1. Covalent Bonding and Lewis Structures
Dr Seemal Jelani
4/6/2017

2. Covalent Bonds
Dr Seemal Jelani
4/6/2017

3. When two or more atoms of the same or similar electronegativities react, a complete transfer of electrons does not occur
In these instances the atoms achieve noble gas configurations by sharing electrons
Dr Seemal Jelani
4/6/2017

4. Covalent bonds
Form by sharing of electrons between atoms of similar electronegativities to achieve the configuration of a noble gas.
Molecules are composed of atoms joined exclusively or predominantly by covalent bonds
Dr Seemal Jelani
4/6/2017

5. Molecules may be represented by electron-dot formulas or, more conveniently, by bond formulas where each pair of electrons shared by two atoms is represented by a line.
Dr Seemal Jelani
4/6/2017

6. Electrons of an atom are of two types:
Types of electron
Electrons of an atom are of two types:
Core electrons and Valence electrons
Only the valence electrons are shown in Lewis dot-line structures
Dr Seemal Jelani
4/6/2017

7. The number of valence electrons is equal to the group number of the element for the representative elements
Dr Seemal Jelani
4/6/2017

8. How to distribute electrons
For atoms the first four dots are displayed around the four “sides” of the symbol for the atom.
Dr Seemal Jelani
4/6/2017

9. 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
4/6/2017

10. The more bonds the higher the valence.
Covalent compounds are produced by sharing of one or more pairs of electrons by two atoms.
The valence capacity of an atom is the atom’s ability to form bonds with other atoms.
The more bonds the higher the valence.
Dr Seemal Jelani
4/6/2017

11. 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
4/6/2017

12. 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
4/6/2017

13. 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
4/6/2017

14. 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
4/6/2017 2

15. can share those electrons in a covalent bond.
Covalent Bonding in H2
Two hydrogen atoms, each with 1 electron, H .
can share those electrons in a covalent bond. H :
Sharing the electron pair gives each hydrogen an electron configuration analogous to helium.
Dr Seemal Jelani
4/6/2017 2

16. Covalent Bonding in F2
Two fluorine atoms, each with 7 valence electrons .. .. F . :
can share those electrons in a covalent bond. F : ..
Sharing the electron pair gives each fluorine an electron configuration analogous to neon.
Dr Seemal Jelani
4/6/2017 2

17. The Octet Rule
In forming compounds, atoms gain, lose, or share electrons to give a stable electron configuration characterized by 8 valence electrons. F : ..
The octet rule is the most useful in cases involving covalent bonds to C, N, O, and F.
Dr Seemal Jelani
4/6/2017 2

18. Example
Combine carbon (4 valence electrons) and four fluorines (7 valence electrons each) C . F : .. .
to write a Lewis structure for CF4. : F .. C
The octet rule is satisfied for carbon and each fluorine.
Dr Seemal Jelani
4/6/2017 4

19. Example
It is common practice to represent a covalent bond by a line. We can rewrite : F .. C .. C F .. : as
Dr Seemal Jelani
4/6/2017 4

20. Double Bonds and Triple Bonds
Dr Seemal Jelani
4/6/2017 5

21. Inorganic examples C : O .. C : O .. Carbon dioxide : N C H : N C H
Hydrogen cyanide
Dr Seemal Jelani
4/6/2017 10

22. Organic examples C H C : .. H Ethylene : C H Acetylene C H 10
Dr Seemal Jelani
4/6/2017 10

23. Formal Charges
Revision
Dr Seemal Jelani
4/6/2017 5

24. 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
4/6/2017

25. Nitric acid
Formal charge of H : .. H O N ..
We will calculate the formal charge for each atom in this Lewis structure.
Dr Seemal Jelani
4/6/2017 19

26. Nitric acid Formal charge of H : .. H O N ..
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
4/6/2017 19

27. Nitric acid Formal charge of O : .. H O N ..
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 = 6.
Dr Seemal Jelani
4/6/2017 19

28. Nitric acid Electron count of O is 6.
Formal charge of O : .. H O N ..
Electron count of O is 6.
A neutral oxygen has 6 electrons.
Therefore, the formal charge of O is 0.
Dr Seemal Jelani
4/6/2017 19

29. Nitric acid
Formal charge of O : .. H O N ..
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
4/6/2017 19

30. Nitric acid
Formal charge of O : .. H O N ..
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
4/6/2017 19

31. Nitric acid
Formal charge of N : .. H O 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
4/6/2017 19

32. Nitric acid
Formal charges : .. H O N + – ..
A Lewis structure is not complete unless formal charges (if any) are shown.
Dr Seemal Jelani
4/6/2017 19

33. An arithmetic formula for calculating formal charge.
group number in periodic table
number of bonds
number of unshared electrons – –
Dr Seemal Jelani
4/6/2017 16

34. "Electron counts" and formal charges in NH4+ and BF4-.. B F : 1 4 N H + 7 – 4
Dr Seemal Jelani
4/6/2017 17

35. Condensed structural formulas
Dr Seemal Jelani
4/6/2017

36. Condensed structural formulas
Lewis structures in which many (or all) covalent bonds and electron pairs are omitted. H O C :
can be condensed to:
CH3CHCH3 OH
(CH3)2CHOH or
Dr Seemal Jelani
4/6/2017 4

37. Bond-line formulas
CH3CH2CH2CH3 is shown as
CH3CH2CH2CH2OH is shown as OH
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
4/6/2017 4

38. Bond-line formulas H Cl C
H2C
CH2 Cl
is shown as
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
4/6/2017 4

39. Constitutional Isomers
Dr Seemal Jelani
4/6/2017 5

40. 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
4/6/2017 4

41. A Historical Note O NH4OCN H2NCNH2 Ammonium cyanate Urea
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
4/6/2017 4

42. Examples of constitutional isomers.. .. C O N H : H O : + H C N – : O : H ..
Nitromethane
Methyl nitrite
Both have the molecular formula CH3NO2 but the atoms are connected in a different order.
Dr Seemal Jelani
4/6/2017 4

43. Resonance
Dr Seemal Jelani
4/6/2017 5

44. Resonance
two or more acceptable octet Lewis structures may be written for certain compounds (or ions)
Dr Seemal Jelani
4/6/2017 22

45. How to Write Lewis Structures
If an atom lacks an octet, use electron pairs on an adjacent atom to form a double or triple bond.
Example: Nitrogen has only 6 electrons in the structure shown. .. C O N H :
Dr Seemal Jelani
4/6/2017 2

46. How to Write Lewis Structures
Step If an atom lacks an octet, use electron pairs on an adjacent atom to form a double or triple bond.
Example: All the atoms have octets in this Lewis structure. .. C O N H :
Dr Seemal Jelani
4/6/2017 2

47. How to Write Lewis Structures
Step Calculate formal charges.
Example: None of the atoms possess a formal charge in this Lewis structure. .. C O N H :
Dr Seemal Jelani
4/6/2017 2

48. How to Write Lewis Structures
Step Calculate formal charges.
Example: This structure has formal charges; is less stable Lewis structure. .. C O N H : + –
Dr Seemal Jelani
4/6/2017 2

49. Resonance Structures of Methyl Nitrite
same atomic positions
differ in electron positions .. C O N H : .. C O N H : + –
more stable Lewis structure
less stable Lewis structure
Dr Seemal Jelani
4/6/2017 24

50. Resonance Structures of Methyl Nitrite
same atomic positions
differ in electron positions .. C O N H : .. C O N H : + –
more stable Lewis structure
less stable Lewis structure
Dr Seemal Jelani
4/6/2017 24

51. Why Write Resonance Structures?
Dr Seemal Jelani
4/6/2017

52. Electrons in a single Lewis structure are assigned to specific atoms
Electrons in molecules are often delocalized between two or more atoms.
Electrons in a single Lewis structure are assigned to specific atoms
A single Lewis structure is insufficient to show electron delocalization.
Composite of resonance forms more accurately represents electron distribution.
Dr Seemal Jelani
4/6/2017 25

53. Example Ozone (O3) + O Expect: one short bond and one long bond
Lewis structure of ozone shows one double bond and one single bond O ••
• • – +
Expect: one short bond and one long bond
Reality: bonds are of equal length (128 pm)
Dr Seemal Jelani
4/6/2017 26

54. Example Ozone (O3) O + Resonance: O + O +
Lewis structure of ozone shows one double bond and one single bond O ••
• • – +
Resonance: O ••
• • – + O ••
• • – +
Dr Seemal Jelani
4/6/2017 26

55. 3.7 The Shapes of Some Simple Molecules
Dr Seemal Jelani
4/6/2017 1

56. Dr Seemal Jelani
4/6/2017

57. Dr Seemal Jelani
4/6/2017

58. Methane tetrahedral geometry H—C—H angle = 109.5° 8 Dr Seemal Jelani
4/6/2017 8

59. Methane tetrahedral geometry each H—C—H angle = 109.5° 8
Dr Seemal Jelani
4/6/2017 8

60. 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
4/6/2017 11

61. but notice the tetrahedral arrangement of electron pairs
Water
bent geometry
H—O—H angle = 105° H H O : ..
but notice the tetrahedral arrangement of electron pairs
Dr Seemal Jelani
4/6/2017 9

62. Ammonia H H N : H trigonal pyramidal geometry H—N—H angle = 107°
but notice the tetrahedral arrangement of electron pairs
Dr Seemal Jelani
4/6/2017 9

63. Boron Trifluoride F—B—F angle = 120°
trigonal planar geometry allows for maximum separation of three electron pairs
Dr Seemal Jelani
4/6/2017 13

64. Multiple Bonds
Four-electron double bonds and six-electron triple bonds are considered to be similar to a two-electron single bond in terms of their spatial requirements.
Dr Seemal Jelani
4/6/2017 15

65. Formaldehyde: CH2=O O C H H—C—H and H—C—O angles are close to 120°
trigonal planar geometry C O H
Dr Seemal Jelani
4/6/2017 13

66. Figure 1.12: Carbon Dioxide
O—C—O angle = 180°
linear geometry O C
Dr Seemal Jelani
4/6/2017 14

67. Dr Seemal Jelani
4/6/2017

68. Dr Seemal Jelani
4/6/2017

69. 3.7: Polar Covalent Bonds and Electronegativity
Dr Seemal Jelani
4/6/2017 5

70. Electronegativity
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
4/6/2017 2

71. Pauling Electronegativity Scale
Electronegativity increases from left to right in the periodic table.
Electronegativity decreases going down a group.
Dr Seemal Jelani
4/6/2017 5

72. Dr Seemal Jelani
4/6/2017

73. Generalization
The greater the difference in electronegativity between two bonded atoms; the more polar the bond. F : ..
H—H : N
nonpolar bonds connect atoms of the same electronegativity
Dr Seemal Jelani
4/6/2017 7

74. Generalization
The greater the difference in electronegativity between two bonded atoms; the more polar the bond. d- d+ O .. H d+ d- d- F : .. H d+ d- : O C O : .. ..
polar bonds connect atoms of different electronegativity
Dr Seemal Jelani
4/6/2017 7

75. 3.7 Molecular Dipole Moments
Dr Seemal Jelani
4/6/2017 1

76. (expressed in Debye units)
Dipole Moment
A substance possesses a dipole moment if its centers of positive and negative charge do not coincide.
m = e x d
(expressed in Debye units)
+ —
not polar
Dr Seemal Jelani
4/6/2017 17

77. Dr Seemal Jelani
4/6/2017

78. Dr Seemal Jelani
4/6/2017

79. Dr Seemal Jelani
4/6/2017

80. (expressed in Debye units)
Dipole Moment
A substance possesses a dipole moment if its centers of positive and negative charge do not coincide.
m = e x d
(expressed in Debye units) — +
polar
Dr Seemal Jelani
4/6/2017 17

81. Molecular Dipole Moments
molecule must have polar bonds
necessary, but not sufficient
need to know molecular shape
because individual bond dipoles can cancel
Dr Seemal Jelani
4/6/2017 18

82. Molecular Dipole Moments
Carbon dioxide has no dipole moment; m = 0 D
Dr Seemal Jelani
4/6/2017 18

83. Comparison of Dipole Moments
Carbon tetrachloride
Dichloromethane
m = 0 D
m = 1.62 D
Dr Seemal Jelani
4/6/2017 20

84. Carbon tetrachloride m = 0 D
Resultant of these two bond dipoles is
Resultant of these two bond dipoles is
m = 0 D
Carbon tetrachloride has no dipole moment because all of the individual bond dipoles cancel.
Dr Seemal Jelani
4/6/2017 20

85. Dichloromethane
Resultant of these two bond dipoles is
Resultant of these two bond dipoles is
m = 1.62 D
The individual bond dipoles do not cancel in dichloromethane; it has a dipole moment.
Dr Seemal Jelani
4/6/2017 20