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Lewis Structures

Lewis Structures. visualize molecular structure. assume most atoms will have. octet of electrons. 1. Determine number of valence electrons. 2. Place least electronegative atom at center. Using e - pairs , bond central atom to surrounding atoms.

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Lewis Structures

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  1. Lewis Structures visualize molecular structure assume most atoms will have octet of electrons 1. Determine number of valence electrons 2. Place least electronegative atom at center • Using e- pairs, bond central atom to surrounding • atoms 4. Complete octets on surrounding atoms 5. Place remaining e- on central atom. Use multiple bonds to complete octet, if necessary

  2. : : : : : F F N : : : : F : nitrogen trifluoride NF3 valence e- N = 5 5 + (3 x 7) = 26 e- F = 7 central atom = N bond F to N : : : : : : : F F N 6 e- complete octets : : : : : F 24 e- surrounding atoms : 26 e- complete octet central atom include lone pairs

  3. C O bond O C O O : : : : : : : : : : O C O O C O O C O : : : : : : : : : : : : O O O : : carbonate ion CO32- 4 + (6 x 3) = 22e- valence e- C = 4 charge = 2- O = 6 24e- : : C=O bond shorter : : : : O C O 6e- stronger : : : 24e- : : O : resonance structures

  4. : . .. .. . . . . . . C N O . .. . 6 + (2 x 1) = 12e- formaldehyde 4 + CH2O 4 4 - 0 - 4 = 0 - 2 - 3 = -1 : : : : : : : which is best structure? H C H H C O H : : : : formal charge 6 - 2 - 3 = 1 O : compare e- in isolated atom to 6 - 4 - 2 = 0 e- in Lewis structure - ½ shared e- formal charge = - lone pair e- valence e- formal charge = 0 preferred

  5. O N O H O O N O H O O N O H O nitric acid HNO3 valence e- N = 5 5 + (3 x 6) + 1 = 24 e- O = 6 oxy acid O-H 1 H = : : : : : : O N O H 8 e- central atom = N : : : : : 24 e- O bond O to N : +1 0 bond H 0 complete octets -1 +1 +1 -1 surrounding atoms complete octet central atom -1

  6. : : : : : N O : Exceptions to octet rule 1. Molecules with odd number of electrons Lewis structures electron pairs unpaired e- free radical start with 12e- NO 5 + 6 = 11e- : : : : : N O : 5 - 3 - 2 = 0 5 - 4 - 2 = -1 6 - 4 - 2 = 0 6 - 3 - 2 = +1

  7. Exceptions to octet rule 2. Incomplete octets (fewer than 8 e-) B Be (Al) boron trifluoride BF3 3 + (3 x 7) = 24e- : : : : F F B : : : F : :

  8. : : : : : Cl : : Cl Cl : : : : : : Cl Cl : : Exceptions to octet rule 3. Expanded octet more than 8 e- must have d orbitals only Period 3 and above phosphorous pentachloride PCl5 5 + (5 x 7) = 40e- P

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