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Formal Charges

Formal Charges. Formal charge is the charge calculated for an atom in a Lewis structure on the basis of an equal sharing of bonded electron pairs. :. O. N. H. O. :. :. O. Nitric acid. Formal charge of H. We will calculate the formal charge for each atom in this Lewis structure. :. O.

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Formal Charges

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  1. Formal Charges Dr Seemal Jelani

  2. Formal charge is the charge calculated for an atom in a Lewis structure on the basis of an equal sharing of bonded electron pairs. Dr Seemal Jelani

  3. .. : O .. N H O .. : : O Nitric acid Formal charge of H • We will calculate the formal charge for each atom in this Lewis structure. .. Dr Seemal Jelani

  4. .. : O .. N H O .. : : O Nitric acid Formal charge of H • Hydrogen shares 2 electrons with oxygen. • Assign 1 electron to H and 1 to O. • A neutral hydrogen atom has 1 electron. • Therefore, the formal charge of H in nitric acid is 0. .. Dr Seemal Jelani

  5. .. : O .. N H O .. : : O Nitric acid Formal charge of O • Oxygen has 4 electrons in covalent bonds. • Assign 2 of these 4 electrons to O. • Oxygen has 2 unshared pairs. Assign all 4 of these electrons to O. • Therefore, the total number of electrons assigned to O is 2 + 4 = 6. .. Dr Seemal Jelani

  6. .. : O .. N H O .. : : O Nitric acid Formal charge of O • Electron count of O is 6. • A neutral oxygen has 6 electrons. • Therefore, the formal charge of O is 0. .. Dr Seemal Jelani

  7. .. : O .. N H O .. : : O Nitric acid Formal charge of O • Electron count of O is 6 (4 electrons from unshared pairs + half of 4 bonded electrons). • A neutral oxygen has 6 electrons. • Therefore, the formal charge of O is 0. .. Dr Seemal Jelani

  8. .. : O .. N H O .. : : O Nitric acid Formal charge of O • Electron count of O is 7 (6 electrons from unshared pairs + half of 2 bonded electrons). • A neutral oxygen has 6 electrons. • Therefore, the formal charge of O is -1. .. Dr Seemal Jelani

  9. .. : O .. N H O .. : : O Nitric acid Formal charge of N • Electron count of N is 4 (half of 8 electrons in covalent bonds). • A neutral nitrogen has 5 electrons. • Therefore, the formal charge of N is +1. – .. Dr Seemal Jelani

  10. .. : O .. N H O .. : : O Nitric acid Formal charges • A Lewis structure is not complete unless formal charges (if any) are shown. + – .. Dr Seemal Jelani

  11. Formal Charge An arithmetic formula for calculating formal charge. Formal charge = group numberin periodic table number ofbonds number ofunshared electrons – – Dr Seemal Jelani

  12. .. 1 H : : F + .. .. : : N B F F H H .. .. : : F H .. 4 "Electron counts" and formal charges in NH4+ and BF4- 7 – 4 Dr Seemal Jelani

  13. Dr Seemal Jelani Chem-241 Hydronium ion assigned 5 valence electrons .. +Formal charge of +1 H O H H 6-(3+2) 6-5 + 1

  14. Dr Seemal Jelani Chem-241 • Bicarbonate assigned 7 valence electrons: Ö: - charge of -1 H Ö C- Ö: ¨ ¨ 6-(6+1) 6-7 -1

  15. Condensed structural formulas Dr Seemal Jelani

  16. H H H C H C C H : H : H O (CH3)2CHOH or CH3CHCH3 H OH Condensed structural formulas • Lewis structures in which many (or all) covalent bonds and electron pairs are omitted. can be condensed to: Dr Seemal Jelani

  17. CH3CH2CH2CH3 is shown as CH3CH2CH2CH2OHis shown as OH Bond-line formulas • Omit atom symbols. Represent structure by showing bonds between carbons and atoms other than hydrogen. • Atoms other than carbon and hydrogen are called heteroatoms. Dr Seemal Jelani

  18. H Cl Cl C H2C CH2 H2C CH2 C H H Bond-line formulas • Omit atom symbols. Represent structure by showing bonds between carbons and atoms other than hydrogen. • Atoms other than carbon and hydrogen are called heteroatoms. is shown as Dr Seemal Jelani

  19. Constitutional Isomers Dr Seemal Jelani

  20. Constitutional isomers • Isomers are different compounds that have the same molecular formula. • Constitutional isomers are isomers that differ in the order in which the atoms are connected. • An older term for constitutional isomers is “structural isomers.” Dr Seemal Jelani

  21. O H2NCNH2 A Historical Note NH4OCN Urea Ammonium cyanate • In 1823 Friedrich Wöhler discovered that when ammonium cyanate was dissolved in hot water, it was converted to urea. • Ammonium cyanate and urea are constitutional isomers of CH4N2O. • Ammonium cyanate is “inorganic.” Urea is “organic.” Wöhler is credited with an important early contribution that helped overturn the theory of “vitalism.” Dr Seemal Jelani

  22. H .. .. : H C O N O .. .. H Examples of constitutional isomers .. • Both have the molecular formula CH3NO2 but the atoms are connected in a different order. H : O + N H C – : : O H .. Nitromethane Methyl nitrite Dr Seemal Jelani

  23. Dr Seemal Jelani Shapes

  24. Dr Seemal Jelani

  25. Methane • tetrahedral geometry • H—C—H angle = 109.5° Dr Seemal Jelani

  26. Methane • tetrahedral geometry • each H—C—H angle = 109.5° Dr Seemal Jelani

  27. Valence Shell Electron Pair Repulsions • The most stable arrangement of groups attached to a central atom is the one that has the maximum separation of electron pairs(bonded or nonbonded). Dr Seemal Jelani

  28. Water • bent geometry • H—O—H angle = 105° H H : O .. but notice the tetrahedral arrangement of electron pairs Dr Seemal Jelani

  29. Ammonia • trigonal pyramidal geometry • H—N—H angle = 107° H H : N H but notice the tetrahedral arrangement of electron pairs Dr Seemal Jelani

  30. Boron Trifluoride • F—B—F angle = 120° • trigonal planar geometry allows for maximum separationof three electron pairs Dr Seemal Jelani

  31. H O C H Formaldehyde: CH2=O • H—C—H and H—C—Oangles are close to 120° • trigonal planar geometry Dr Seemal Jelani

  32. O O C Figure 1.12: Carbon Dioxide • O—C—O angle = 180° • linear geometry Dr Seemal Jelani

  33. Polar Covalent Bondsand Electronegativity Dr Seemal Jelani

  34. Electronegativity is a measure of an element to attract electrons toward itself when bonded to another element. • An electronegative element attracts electrons. • An electropositive element releases electrons. Dr Seemal Jelani

  35. Pauling Electronegativity Scale Dr Seemal Jelani

  36. Electronegativity increases • from left to right in the periodic table • Electronegativity decreases going down a group. Dr Seemal Jelani

  37. Dr Seemal Jelani

  38. Generalization .. .. : : : : N N F F .. .. H—H Nonpolar bonds connect atoms of the same electronegativity Dr Seemal Jelani

  39. d- .. .. d- d+ d+ d+ : H H O H F .. .. Generalization • The greater the difference in electronegativitybetween two bonded atoms; the more polar the bond. d+ d- d- : : C O O .. .. polar bonds connect atoms ofdifferent electronegativity Dr Seemal Jelani

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