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LEWIS STRUCTURES

LEWIS STRUCTURES. Lewis “dot-line” representations of atoms and molecules Electrons of an atom are of two types: Core electrons and Valence electrons Only the valence electrons are shown in Lewis dot-line structures.

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LEWIS STRUCTURES

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  1. LEWIS STRUCTURES Dr Seemal Jelani

  2. Lewis “dot-line” representations of atoms and molecules • Electrons of an atom are of two types: • Coreelectrons and Valenceelectrons • Only the valence electrons are shown in Lewis dot-line structures Dr Seemal Jelani

  3. The number of valence electrons is equal to the group number of the element for the representative elements • For atoms the first four dots are displayed around the four “sides” of the symbol for the atom. Dr Seemal Jelani

  4. If there are more than four electrons, the dots are paired with those already present until an octet is achieved • Ionic compounds are produced by complete transfer of an electron from one atom to another Dr Seemal Jelani

  5. Covalent compounds are produced by sharing of one or more pairs of electrons by two atoms. • The valence capacityof an atom is the atom’s ability to form bonds with other atoms. • The more bonds the higher the valence. Dr Seemal Jelani

  6. The valence of an atom is not fixed, but some atoms have typical valences which are most common: Carbon: valence of 4 Nitrogen: valence of 3 Oxygen: valence of 2 Fluorine: valence of 1 Dr Seemal Jelani

  7. Covalent bonding and Lewis structures • Covalent bonds are formed from sharing of electrons by two atoms • Molecules possess only covalent bonds • The bedrock rule for writing Lewis structures for the first full row of the periodic table is the octet rule for C, N, O and F Dr Seemal Jelani

  8. C, N, O and F atoms are always surrounded by eight valence electrons • For hydrogen atoms, the doublet rule is applied: H atoms are surrounded by two valence electrons. Dr Seemal Jelani

  9. The Lewis Model of Chemical Bonding • In 1916 G. N. Lewis proposed that atoms combine in order to achieve a more stable electron configuration. • Maximum stability results when an atom is isoelectronic with a noble gas. • An electron pair that is shared between two atoms constitutes a covalent bond. Dr Seemal Jelani

  10. . . H H : H H Covalent Bonding in H2 Two hydrogen atoms, each with 1 electron, • Sharing the electron pair gives each hydrogen an electron configuration analogous to helium. can share those electrons in a covalent bond. Dr Seemal Jelani

  11. . .. . . . : C F .. . .. : : F .. .. .. : : : F C : F .. .. .. : : F .. Example Combine carbon (4 valence electrons) and four fluorines (7 valence electrons each) to write a Lewis structure for CF4. The octet rule is satisfied for carbon and each fluorine. Dr Seemal Jelani

  12. .. : : F .. .. .. : : : F C : F .. .. .. : : F .. : : F .. .. : : C F F .. .. : : F .. Example It is common practice to represent a covalentbond by a line. We can rewrite .. as Dr Seemal Jelani

  13. Double Bonds andTriple Bonds Dr Seemal Jelani

  14. .. .. .. .. : : : : : : : : C C O O O O : : : : : : H H C N C N Inorganic examples Carbon dioxide Hydrogen cyanide Dr Seemal Jelani

  15. H H H H .. .. : : : : H H C C C C H H : : : : : H H H H C C C C Organic examples Ethylene Acetylene Dr Seemal Jelani

  16. Formal Charges Revision Dr Seemal Jelani

  17. 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

  18. .. : 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

  19. .. : 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

  20. .. : 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

  21. .. : 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

  22. .. : 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

  23. .. : 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

  24. .. : 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

  25. .. : 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

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

  27. .. 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

  28. Condensed structural formulas Dr Seemal Jelani

  29. 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

  30. 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

  31. 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

  32. Constitutional Isomers Dr Seemal Jelani

  33. 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

  34. 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

  35. 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

  36. Dr Seemal Jelani Shapes

  37. Dr Seemal Jelani

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

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

  40. 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

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

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

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

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

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

  46. Polar Covalent Bondsand Electronegativity Dr Seemal Jelani

  47. 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

  48. Pauling Electronegativity Scale Dr Seemal Jelani

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

  50. Dr Seemal Jelani

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