How to write the Lewis structure of a molecule

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Steps to write the Lewis structure of a molecule

1. Find the number of valence electrons for each of the atoms in the molecule.

The number of valence electrons is usually the same as the group number where the element is located.

The number of valence electrons for alkaline metals is 1 (group 1)

The number of valence electrons for alkalineearth metals is 2 (group 2)

The number of valence electrons for group 3 elements is 3

The number of valence electrons for group 4 elements is 4

The number of valence electrons for group 5 elements is 5

The number of valence electrons for group 6 elements is 6

The number of valence electrons for group 7 elements is 7

Example: methane

• Methane: CH4

• C is located in group 4, thus possess 4 valence electrons

• H is located in group 1, thus possess 1 valence electron

2. Count the total number of valence electrons in the molecule:

Sum the valence electrons from each atom in the molecule

For methane CH4

• Total valence e. from C: 44 e. x 1 C = 4

• Total valence e. from H: 41 e. x 4H = 4

Total valence e. in the molecule: 4 + 4 = 8

3. Use the octet rule to figure how many octet electrons the atoms should have:

Most atoms need 8 electrons to complete their valence shield

Hydrogen is an exception and needs only 2

For methane CH4

• Carbon should have 8 valence electrons

• Each hydrogen should have 2 valence electrons

4. Draw an initial sketch of the molecule by placing the symbol of each atom in a diagram

For methane:H

H C HH

5. Place the valence electrons of each atom around them, starting from the outer atoms

For methane CH4

• Hydrogen has 1 valence electron:

C

H

H

H

H

For methane CH4

• Carbon has 4 valence electrons:

C

H

H

H

H

6. Each bond in a molecule is made up from two electrons.

Each atom in the molecule need to be bonded together to its nearest atom .

For methane CH4

• Form single bonds between the atoms:

C

H

H

H

H

C

H

H

H

H

For methane CH4

• Form single bonds between the atoms:

C

H

H

H

H

For methane CH4

• Form single bonds between the atoms:

For methane CH4

• Form single bonds between the atoms:

C

H

H

H

H

For methane CH4

Now the structure is complete:

C

H

H

H

H

7. At the end, each atom should be surrounded by eight electrons (following the octet rule)

Remember that H only needs 2 electrons

For methane CH4

• Carbon is surrounded by 8 electrons

C

H

H

H

H

For methane CH4

• Each hydrogen is surrounded by 2 electrons

C

H

H

H

H

Remember these simple rules when drawing Lewis structures:

• Hydrogen and the halogens form single bonds• The oxygen family forms single or double

bonds. They need at least 2 bonds.• The nitrogen family forms single, double or

triple bonds. They need at least 3 bonds.• The carbon family needs 4 bonds.

Practice writing the Lewis structure of any molecule!

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