1. Gateway to Understanding Molecular Structure
Lewis Dot Structures
Gateway to Understanding Molecular Structure

2. Molecular Structure & Bonding
A molecular structure, unlike a simple molecular formula, indicates the exact 3-D nature of the molecule. It indicates which atoms are bonded to which atoms, and the 3-D orientation of those atoms relative to each other.

3. Molecular Formula vs. Molecular Structure
Molecular formula – H2O
Molecular structure: O
H H

4. Molecular Structure Two issues: What is stuck to what?
How are they oriented?

5. What is stuck to what?
The first thing you need to do in drawing a molecular structure is to figure out which atom sticks to which other atoms to generate a skeletal model of the molecule.
The skeletal model is called a Lewis Dot Structure.

6. Lewis Dot Structures
The first step towards establishing the full 3-D geometry of a molecule is determining what is stuck to what and how each atom is connected.
Lewis Dot Structures provide this information.

7. Two Rules
Total # of valence electrons – the total number of valence electrons must be accounted for, no extras, none missing.
Octet Rule – every atom should have an octet (8) electrons associated with it. Hydrogen should only have 2 (a duet).

8. What’s a “valence electron”?
It’s an electron in the outermost shell of an atom. When two atoms bump, it’s the valence shells that hit first.
“Core” electron – not on the outside.

9. The Bohr Model
Nucleus e- n p p n e- n p n e-

10. Electronic Structure of Atoms
Since the electrons are so important, understanding the electronic structure of atoms is critical to understanding why atoms react with each other.
The first thing we need to do is move beyond classical physics and the Bohr Model

11. What’s wrong with the Bohr Model?
The electrons should collapse into the nucleus
Nucleus e- n p p n e- n p n e-

12. It doesn’t 
The electron “orbits” are stable and electrons can move between them by absorbing light (higher energy orbitals) or emitting light (moving into lower energy orbitals).

13. Quantum electronic structure
The solution to the electron paradox is that the world of the atom is not “classical” but “quantum mechanical”.
In a quantum world, only certain discrete energy levels are allowed. You cannot slowly decay in orbit until you crash into the nucleus.

14. Can’t do it! Has to jump!
Nucleus e- n p p n e- n p n e-

15. Can’t do it! Has to jump!
Nucleus e- n p p n e- n p n e-

16. Electron Orbitals
Electron orbitals are diffuse. The electron is not a hard little pellet, but a “probability cloud”.
Electron orbitals are 95% probability intervals.
Allowed electron orbitals are determined by 4 quantum numbers.

17. Electron Orbitals – s-orbital

18. p-orbital

19. Electron Orbitals
Every electron is represented by 4 quantum numbers. These electron Quantum numbers are:
n = principal quantum number (kind of like the Bohr orbit)
l = angular momentum quantum number – gives the shape
ml = magnetic quantum number – determines the number of orbitals of a given shape (2l+1)
ms = spin quantum number – determines number of electrons in each orbital (2)

20. Allowed Quantum numbers
l = 0, 1, 2, 3, 4…(n-1)
There are as many different types of orbitals as “n”.
ml = -l, -l+1…-1, 0, 1…l-1, l
There are 2l+1 orbitals of a given type (l)
ms = -1/2, 1/2
There are two electrons in each orbital.

21. Possible Quantum numbers
n = 1, l=0, ml = 0, ms=-1/2
n = 1, l=0, ml = 0, ms=+1/2
n = 2, l=0, ml = 0, ms=-1/2
n = 2, l=0, ml = 0, ms=+1/2
n = 2, l=1, ml = -1, ms=-1/2
n = 2, l=1, ml = -1, ms=+1/2
n = 2, l=1, ml = 0, ms=-1/2
n = 2, l=1, ml = 0, ms=+1/2
n = 2, l=1, ml = 1, ms=-1/2
n = 2, l=1, ml = 1, ms=+1/2

22. Possible Quantum numbers
n = 1 l=0 ml = 0 ms=-1/2
ms=+1/2
n = 2 l=0 ml = 0 ms=-1/2
l=1 ml = -1 ms=-1/2
ml = 0 ms=-1/2
ml = 1 ms=-1/2

23. What do these numbers mean?
n is like the Bohr orbit number. It gives the “shell” the electron is in.
l is the orbital number, it specifies the type of orbital within the same shell.
ml gives the orientation of the orbital – these are different flavors of the same orbital
ms is the magnetic spin of the electron (think N and S pole) – this is specific to the electron not the orbital

24. Shorthand Notation Orbitals are specified by letters:
l=0 is an s orbital
l=1 is a p orbital
l=2 is a d orbital
l=3 is an f orbital
l=4 is a g orbital (then h, i, j, k…)

25. Shorthand notation n=1, l=0 is called a 1s orbital
n=2, l=1 is called a 2p
n=3, l=2 is called a 3d
The number of electrons in each orbital are indicated as a superscript.
1s2 means 2 electrons are in the 1s orbital
3d7 means 7 electrons are in the 3d orbital

26. d-Orbitals

27. Rules Governing Electrons
Pauli Exclusion Principle - No two electrons in an atom can have the same 4 quantum numbers
Lowest energy orbitals fill first
Hund’s rule – Electrons pair up as a last resort
An orbital being full or half-full is good! (lower in energy)

28. Energy of the Orbitals 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 5g
6s 6p 6d 6f 6g 6h
7s 7p 7d 7f 7g 7h 7i

29. Pauli Exclusion Principle
This determines the number of orbitals in a shell and the total number of electrons that fit in each orbit.
There is only 1 orbital (s) in the 1st shell of only 1 type which can hold, at most, 2 electrons.
There are 2 different orbitals (s, p) in the 2nd shell. There is 1 type of s (always) and 3 types of p (always). Each type can hold 2 electrons. So, at most, the 2nd shell can hold 8 electrons.
Etc.

30. Electron Configurations
If you need to figure out the electron configuration, you just count the electrons and start filling from lowest energy to highest.
For example, consider C
Carbon has 6 electrons, where do they go.

31. Carbon C – 6 electrons 1s is the lowest energy orbital, it takes 2
2s is the next lowest, it also takes 2
2p comes next, it can take up to 6, so it gets the last 2 electrons
1s22s22p2

32. What’s the electron configuration of Mg?

34. Mg = 12 electrons
1s gets 2
2s gets 2
2p gets 6
3s gets 2
1s22s22p63s2

35. Clicker question What is the ground state electron configuration of N?
A. 1s22s5
B. 1s22s22p3
C. 1s22s22p5
D. 1s22p5

36. Clicker question
What is the ground state electron configuration of As?
A. 1s22s22p63s23p83d104p3
B. 1s22s22p63s23p64s24p3
C. 1s22s22p63s23p64s23d104p3
D. 1s22s22p63s23p63d104p5

38. Clicker question
What is the ground state electron configuration of Cr?
A. 1s22s22p63s23p83d4
B. 1s22s22p63s23p64s23d4
C. 1s22s22p63s23p64s13d5
D. 1s22s22p63s23p63d6

40. Core vs. Valence Electrons
Core electrons – completed shells
Valence electrons – “outer” or incomplete shells
Only the valence electrons affect the chemistry of an atom.

41. What’s the valence configuration of…
Ca?
[Ar]4s2
Mo?
[Kr]5s14d5
Ga?
[Ar]4s23d104p1 (3 valence electrons)

43. What about Fe3+? Fe (atomic number 26) [Ar]4s23d6
Take away 3 electrons…
[Ar]4s23d3 OR
[Ar]3d5

44. Ions are different…
Electrons go into the shells in the order we indicated, but to form an ion by removing electrons, they come out in a different order. N=4 electrons come out before N=3. N=5 electrons come out before N=4.
In other words Ns electrons come out before (N-1)d electrons.

45. What about Fe3+? Fe (atomic number 26) [Ar]4s23d6
Take away 3 electrons…the 4s electrons come out before 3d, so…
[Ar]4s23d3 - NOT
[Ar]3d5 - correct

46. Molecular Structure & Bonding
A molecular structure, unlike a simple molecular formula, indicates the exact 3-D nature of the molecule. It indicates which atoms are bonded to which atoms, and the 3-D orientation of those atoms relative to each other.

47. Molecular Formula vs. Molecular Structure
Molecular formula – H2O
Molecular structure: O
H H

48. Molecular Structure Two issues: What is stuck to what?
How are they oriented?

49. What is stuck to what?
The first thing you need to do in drawing a molecular structure is to figure out which atom sticks to which other atoms to generate a skeletal model of the molecule.
The skeletal model is called a Lewis Dot Structure.

50. Lewis Dot Structures
The first step towards establishing the full 3-D geometry of a molecule is determining what is stuck to what and how each atom is connected.
Lewis Dot Structures provide this information.

51. Two Rules
Total # of valence electrons – the total number of valence electrons must be accounted for, no extras, none missing.
Octet Rule – every atom should have an octet (8) electrons associated with it. Hydrogen should only have 2 (a duet).

52. Determining the number of valence electrons:
Full d-orbitals do not count as valence electrons. They belong to the inner shell.
For example:
As is [Ar]4s23d104p3
This is FIVE (5) valence electrons. The 3d is part of the inner shell (n=3) which is full.

53. How many valence electrons does Ge have?
B. 14
C. 3
D. 4
E. 5

55. Take a look at Ge electron structure
[Ar]4s23d104p2
Full d-orbitals don’t count. So there are 4 valence electrons.

56. How many valence electrons does Ti have?
B. 2
C. 3
D. 4
E. 5

57. How many valence electrons does Te have?15 16 3 5 6

58. Total Number of Valence Electrons
The total number of available valence electrons is just the sum of the number of valence electrons that each atom possesses (ignoring d-orbital electrons)
So, for H2O, the total number of valence electrons = 2 x 1 (each H is 1s1) + 6 (O is 2s22p4) = 8
CO2 has a total number of valence electrons = 4 (C is 2s22p2) + 2 * 6 (O is 2s22p4) = 16

59. Central Atom In a molecule, there are only 2 types of atoms:
“central” – bonded to more than one other atom.
“terminal” – bonded to only one other atom.
You can have more than one central atom in a molecule.

60. How many central atoms in ethanol?1 2 3 4 5

61. Bonds Bonds are pairs of shared electrons.
Each bond has 2 electrons in it.
You can have multiple bonds between the same 2 atoms. For example:
C-O
C=O
C O
Each of the lines represents 1 bond with 2 electrons in it.

62. Lewis Dot Structure
Each electron is represented by a dot in the structure .
:Cl:
That symbol with the dots indicate a chlorine atom with 7 valence electrons.

63. Drawing Lewis Dot Structures
Determine the total number of valence electrons.
Determine which atom is the “central” atom.
Stick everything to the central atom using a single bond.

64. Dot structure for H2O 1. Total number of valence electrons:
6 + (2 x 1) =8
2. Central Atom – typically, the central atom will be leftmost and/or bottommost in the periodic table. It is the atom that wants more than one thing stuck to it. H is NEVER the central atom.
3. Stick all terminal atoms to the central atom using a single bond.

65. Dot structure for H2O
H – O - H

66. Drawing Lewis Dot Structures
Determine the total number of valence electrons.
Determine which atom is the “central” atom.
Stick everything to the central atom using a single bond.
Fill the octet of every atom by adding dots.
Verify the total number of valence electrons in the structure.

67. Dot structure for H2O .. H – O – H ¨
That is a total of 8 valence electrons used: each bond is 2, and there are 2 non-bonding pairs.

68. Drawing Lewis Dot Structures
Determine the total number of valence electrons.
Determine which atom is the “central” atom.
Stick everything to the central atom using a single bond.
Fill the octet of every atom by adding dots.
Verify the total number of valence electrons in the structure.
Add or subtract electrons to the structure by making/breaking bonds to get the correct # of valence electrons.
Check the “formal charge” of each atom.

69. Formal Charge of an atom
“Formal charge” isn’t a real charge. It’s a pseudo-charge on a single atom.
Formal charge = number of valence electrons – number of bonds – number of non-bonding electrons.
Formal charge (FC) is ideally 0, acceptably +/-1, on occasion +/- 2. The more 0s in a structure, the better.
The total of all the formal charges of each atom will always equal the charge on the entire structure (0 for neutral molecules).

70. Dot structure for H2O .. H – O – H ¨ FC (H) = 1-1-0 = 0
FC (O) = 6 – 2 – 4 = 0
This is excellent, all the FCs are 0!

71. Another example
Let’s try CO2

72. Drawing Lewis Dot Structures
Determine the total number of valence electrons.
Determine which atom is the “central” atom.
Stick everything to the central atom using a single bond.
Fill the octet of every atom by adding dots.
Verify the total number of valence electrons in the structure.
Add or subtract electrons to the structure by making/breaking bonds to get the correct # of valence electrons.
Check the “formal charge” of each atom.

74. CO2
CO2
Total number of valence electrons = 4 from carbon + 2x6 from oxygen = 16
Central Atom?
Either C or O could be a central atom. C is more likely (to the left, to the left, to the left…)

75. CO2 CO2 16 total valence electrons O – C – O Fill out the octets
:O – C - O:
¨ ¨ ¨

76. Drawing Lewis Dot Structures
Determine the total number of valence electrons.
Determine which atom is the “central” atom.
Stick everything to the central atom using a single bond.
Fill the octet of every atom by adding dots.
Verify the total number of valence electrons in the structure.
Add or subtract electrons to the structure by making/breaking bonds to get the correct # of valence electrons.
Check the “formal charge” of each atom.

77. CO2 16 total valence electrons .. .. .. :O – C - O: ¨ ¨ ¨
:O – C - O:
¨ ¨ ¨
Structure has 20 electrons in it. Too many!
I need to lose 4 electrons. What’s the best way to do that?
Make 2 bonds – each new bond costs 2 electrons

78. CO2 :O = C = O: ¨ ¨ Structure has 16 electrons in it. Just right!
¨ ¨
Structure has 16 electrons in it. Just right!
Notice, this works because there are 2 ways to count the electrons:
When I count the total # of electrons, I count each electron once.
When I count the electrons for each atom, I count the bond twice (once for each atom in the bond)

79. CO2 :O = C = O: ¨ ¨ Is this the only structure I could have drawn?
¨ ¨
Is this the only structure I could have drawn?
I only needed two new bonds, I didn’t specify where they needed to go! ..
:O C - O:
:O - C O:
Which is correct?

80. Choosing between different structures?
The first test is formal charge:
:O = C = O:
¨ ¨
FC (O) = 6 – 2 – 4 = 0
FC (C) = 4 – 4 – 0 = 0 ..
:O C - O:
FC (left O) = 6 – 3 – 2 = 1
FC (right O) = 6 – 1 – 6 = -1
Based on formal charge the upper structure is the better one.

81. Are these even different?..
:O C - O:
:O - C O:
Depends on what I mean by different!

82. Are they different? .. :O1 C – O2 : ¨ :O1 - C O2 :
If I label them, I can see a difference. (Isotopic labeling).
If I don’t label them, they are interchangeable, just rotate the top one to get the bottom one.

83. Resonance .. :O1 C – O2 : ¨ :O1 - C O2 :
Structures that are identical, but differ only in the arrangement of bonds are called resonance structures.
Resonance is always GOOD!

84. Resonance
When you have resonance, the real structure is not any one of the individual structures but the combination of all of them.
You can always recognize resonance – there are double or triple bonds involved.
If you take the 3 different CO2 structures, the “average” is the original one we drew with 2 double bonds.

85. Resonance
Resonance is indicated by drawing all resonance structures, separated by “ ”
:O C - O: :O - C O: :O = C = O:
¨ ¨ ¨ ¨
But this is not necessary in this case, as the last structure is also the combination of the 3 structures

86. Nitrite ion Draw the Lewis Dot structure for NO2-
How many valence electrons?
N has 5, O has 6, but there’s one extra (it’s an ion!)
5 + 2 (6) = 17 valence electrons + 1 extra = 18 valence electrons

87. Nitrite LDS What’s the central atom? Nitrogen O – N – O .. .. ..
:O – N - O:
¨ ¨ ¨
Total number of electrons?
20 electrons – too many

88. Nitrite LDS .. .. .. :O – N - O: ¨ ¨ ¨ How do you fix the problem?
:O – N - O:
¨ ¨ ¨
How do you fix the problem?
Make a bond
:O = N - O:
What do you think?
RESONANCE

89. Nitrite LDS .. .. .. .. .. .. :O = N - O: :O - N = O: ¨ ¨
:O = N - O: :O - N = O:
¨ ¨
What’s the real structure look like?
It’s an average of those 2. Kind of 1-1/2 bonds between each N and O! In fact, if you measure the bond angles in nitrite, you find that they are equal (a double bond would be shorter than a single bond)

90. Draw the best Lewis Dot Structure for Cl2CO

92. Double bond between C and O or C and Cl?
A.. C and O B. C and Cl C. Doesn’t matter D. I don’t care anymore

93. Draw the best Lewis Dot Structure for CO32-

94. Exceptions to the Octet Rule
There are exceptions to the octet rule:
Incomplete octets – less than 8 electrons.
Expanded octets – more than 8 electrons

95. Incomplete Octets
The most common elements that show incomplete octets are B, Be besides H.
So, for example, BCl3 has the Lewis structure:
: Cl – B – Cl:
¨ | ¨
: Cl :
Total valence electrons is correct at 24.
FC (B) = – 0 = 0
FC (Cl) = = 0

96. Expanded Octets
The most common atoms to show expanded octets are P and S. It is also possible for some transition metals.
An example of an expanded octet would be PCl5:
:Cl: :Cl: Total valence e- = 40
:Cl – P - Cl : FC(P) = 5 – 5 – 0 =0
¨ | ¨
: Cl: FC (Cl) = 7 – 1 – 6 = 0