Lewis Formulas (DP IB Chemistry): Revision Note
Lewis Formulas
Lewis formulas are also known as Lewis structures or electron dot diagrams
They show all valence electrons in a covalently bonded species
This includes bonding pairs and lone pairs
A pair of electrons can be represented by:
Dots
Crosses
A combination of dots and crosses
A line
For example, chlorine can be shown as:

Note: Cl–Cl is not a Lewis formula, since it does not show all the electron pairs
Steps for drawing Lewis formulas
Count the total number of valence electrons:
This is for all atoms in the molecule
Draw the skeletal positions:
This is to show how the atoms are connected
The central atom, usually the least electronegative, should be in the middle
Add electron pairs:
First, add the bonding pairs between each pair of bonded atoms
Then, add lone pairs to complete octets (or duets for hydrogen), starting with outer atoms
Check your structure:
Confirm that all valence electrons have been used
Ensure all atoms (except exceptions) have full octets, or duets for hydrogen
Worked Example
How many electrons are in the 2-aminoethanoic acid molecule?

Answer:
Add valence electrons for each atom:
C (×2) = 2 × 4 = 8
H (×5) = 5 × 1 = 5
O (×2) = 2 × 6 = 12
N (×1) = 5
Total = 30 electrons
Include both bonding and lone pairs
Remember to count lone pairs on O and N
Each single bond contains 2 electrons
Lone pairs are especially important on N and O, which follow the octet rule
Conclusion:
The total number of valence electrons is 30
Drawing Lewis formulas
Applying the steps to draw full Lewis formulas for some simple molecules:
Methane, CH4
Count the total number of valence electrons:
C = 4 electrons
H = 1 electron
CH4 = (1 x C) + (4 x H) = (1 x 4) + (4 x 1) = 8 electrons total
Draw the skeletal positions:
Carbon is the central atom surrounded by four hydrogen atoms

Add electron pairs:
First, add the bonding pairs
Each C-H bond contains one pair of electrons
Then, add lone pairs
The bonding pairs account for all electrons
So, there are no lone pairs

Check your structure:
All valence electrons used
Carbon has a full octet
Hydrogen has duets
Ammonia, NH3
Count the total number of valence electrons:
N = 5 electrons
H = 1 electron
NH3 = (1 x N) + (3 x H) = (1 x 5) + (3 x 1) = 8 electrons total
Draw the skeletal positions:
Nitrogen is the central atom surrounded by three hydrogen atoms

Add electron pairs:
First, add the bonding pairs
Each N-H bond contains one pair of electrons
Then, add lone pairs
The bonding pairs account for 6 electrons
So, there is one lone pair on nitrogen

Check your structure:
All valence electrons used
Nitrogen has a full octet
Hydrogen has duets
Tetrachloromethane, CCl4
Count the total number of valence electrons:
C = 4 electrons
Cl = 7 electrons
CCl4 = (1 x C) + (4 x Cl) = (1 x 4) + (4 x 7) = 32 electrons total
Draw the skeletal positions:
Carbon is the central atom surrounded by four chlorine atoms

Add electron pairs:
First, add the bonding pairs
Each C-Cl bond contains one pair of electrons
Then, add lone pairs
The bonding pairs account for 8 electrons and complete the octet for C
There are 24 remaining electrons
This means that there is a total of 12 lone pairs of electrons
Each chlorine atom has 3 lone pairs

Check your structure:
All valence electrons used
Carbon has a full octet
Chlorine has octets
Worked Example
Draw the Lewis structures for:
Water, H2O
Carbon dioxide, CO2
Hydrogen cyanide, HCN
Answers:
Water, H2O
H2O = (2 x H) + (1 x O) = (2 x 1) + (1 x 6) = 8 electrons total
Oxygen is the central atom surrounded by two hydrogen atoms
Each O-H bond contains one pair of electrons
There are 2 lone pairs on the oxygen

Carbon dioxide, CO2
CO2 = (1 x C) + (2 x O) = (1 x 4) + (2 x 6) = 16 electrons total
Carbon is the central atom surrounded by two oxygen atoms
Each C=O bond contains two pairs of electrons
Each oxygen atom has 2 lone pairs

Hydrogen cyanide, HCN
HCN = (1 x H) + (1 x C) + (1 x N) = (1 x 1) + (1 x 4) + (1 x 5) = 10 electrons total
Carbon is the central atom surrounded by one hydrogen and one nitrogen atom
The C-H bond contains one pair of electrons
The C
N bond contains three pairs of electrons
The nitrogen atom has 1 lone pair

When the octet rule does not apply
Most elements tend to follow the octet rule, aiming for 8 electrons in their valence shell to match a noble gas configuration
However, several elements can form stable compounds with fewer than 8 electrons:
Hydrogen (H):
Stable with 2 electrons (1s2), like helium
Lithium (Li):
Loses one electron to become Li+ (1s2)
Beryllium (Be):
Often forms compounds with just 4 valence electrons
Boron (B) and Aluminium (Al):
Can form stable compounds with only 6 valence electrons
Two examples of stable molecules with incomplete octets that do not follow the typical octet rule are:
Beryllium chloride, BeCl2
BeCl2 = (1 x Be) + (2 x Cl) = (1 x 2) + (2 x 7) = 16 electrons total
Beryllium is the central atom surrounded by two chlorine atoms
Each Be-Cl bond contains one pair of electrons
Be has only 4 electrons around it (2 bonding pairs)
Be does not achieve an octet, but the molecule is stable
Each chlorine atom has 3 lone pairs
Each chlorine has a complete octet
BeCl2 is stable despite beryllium not reaching 8 electrons

Boron trifluoride, BF3
BF3 = (1 x B) + (3 x F) = (1 x 3) + (3 x 7) = 24 electrons total
Boron is the central atom surrounded by three fluorine atoms
Each B-F bond contains one pair of electrons
B has only 6 electrons around it (3 bonding pairs)
B does not achieve an octet, but the molecule is stable
Each fluorine atom has 3 lone pairs
Each fluorine has a complete octet
BF3 is stable despite boron not reaching 8 electrons

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