In the above structure, we use (6 electrons × 3 oxygen atoms)=18 valence electrons, selected from the total remaining 20 valence electrons.

(20 - 18)=2 valence electrons

Now there are only two valence electrons left.

Complete the octagon of the central atom, and form covalent bonds if necessary

Now that we have completed the octet of the surrounding atom, we need to complete the octet of the central atom.

As you can see in the fourth step of the structure, bromine is the central atom, which is connected to three oxygen atoms through a single bond. This means that it has shared six valence electrons through three single bonds.

So bromine needs only two electrons to complete octet. Because we have two valence electrons left, we divide these two by bromine.

If you look at the above structure, we will find that all atoms (oxygen and bromine) have completed their octets very comfortably, because each of them has 8 valence electrons that can be shared.

At the same time, we used 26 valence electrons of  bromate anion.

Now we only need to test the stability of the above structure through the concept of formal charge.

Check the stability with the aid of the concept of formal charge

The structure with formal charge close to zero or zero is the best and most stable Lewis structure.

Calculate the formal charge of the atom. Use the formula given below-

Formal charge=(valence electron - lone pair electron - 1/2 shared electron)

We will calculate the formal charge of the fifth step structure to verify its stability.

For bromine atom-

Di's valence electron=7

On bromine, lone pair electrons=2

Common electron pair around bromine=6 (3 single bonds)

F.C. for bromine atom=(7 - 2 - 6/2)=+2

Therefore, the final Lewis structure of bromate anion contains two double bonds and a single bond. Each oxygen atom has 8 valence electrons, and the outermost layer of the bromine center atom has 12 valence electrons, because it has the ability to expand the octet.

The total charge of bromate anion is - 1

Since the bromate anion molecule also contains a negative ion, we need to put brackets around the BrO3 lewis structure and display a negative ion outside the brackets.

What is the molecular geometry of bromate anion?

The molecular geometry of bromate anion is a triangular pyramid, because its central atom (bromine) is attached to three oxygen atoms, and it also contains a lone pair. The existence of the lone pair on the bromine atom pushes all three oxygen atoms downward, because according to VSEPR theory, there is a repulsive force between the electron pairs.

So the final shape of bromate anion looks like a triangular pyramid.

According to VSEPR theory, lone pair electrons and bond pair electrons will generate repulsive force, and try to maintain distance, so the repulsive force between electrons becomes minimum.

In the above structure, the lone electron pairs on the bromine atom repel the adjacent bonding electron pairs, so they are further pushed down to form the triangular pyramid molecular geometry of BrO3 -.

In theory, we can use AXN method and VSEPR diagram to determine the shape of BrO3 -.

Now we will use this method to find the molecular geometry of BrO3 -.

AXN symbol of BrO3 molecule:

A represents the central atom, so according to the BrO3 lewis structure, bromine is the central atom. A=Bromine

X is the bond atom connected to the central atom. We know that bromine is connected to three oxygen atoms. Therefore, X=3

N is the lone pair on the central atom, and there is one lone pair on the bromine atom. Therefore, N=1

So the AXN symbol of BrO3 molecule becomes AX3N1.

Therefore, according to the VSEPR diagram, if the central atom of a molecule contains an isolated pair and is covered by three surrounding atoms, the molecular shape of the molecule is essentially a triangular pyramid.

Therefore, the molecular or geometric shape of BrO3 - is a triangular cone.