1. LEWIS STRUCTURES
Dr Seemal Jelani
5/7/2019

2. 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
Dr Seemal Jelani
5/7/2019

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
5/7/2019

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
5/7/2019

5. 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
5/7/2019

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
5/7/2019

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
5/7/2019

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
5/7/2019

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
5/7/2019 2

10. 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.
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5/7/2019 2

11. 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.
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5/7/2019 4

12. Example
It is common practice to represent a covalent bond by a line. We can rewrite : F .. C .. C F .. : as
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5/7/2019 4

13. Double Bonds and Triple Bonds
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5/7/2019 5

14. Inorganic examples C : O .. C : O .. Carbon dioxide : N C H : N C H
Hydrogen cyanide
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15. Organic examples C H C : .. H Ethylene : C H Acetylene C H 10
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5/7/2019 10

16. Formal Charges
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5/7/2019 5

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
5/7/2019

18. Nitric acid
Formal charge of H : .. H O N ..
We will calculate the formal charge for each atom in this Lewis structure.
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5/7/2019 19

19. 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.
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5/7/2019 19

20. 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.
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5/7/2019 19

21. 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.
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5/7/2019 19

22. 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
5/7/2019 19

23. 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
5/7/2019 19

24. 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
5/7/2019 19

25. Nitric acid
Formal charges : .. H O N + – ..
A Lewis structure is not complete unless formal charges (if any) are shown.
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26. An arithmetic formula for calculating formal charge.
group number in periodic table
number of bonds
number of unshared electrons – –
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27. "Electron counts" and formal charges in NH4+ and BF4-.. B F : 1 4 N H + 7 – 4
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5/7/2019 17

28. .. + Formal charge of +1 H O H H 6-(2+3) 6-5 + 1
Hydronium ion assigned 5 valence electrons
Formal charge of +1
H O H H
6-(2+3)
6-5
+ 1
Dr Seemal Jelani Chem-241
5/7/2019

29. Bicarbonate assigned 7 valence
electrons: Ö:
charge of -1
H Ö C- Ö: ¨ ¨
6-(6+1)
6-7 -1
Dr Seemal Jelani Chem-241
5/7/2019

30. Condensed structural formulas
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5/7/2019

31. 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
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5/7/2019 4

32. 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.
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5/7/2019 4

33. 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.
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5/7/2019 4

34. Constitutional Isomers
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35. 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.”
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5/7/2019 4

36. A Historical Note O NH4OCN H2NCNH2 Ammonium cyanate Urea CH4N2O 4
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5/7/2019 4

37. Ammonium cyanate and urea are constitutional isomers of CH4N2O.
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
5/7/2019

38. 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.
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5/7/2019 4

39. Shapes
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5/7/2019

40. Dr Seemal Jelani
5/7/2019

41. Methane tetrahedral geometry H—C—H angle = 109.5° 8 Dr Seemal Jelani
5/7/2019 8

42. Methane tetrahedral geometry each H—C—H angle = 109.5° 8
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5/7/2019 8

43. 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).
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5/7/2019 11

44. 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
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5/7/2019 9

45. Ammonia H H N : H trigonal pyramidal geometry H—N—H angle = 107°
but notice the tetrahedral arrangement of electron pairs
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5/7/2019 9

46. Boron Trifluoride F—B—F angle = 120°
trigonal planar geometry allows for maximum separation of three electron pairs
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5/7/2019 13

47. Formaldehyde: CH2=O O C H H—C—H and H—C—O angles are close to 120°
trigonal planar geometry C O H
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5/7/2019 13

48. Figure 1.12: Carbon Dioxide
O—C—O angle = 180°
linear geometry O C
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5/7/2019 14

49. Polar Covalent Bonds and Electronegativity
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5/7/2019 5

50. element to attract electrons toward itself
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
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5/7/2019 2

51. Pauling Electronegativity Scale
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5/7/2019 5

52. Electronegativity increases from left to right in the periodic table
decreases going down a
group.
Dr Seemal Jelani
5/7/2019

53. Dr Seemal Jelani
5/7/2019

54. Nonpolar bonds connect atoms of the same electronegativity
Generalization F : .. : N
H—H
Nonpolar bonds connect
atoms of the same
electronegativity
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5/7/2019 7

55. 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
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5/7/2019 7