【CHEMISTRY】How to Draw Lewis Structure and Predict the Shape (Parent/ Molecular Geometry)

How to Draw Lewis Structure and Predict the Shape (Parent/ Molecular Geometry)

Memorize it!

Electron domain	Bonding pair	Lone pair	Molecular geometry
2	2	0	Linear
3	3	0	Trigonal planar
	2	1	Bent/ V-shaped
4	4	0	Tetrahedral
	3	1	Trigonal pyramidal
	2	2	Bent/ V-shaped
5	5	0	Trigonal bipyramidal
	4	1	See-saw
	3	2	T-shaped
	2	3	Linear
6	6	0	Octahedral
	5	1	Square pyramidal
	4	2	Square planar

Exp 1: CCl₄

Step 1: Determine the total number of valence electrons.

Valence electron of C atom = 4

Valence electron of 4 Cl atom = 4(7) = 28

Total valence electron = 28 + 4 = 32

Step 2: Arrange the atoms. Determine which one is central atom (normally is the least electronegative)

So, in this case is C atom.

Step 3: Add enough electrons to each of the terminal (旁边) atoms – make it become octet

Step 4: Place any left electron to the central atom.

In this case, bonding electron = 4(2) = 8 有四个bond, 每个bond 有两个 electron

                    Terminal atoms 的 valence electron = 6(4) = 24 旁边的那些electron

                    Total valence electron = 32

Check 回上面， 发现没有剩下electron 给 central atom. 32 - 32 = 0

Step 5: Minimise the formal charge. 【这个在中六并没有学到，但我觉得蛮实用的东西】

The sum of the formal charge must be equal to the overall charge of molecules/ ions.

Let say CCl₄ is a neutral molecule, thus the sum of formal charge of S and O has to be 0.

Formal charge = Valence electron – (lone pair electron +  bonding electron)

Formal charge of C atom = 4 – (0 + ½(8) )

                                         = 4 – 4

                                         = 0

Formal charge of Cl atom = 7 – (6 + ½(2))

                                          = 7 – 7

                                          = 0

Sum of formal charge = 0    Check back it is a neutral molecule, thus correct lewis structure.

Conclusion: Parent geometry: Tetrahedral                   electron domain is 4

                   Molecular geometry: Tetrahedral              4 bonding pair + 0 lone pair

 

Exp 2: SO₂

Step 1: Determine the total number of valence electron.

Valence electron of S = 6

Valence electron of O = 2(6) = 12

Total valence electron = 6 + 12 = 18

Step 2: Arrange the atoms. Determine which one is central atom (normally is the least electronegative)

S is the least electronegative, so it is the central atom.

Step 3: Add enough electrons to each of the terminal atoms – make it become octet

Step 4: Place any left electron to the central atom.

Bonding electron = 2(2) = 4

Valence electron of terminal atoms = 2(6) = 12

Electron to be placed to central atom, S = 18 – (4 + 12) = 2

Step 5: Minimise the formal charge.

Formal charge of S atom = 6 – (2 + ½(4) )

                                         = +2

Formal charge of O atom = 6 – (6 + ½(2) )

                                         = -1

Sum of formal charge = +2 - 1 = +1

It is a neutral molecule, but we get +1, indicates that this is the incorrect lewis structure.

*Use the lone pair electrons from the terminal atom to form multiple bonds with the central atom.

Check the formal charge again.

Formal charge of S atom = 6 – (2 + ½(8) )

                                         = 0

Formal charge of O atom = 6 – (4 + ½(4) )

                                         = 0

Conclusion: Parent geometry: Trigonal planar               3 electron domain

                    Molecular geometry: Bent/ V-shaped         2 bonding pair + 1 lone pair

*When calculating bonding pair, doesn’t matter whether it is single/ double/ triple bond

 

Exp 3: CN^-

Step 1: Determine the total number of valence electron.

Valence electron of C = 4

Valence electron of N = 5

Charge -1 means add one more electron.

Total valence electron = 4 + 5 + 1 = 10

Step 2: Arrange the atoms. (only 2 atoms, so doesn’t matter) 

Step 3: Add valence electrons to the more electronegative atom first.

 

Step 4: Place left electron to the other atom.

Step 5: Check for octet.

 

 基本上每一个molecule/ ions 跟着这些步骤就能画出它的 lewis structure。

希望这些有帮到你们哦！