Br3– (tribromide) has three bromine atoms. In the lewis structure of Br3–, there are two single bonds around the bromine atom, with two other bromine atoms attached to it, and on each bromine atom, there are three lone pairs.
Also, there is a negative (-1) charge on the center bromine atom.
Here’s how you can draw the Br3– lewis structure step by step.
Step #1: draw sketch
Step #2: mark lone pairs
Step #3: mark charges (if there are)
Let’s break down each step in detail.
#1 Draw Sketch
- First, determine the total number of valence electrons
Since Br3– has three bromine atoms, so…
Valence electrons of three bromine atoms = 7 × 3 = 21
Now the Br3– has a negative (-1) charge, so we have to add one more electron.
So the total valence electrons = 21 + 1 = 22
- Second, find the total electron pairs
We have a total of 22 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 = 22 ÷ 2 = 11
- Third, determine the central atom
Here, there are three atoms and all atoms are bromine, so we can assume any one as the central atom.
Let’s assume that the central atom is center bromine.
- And finally, draw the rough sketch
#2 Mark Lone Pairs
Here, we have a total of 11 electron pairs. And two Br — Br bonds are already marked. So we have to only mark the remaining nine electron pairs as lone pairs on the sketch.
Also remember that 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 left bromine and right bromine.
So for each bromine, there are three lone pairs.
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 left bromine and right bromine atom, formal charge = 7 – 6 – ½ (2) = 0
For center bromine atom, formal charge = 7 – 6 – ½ (4) = -1
Here, the center bromine atom has a charge, so mark it on the sketch as follows:
In the above structure, you can see that the central atom (center bromine) forms an octet. Hence, the octet rule is satisfied.
Now there is still a negative (-1) charge on the center bromine 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 bromine.
Therefore, this structure is the most stable lewis structure of Br3–.
And since the Br3– has a negative (-1) charge, mention that charge on the lewis structure by drawing brackets as follows:
Next: Br2 Lewis Structure