons in the outermost orbit.the gases do not enter into chemical combination and,therefore, are assumed to have complete or stable orbits.the atoms of all other elements have incomplete outermost orbits and tend to complete them by chemical combination with other atoms.G.N.lewis proposed that it is the urge of atoms to complete their outermost orbit of electrons as in the inert gases, which is responsible for chemical  combination. in other word, chemical combination  between two atoms resulte from the  redistribution of electrons between them so that both the atom complete their outermost orbits or acquire stable electronic configuration possessed by their inert gases.

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