How to write lewis structure

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How to write lewis structure

  1. 1. Writing Lewis Structures
  2. 2. Rules for writing structures(1) you must know the order in which theatoms are connectedThis is normally determined by experimentand is referred to as the constitution of amoleculeGeneral rule: the atom with the lowest electronaffinity is usually the central atom
  3. 3. Levels of StructureElemental Composition Empirical Formula Molecular Formula Constitution Configuration Conformation
  4. 4. Rules for writing structures (1) you must know the order in which the atoms are connected This is normally determined by experiment and is referred to as the constitution of a moleculeExample: hypochlorous acid has themolecular formula HClO. But the atoms areconnected in the order of HOCl
  5. 5. Rules for writing structures (2) Count the number of valence electrons For main group elements this is the same as the group number in the periodic table Example: hypochlorous acid : HOCl H 1 electron O 6 electron Cl 7 electron total 14 valence electrons
  6. 6. Rules for writing structures (3)write out the constitution in a form that shows the covalent bonds and count the number of electrons in covalent bonds Example: hypochlorous acid H O Cl 4 electrons in covalent bonds 14 valence electrons 10 electrons remain to be assigned
  7. 7. Rules for writing structures (4)assign remaining electrons so as to complete the octets of as many atoms as possible. Example: hypochlorous acid HOCl (4 electrons in covalent bonds + 10 more electrons assigned as shown) : : H O Cl : : :
  8. 8. Rules for writing structures (5) when the number of electrons is insufficient to complete the octetes of all of the atoms, assign them to atoms in order of decreasing atom electronegativity.Example: nitrous acid HNO2 ( HONO ) : : H O N O: : : :18 valence electrons Need to assign 12 electons in addition to 6 found in three bonds
  9. 9. Rules for writing structures (6) use unshared pairs for double bonds if this will satisfy octet rule. Example: nitrous acid HONO : : : H O N O: H O N O : : : : : : :
  10. 10. Another example Example: hyponitrous acid HONNOH 24 valence electrons H O N N O H : : : : H O N N O H : : :
  11. 11. Another example Example: hyponitrous acid HONNOH 24 valence electrons H O N N O H : : : : H O N N O H : :
  12. 12. IonsSubtract one electron for each positivecharge Ammonium ion ( NH4+ ) Number of electrons = 5 + 4 -1 = 8 H + H N H H
  13. 13. Ionsadd one electron for each negative charge Tetrafluoroborate ( BF4- ) Number of electrons = 3 + 28+1 = 32 : : F: - : : :F B F : : : :F : :
  14. 14. Rules for writing structures (7) assign formal charges Example : nitric acid HNO3 (HONO2) : :O + : H O N : O : : - :
  15. 15. Formal Charge and Lewis Structure
  16. 16. Formal Charge number of valence= electrons in neutral - electron count of atom atomelectron count = number of one- half the electrons + number of shared “owned” by atom electrons
  17. 17. electron counts and formal charges inNH4+ and BF4- 1 7 + : H : F: - : : H N H :F B F : : : H :F : : 4 4
  18. 18. “Electron counts” in nitric acid 6 1 : : :O H O N 4 : O : : : 6 7
  19. 19. Formal charges in nitric acid : :O : 1+ H O N 4 : O : : : 1- 7
  20. 20. Lewis Structure of nitric acid : : :O H O N + : O : : : -
  21. 21. Formal Chargedoes not represent the real charge on anatom in the moleculeit can however be used to determine thevalidity of a molecules Lewis structure Try to minimize formal charge in your Lewis structures Avoid positive values of formal charge on highly electronegative elements
  22. 22. Recall : Rules for writing Lewis Structures(6) use unshared pairs for double bonds if thiswill satisfy octet rule. Example : nitrous acid HONO : : :H O N O: H O N O : : : : : : : : : + - : : : H O N O: H O N O: : : : :
  23. 23. Formal Charge + - : : :H O N O : H O N O: : : : : :More stable Lewis Less stable LewisStructure Structure
  24. 24. Formal ChargeThe sum of the formal charges of allatoms in a given molecule or ion mustequal the overall charge on that species
  25. 25. Two ConventionsOxidation StatesFormal Charges formal charges are closer to actual atomic charges than are oxidation states but are still only estimations of molecular atomic charge
  26. 26. Lewis Structure of nitric acid formal charge oxidation state : : :O H O N + +5 : O : : : - -2 oxidation state = electrons lost or gained plus formal charge
  27. 27. The Concept of Resonance
  28. 28. ResonanceTwo or more Lewis structures may bewritten for certain compounds (or ions )
  29. 29. Recall : Rules for writing Lewis Structures(6) use unshared pairs for double bonds if thiswill satisfy octet rule. Example :bicarbonate HOOCO - : :O: C : : :O O H - : :
  30. 30. Recall : Rules for writing Lewis Structures(6) use unshared pairs for double bonds if thiswill satisfy octet rule. Example :bicarbonate HOOCO - - : :O: : O: C C : : : :O O H O O H - : : :
  31. 31. What writing resonance structuresaccomplishesElectrons in molecules are often delocalizedbetween two or more atoms.electrons in a single Lewis structure are assignedto specific atoms “localization”.a single Lewis Structure is insufficient to showelectron delocalization.a composite of resonance forms more accuratelydepicts electron distribution
  32. 32. Example - :O: : :O: - : :O: +N N -: O : O: N + +: : ::O O: : :O O: : - - :-: : : Nitrate ion
  33. 33. Exceptions to the octet rule
  34. 34. Counting only valence electrons elements in the second period can never have more than 8 electrons but can have fewer than 8elements in the third period can havemore than 8 electrons
  35. 35. Less than 8 electrons : :F : B : : :F F : : :
  36. 36. more than 8 electrons : : F : : : :F F : : : : S : :F F : :F : : : :
  37. 37. Coordinate Covalent Bond : :F: H : :F B + : N H : :F: H :A covalent bond in which one of the atomsdonates both electrons
  38. 38. Coordinate Covalent Bond : :F: H : - + :F B N H : :F: H :A covalent bond in which one of the atomsdonates both electrons The distinction is useful for keeping track of electrons and assigning formal charge
  39. 39. Odd Electron Molecules + - : : : : O N O N O: . . : : :Some molecules contain an odd number ofelectrons ( NO ) and ( NO2 ) notable examples The octet is not complete

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