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CHEMICAL BONDING AND MOLECULAR STRUCTURE, , ‘The attractive force that binds the atoms together in a molecule is called a chemical bond. Its formed either by, the transfer of electrons or by the sharing of electrons., Lewis symbols, , {In the formation of a chemical bond, only the outermost electrons participate and these electrons are called, valence electrons. The inner electrons are well protected and they are called core electrons., , ‘G.N Lewis introduced simple notations to represent valence electrons in an atom. These notations are called Lewis, ‘symbols or Lewis notations. Here the nucleus and the inner electrons are represented by the symbol of the element and, the valence electrons by dots or crosses., , Eg for sodium [,,Na-2,8,1], the Lewis symbol is Na and for chlorine ,,C1-2,8,7], the Lewis symbol is CL, , Lewis symbols are informative. It gives the valency of an atom, which is either equal to the number of dots or, crosses or equal to 8 ~no. of dots or crosses., , Octet rule, , This rule was proposed by Lewis and Kossel. According to this rule, atoms undergo chemical reaction in order to, ‘attain octet of electrons inthe valence shell. Or, toms containing & electrons in theie valence shell are stable. To attain, ‘octet, they share electrons or exchange electrons., lonic or Electrovalent Bond, , ‘A bond formed by the transfer of electron between 2 atoms is called ionic or electrovolent bond. Or, itis the force, of attraction between two oppositely charged ions., , Eg formation of NaCl, , Na+, ——+ Na'cr, , 281 287, , | Na atom, there is only one electron inthe valence shel) and in i, there are 8 electronsin the valence shell. In, order to attain octet state, Na atom loses one electron and fort N=” lon and Cl atom gains one electron and form C ion., ‘These two ions attract each other by electrostatic force calle’ ionic bond., , Na +» Na’ +e, , 281 28, d+e +a, 287 288, , Nas —e Neo