PS3- Lewis structure

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PS₃^– has one phosphorus atom and three sulfur atoms.

In PS₃^– Lewis structure, there is one single bond and two double bonds around the phosphorus atom, with three sulfur atoms attached to it. The sulfur atom with a single bond has three lone pairs, and the sulfur atom with double bonds has two lone pairs.

Also, there is a negative (-1) charge on the sulfur atom with a single bond.

Steps

To properly draw the PS₃^– Lewis structure, follow these steps:

#1 Draw a rough sketch of the structure
#2 Next, indicate lone pairs on the atoms
#3 Indicate formal charges on the atoms, if necessary
#4 Minimize formal charges by converting lone pairs of the atoms
#5 Repeat step 4 if necessary, until all charges are minimized

Let’s break down each step in more detail.

#1 Draw a rough sketch of the structure

• First, determine the total number of valence electrons

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

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

Since PS₃^– has one phosphorus atom and three sulfur atoms, so…

Valence electrons of one phosphorus atom = 5 × 1 = 5
Valence electrons of three sulfur atoms = 6 × 3 = 18

Now the PS₃^– has a negative (-1) charge, so we have to add one more electron.

So the total valence electrons = 5 + 18 + 1 = 24

Learn how to find: Phosphorus valence electrons and Sulfur valence electrons

• 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 phosphorus is less electronegative than sulfur, assume that the central atom is phosphorus.

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

• And finally, draw the rough sketch

#2 Next, indicate lone pairs on the atoms

Here, we have a total of 12 electron pairs. And three P — S 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 (phosphorus and sulfur) are the period 3 elements, so they can keep more than 8 electrons in their last shell.

Always start to mark the lone pairs from outside atoms. Here, the outside atoms are sulfurs.

So for each sulfur, there are three lone pairs, and for phosphorus, there is zero lone pair because all nine electron pairs are over.

Mark the lone pairs on the sketch as follows:

#3 Indicate formal charges on the atoms, if necessary

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 (6) = +2

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

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

The above structure is not a stable Lewis structure because both phosphorus and sulfur 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 sulfur atom to make a new P — S bond with the phosphorus atom as follows:

#5 Repeat step 4 (minimize charges again)

Since there are charges on phosphorus and sulfur atoms, again convert a lone pair of the sulfur atom to make a new P — S 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 (sulfurs) also form an octet. Hence, the octet rule is satisfied.

Now there is still a negative (-1) charge on the sulfur atom.

This is okay, because the structure with a negative charge on the most electronegative atom is the best Lewis structure. And in this case, the most electronegative element is sulfur.

Also, the above structure is more stable than the previous structures. Therefore, this structure is the most stable Lewis structure of PS₃^–.

And since the PS₃^– has a negative (-1) charge, mention that charge on the Lewis structure by drawing brackets as follows:

Next: SOF₂ Lewis structure

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External links

• https://www.chegg.com/homework-help/questions-and-answers/consider-lewis-structure-ps3-ion-right–many-total-electrons-used-construct-structure-ion–q103924829
• https://www.bartleby.com/questions-and-answers/consider-the-molecule-ps3-1.-how-many-effective-pairs-are-around-the-central-atom/f8ed6467-2bcc-4d1c-87db-6c734906c7e1