C. Mark Zheng © 2008. All rights reserved. How to Draw Lewis Dot Structures (Demo)

C. Mark Zheng © 2008. All rights reserved.

How to Draw Lewis Dot Structures(Demo)


How to Draw Lewis Dot Structures(Demo)

Hydrogen Cyanide (HCN)


Step 1: Identify the central atom and draw the outer atoms around it.





Carbon is the central atom



C



CH



CH N


Step 2: Connect each outer atom to the central atom using a dash.



CH N



CH N



CH N


Step 3: Count the total number of valence electrons.

.



CH N



CH N

H = 1 e-



CH N

H = 1 e-

C = 4 e-



CH N

H = 1 e-

C = 4 e-

N = 5 e-



CH N

H = 1 e-

C = 4 e-

+ N = 5 e-



CH N

H = 1 e-

C = 4 e-

+ N = 5 e-



CH N

H = 1 e-

C = 4 e-

+ N = 5 e-

Total = 10 e-


Step 4: Fill each outer atom with electrons such that the outer atoms obey the octet rule.



CH N



CH N

Hydrogen



CH N

Hydrogen



CH N

Hydrogen

1



CH N

Hydrogen

1 2



CH N

Hydrogen full (2e- max for H)

1 2



CH N

Nitrogen

1 2



CH N

Nitrogen

1 2



CH N

Nitrogen

1 1 2



CH N

Nitrogen

1 21 2



CH N

Nitrogen need 6 more

1 21 2



CH N

Nitrogen

1 23●1 2



CH N

Nitrogen

1 23 4● ●1 2



CH N

Nitrogen

1 23 4

5

● ●●1 2



CH N

Nitrogen

1 23 4

5

6

● ●●●

1 2



CH N

Nitrogen

1 23 4

5

6

● ●●●

7●

1 2



CH N

Nitrogen

1 23 4

5

6

● ●●●

8 7●●

1 2


Step 5: Apply leftover electrons to the central atom.



CH N1 23 4

5

6

● ●●●

8 7●●

H = 1 e-

C = 4 e-

+ N = 5 e-

Total = 10 e-

1 2



CH N1 23 4

5

6

● ●●●

8 7●●

H = 1 e-

C = 4 e-

+ N = 5 e-

Total = 10 e-

used = 10 e-

1 2



CH N1 23 4

5

6

● ●●●

8 7●●

H = 1 e-

C = 4 e-

+ N = 5 e-

Total = 10 e-

- used = 10 e-

1 2



CH N1 23 4

5

6

● ●●●

8 7●●

H = 1 e-

C = 4 e-

+ N = 5 e-

Total = 10 e-

- used = 10 e-

1 2



CH N1 23 4

5

6

● ●●●

8 7●●

H = 1 e-

C = 4 e-

+ N = 5 e-

Total = 10 e-

- used = 10 e-

left = 0 e-

1 2



CH N1 23 4

5

6

● ●●●

8 7●●

H = 1 e-

C = 4 e-

+ N = 5 e-

Total = 10 e-

- used = 10 e-

left = 0 e-

1 2

No (zero) electron go on the central atom (C)


Step 6: Check each atom to see if the octet rule is obeyed.



CH N1 23 4

5

6

● ●●●

8 7●●

1 2



CH N1 23 4

5

6

● ●●●

8 7●●

1 2

We the electrons were assigned to the outer atoms, we made sure they obey the octet rule; therefore we need only check the central atom – carbon.



CH N● ●

●●

●●



CH N● ●

●●

●●

1



CH N● ●

●●

●●

1 2



CH N● ●

●●

●●

1 2 3



CH N● ●

●●

●●

1 2 3 4



CH N● ●

●●

●●

1 2 3 4

4



CH N● ●

●●

●●

1 2 3 4

4 X



CH N● ●

●●

●●

1 2 3 4

4 X – Does NOT obey the octet rule


Step 7: Move a pair of non-bonding (or lone pair) electrons from the outer atom to form a new bond with the central atom.



CH N● ●

●●

●●



CH N● ●

●●

●●



CH N● ●

●●


Check Octet Rule Again



CH N● ●

●●

1



CH N● ●

●●

1 2



CH N● ●

●●

1 2 3



CH N● ●

●●

1 2 3 4



CH N● ●

●●

1 2 3 4

5



CH N● ●

●●

1 2 3 4

5 6



CH N● ●

●●

1 2 3 4

5 6

6



CH N● ●

●●

1 2 3 4

5 6

6 X



CH N● ●

●●

1 2 3 4

5 6

6 X – Does NOT obey the octet rule



CH N● ●

●●



CH N● ●

●●



CH N ●●


Check Octet Rule Again



CH N ●●



CH N ●●

4 bonds around carbon = 8 e-

+ 0 lone pair e- around carbon = 0 e-

valence electrons around carbon = 8 e-



CH N ●●

4 bonds around carbon = 8 e-

+ 0 lone pair e- around carbon = 0 e-

valence electrons around carbon = 8 e-

Octet rule is obeyed!


Step 8 & 9: Check and minimize formal charge on all atoms.


Step 8 & 9: Check and minimize formal charge on all atoms.F.C. = (normal # of valence e-) – (# non-bonding e- + ½ # of bonding e- )



CH N ●●



CH N ●●

Atom Normal # of Valence

electrons

# of non-bonding electrons

# of bonding electrons

Formal Charge



CH N ●●

H = 1 - ( 0 + ½ (2) ) = 0


electrons



Formal Charge



CH N ●●

H = 1 - ( 0 + ½ (2) ) = 0 C = 4 - ( 0 + ½ (8) ) = 0


electrons



Formal Charge



CH N ●●

H = 1 - ( 0 + ½ (2) ) = 0 C = 4 - ( 0 + ½ (8) ) = 0 N = 5 - ( 2 + ½ (6) ) = 0


electrons



Formal Charge



CH N ●●

H = 1 - ( 0 + ½ (2) ) = 0 C = 4 - ( 0 + ½ (8) ) = 0 N = 5 - ( 2 + ½ (6) ) = 0

All Formal Charges Minimized!


electrons



Formal Charge





Final Structure



Final Structure

CH N ●●



Final Structure

CH N ●●

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C. Mark Zheng © 2008. All rights reserved. How to Draw Lewis Dot Structures (Demo)