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form - FORMAL CHARGE IDEAS Goal Use of FORMAL CHARGE ideas...

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FORMAL CHARGE IDEAS Goal : Use of FORMAL CHARGE ideas to evaluate the RELATIVE IMPORTANCE of the contributions which DIFFERENT RESONANCE STRUCTURES make to the ACTUAL ELECTRONIC STRUCTURE OF THE MOLECULE. Procedure : 1. Write down the Lewis Electron Dot Structure 2. Divide SHARED ELECTRON PAIRS EQUALLY between the bonded atoms (i.e. between the 2 atoms sharing the electrons) 3. Compute the FORMAL CHARGE as follows: FORMAL CHARGE on atom A= No. of VALENCE electrons on A in the ISOLATED, NEUTRAL ATOM No. of VALENCE electrons on A AFTER PERFORMING STEPS 1 AND 2. Example (i): N 2 1. :N N: 2. N N 3. Isolated, neutral N atom has 5 valence electrons; Formal Charge on N = 5 - 5 = 0. Example (ii): SO 3 Note: Sum of Formal Charges = OVERALL (OR NET) CHARGE on the molecule or ion Consider a + 1 and a - 1 formal charge. From Coulomb's Law (P.E. = e 2 /( α r)) and we know that such a pair of unlike charges will have their lowest P.E. when r = o. Thus, SEPARATING such a pair of charges requires WORK (i.e. REQUIRES AN INVESTMENT OF ENERGY).
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