PBr3 Lewis structure

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PBr3 Lewis Structure
PBr3 Lewis structure

PBr3 (phosphorus tribromide) has one phosphorus atom and three bromine atoms.

In the PBr3 Lewis structure, there are three single bonds around the phosphorus atom, with three bromine atoms attached to it. Each bromine atom has three lone pairs, and the phosphorus atom has one lone pair.


Here’s how you can easily draw the PBr3 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

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
Periodic table

In the periodic table, phosphorus lies in group 15, and bromine lies in group 17.

Hence, phosphorus has five valence electrons and bromine has seven valence electrons.

Since PBr3 has one phosphorus atom and three bromine atoms, so…

Valence electrons of one phosphorus atom = 5 × 1 = 5
Valence electrons of three bromine atoms = 7 × 3 = 21

And the total valence electrons = 5 + 21 = 26

  • Second, find the total electron pairs

We have a total of 26 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 = 26 ÷ 2 = 13

  • Third, determine the central atom

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

Since phosphorus is less electronegative than bromine, assume that the central atom is phosphorus.

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

  • And finally, draw the rough sketch
PBr3 Lewis Structure (Step 1)
Rough sketch of PBr3 Lewis structure

#2 Mention lone pairs on the atoms

Here, we have a total of 13 electron pairs. And three P — Br bonds are already marked. So we have to only mark the remaining ten electron pairs as lone pairs on the sketch.

Also remember that phosphorus is a period 3 element, so it can keep more than 8 electrons in its last shell. And bromine is a period 4 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 bromines.

So for each bromine, there are three lone pairs, and for phosphorus, there is one lone pair.

Mark the lone pairs on the sketch as follows:

PBr3 Lewis Structure (Step 2)
Lone pairs marked, and got the stable Lewis structure of PBr3

#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 – 2 – ½ (6) = 0

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

Here, both phosphorus and bromine atoms do not have charges, so no need to mark the charges.

In the above structure, you can see that the central atom (phosphorus) forms an octet. And the outside atoms (bromines) also form an octet. Hence, the octet rule is satisfied.

Therefore, this structure is the stable Lewis structure of PBr3.

Next: CH3Br Lewis structure

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