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Drawing Lewis Structures

Drawing Lewis Structures. AP Chemistry Bonding Notes 3. Add up all the valence electrons on all atoms For an anions, add electrons equal to the negative charge For cation , subtract the number of electrons equal to the negative charge Identify the central atom

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Drawing Lewis Structures

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  1. Drawing Lewis Structures AP Chemistry Bonding Notes 3

  2. Add up all the valence electrons on all atoms • For an anions, add electrons equal to the negative charge • For cation, subtract the number of electrons equal to the negative charge • Identify the central atom • When the central atom has other atoms bound to it, the central atom is usually written first. • Place the central atom in the center of the molecule and add the other atoms around it • Place one bond between each pair of atoms • Complete the octet for all atoms except for the central atom • Put the remaining electrons on the central atom. • Use multiple bonds if necessary. Guidelines

  3. It is possible to draw more than one Lewis structure with the octet rule obeyed for all the atoms. • To determine which structure is most reasonable, we use formal charge. • The formal charge of an atoms is the charge that an atom ( in a molecule) would have if all the atoms had the same electronegativity Formal Charge

  4. Assign all nonbonding (unshared) electrons to the atom on which they are found Assign half of the bonding electrons to each atom in the bond The formal charge is the number of valence in the isolated atom minus the number of electrons assigned to the atom in the Lewis structure Calculating Formal Charge

  5. Consider CN- (cyanide ion) For Carbon: There are four valence electrons ( from the periodic table In the Lewis Structure there are two non-bonding electrons and three from the triple covalent bond There are five electrons from the Lewis structure Formal Charge: 4-5=-1 Calculating Formal Charge

  6. Consider CN- (cyanide ion) For Nitrogen: There are five valence electrons In the Lewis Structure there are two nonbonding electrons and three from the triple bond There are five electrons in the Lewis Structure Formal Charge= 5-5=0 Calculating Formal Charge

  7. Using formal charge calculations to distinguish between alternative Lewis structures The most stable structure has the lowest formal charge on each atom The most negative formal charge on the most electronegative atom NOTE: Formal charges do not represent REAL charges on Atoms Calculating Formal Charge

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