POCl3 Lewis structure

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POCl₃ (phosphoryl chloride) has one phosphorus atom, one oxygen atom, and three chlorine atoms.

In the POCl₃ Lewis structure, there is one double bond and three single bonds around the phosphorus atom, with one oxygen atom and three chlorine atoms attached to it. Each chlorine atom has three lone pairs, and the oxygen atom has two lone pairs.

Steps

Here’s how you can easily draw the POCl₃ Lewis structure step by step:

#1 Draw a rough skeleton structure
#2 Mention lone pairs on the atoms
#3 If needed, mention formal charges on the atoms
#4 Minimize formal charges by converting lone pairs of the atoms, and try to get a stable Lewis structure
#5 Repeat step 4 again if needed, until all charges are minimized

Now, let’s take a closer look at each step mentioned above.

#1 Draw a rough skeleton structure

• First, determine the total number of valence electrons

In the periodic table, phosphorus lies in group 15, sulfur lies in group 16, and chlorine lies in group 17.

Hence, phosphorus has five valence electrons, sulfur has six valence electrons, and chlorine has seven valence electrons.

Since POCl₃ has one phosphorus atom, one oxygen atom, and three chlorine atoms, so…

Valence electrons of one phosphorus atom = 5 × 1 = 5
Valence electrons of one oxygen atom = 6 × 1 = 6
Valence electrons of three chlorine atoms = 7 × 3 = 21

And the total valence electrons = 5 + 6 + 21 = 32

Learn how to find: Phosphorus valence electrons, Oxygen valence electrons, and Chlorine valence electrons

• Second, find the total electron pairs

We have a total of 32 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 = 32 ÷ 2 = 16

• Third, determine the central atom

We have to place the least electronegative atom at the center.

Since phosphorus is less electronegative than oxygen and chlorine, assume that the central atom is phosphorus.

Therefore, place phosphorus in the center and oxygen and chlorine on either side.

• And finally, draw the rough sketch

#2 Mention lone pairs on the atoms

Here, we have a total of 16 electron pairs. And four bonds are already marked. So we have to only mark the remaining twelve electron pairs as lone pairs on the sketch.

Also remember that both (phosphorus and chlorine) are the period 3 elements, so they can keep more than 8 electrons in their last shell. And oxygen is a period 2 element, so it can not keep more than 8 electrons in its last shell.

Always start to mark the lone pairs from outside atoms. Here, the outside atoms are oxygen and chlorines.

So for oxygen and each chlorine, there are three lone pairs, and for phosphorus, there is zero lone pair because all twelve electron pairs are over.

Mark the lone pairs on the sketch as follows:

#3 If needed, mention formal charges on the atoms

Use the following formula to calculate the formal charges on atoms:

Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons

For phosphorus atom, formal charge = 5 – 0 – ½ (8) = +1

For oxygen atom, formal charge = 6 – 6 – ½ (2) = -1

For each chlorine atom, formal charge = 7 – 6 – ½ (2) = 0

Here, both phosphorus and oxygen atoms have charges, so mark them on the sketch as follows:

The above structure is not a stable Lewis structure because both phosphorus and oxygen atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.

#4 Minimize formal charges by converting lone pairs of the atoms

Convert a lone pair of the oxygen atom to make a new P — O bond with the phosphorus atom as follows:

In the above structure, you can see that the central atom (phosphorus) forms an octet. And the outside atoms (oxygen and chlorines) also form 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 POCl₃.

Next: SCl₄ Lewis structure

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External links

• https://lambdageeks.com/pocl3-lewis-structure/
• https://www.thegeoexchange.org/chemistry/bonding/Lewis-Structures/POCl3-lewis-structure.html
• https://geometryofmolecules.com/pocl3-lewis-structure-molecular-geometry-hybridization-bond-angle/
• https://www.chemistryscl.com/general/phosphorus-oxychloride-POCl3-lewis-structure/index.php