COCl2 (phosgene) has one carbon atom, one oxygen atom, and two chlorine atoms. In the lewis structure of COCl2, there are two single bonds and one double bond around the carbon atom, with two chlorine atoms and one oxygen atom attached to it. Two chlorine atoms with single bonds have three lone pairs, and one oxygen atom with a single bond has two lone pairs.
Here’s how you can draw the COCl2 lewis structure step by step.
Step #1: draw sketch
Step #2: mark lone pairs
Step #3: mark charges
Step #4: minimize charges
Step #5: minimize charges again (if there are)
Let’s break down each step in detail.
#1 Draw Sketch
- First, determine the total number of valence electrons
Hence, carbon has four valence electrons, oxygen has six valence electrons, and chlorine has seven valence electrons.
Since COCl2 has one carbon atom, one oxygen atom, and two chlorine atoms, so…
Valence electrons of one carbon atom = 4 × 1 = 4
Valence electrons of one oxygen atom = 6 × 1 = 6
Valence electrons of two chlorine atoms = 7 × 2 = 14
And the total valence electrons = 4 + 6 + 14 = 24
- Second, find the total electron pairs
We have a total of 24 valence electrons. And when we divide this value by two, we get the value of total electron pairs.
Total electron pairs = total valence electrons ÷ 2
So the total electron pairs = 24 ÷ 2 = 12
- Third, determine the central atom
We have to place the least electronegative atom at the center.
Since carbon is less electronegative than oxygen and chlorine, assume that the central atom is carbon.
Therefore, place carbon in the center and oxygen and chlorine on either side.
- And finally, draw the rough sketch
#2 Mark Lone Pairs
Here, we have a total of 12 electron pairs. And three bonds are already marked. So we have to only mark the remaining nine electron pairs as lone pairs on the sketch.
Also remember that both (carbon and oxygen) are the period 2 elements, so they can not keep more than 8 electrons in their last shell. And chlorine is a period 3 element, so it can keep more than 8 electrons in its last shell.
Always start to mark the lone pairs from outside atoms. Here, the outside atoms are chlorine and oxygen.
So for each chlorine and oxygen, there are three lone pairs, and for carbon, there is zero lone pair because all nine electron pairs are over.
Mark the lone pairs on the sketch as follows:
#3 Mark Charges
Use the following formula to calculate the formal charges on atoms:
Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons
For carbon atom, formal charge = 4 – 0 – ½ (6) = +1
For oxygen atom, formal charge = 6 – 6 – ½ (2) = -1
For each chlorine atom, formal charge = 7 – 6 – ½ (2) = 0
Here, both carbon and oxygen atoms have charges, so mark them on the sketch as follows:
The above structure is not a stable lewis structure because both carbon and oxygen atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.
#4 Minimize Charges
Convert a lone pair of the oxygen atom to make a new C — O bond with the carbon atom as follows:
In the above structure, you can see that the central atom (carbon) forms an octet. Hence, the octet rule is satisfied.
Also, the above structure is more stable than the previous structures. Therefore, this structure is the stable lewis structure of COCl2.
Next: BrF5 Lewis Structure