BH2- Lewis structure

BH₂^– has one boron atom and two hydrogen atoms.

In BH₂^– Lewis structure, there are two single bonds around the boron atom, with two hydrogen atoms attached to it, and on the boron atom, there is one lone pair.

Also, there is a negative (-1) charge on the boron atom.

Alternative method: Lewis structure of BH₂^–

Contents

Rough sketch

First, determine the total number of valence electrons

In the periodic table, boron lies in group 13, and hydrogen lies in group 1.

Hence, boron has three valence electrons and hydrogen has one valence electron.

Since BH₂^– has one boron atom and two hydrogen atoms, so…

Valence electrons of one boron atom = 3 × 1 = 3
Valence electrons of two hydrogen atoms = 1 × 2 = 2

Now the BH₂^– has a negative (-1) charge, so we have to add one more electron.

So the total valence electrons = 3 + 2 + 1 = 6

Learn how to find: Boron valence electrons and Hydrogen valence electrons

Second, find the total electron pairs

We have a total of 6 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 = 6 ÷ 2 = 3

Third, determine the central atom

Here hydrogen can not be the central atom. Because the central atom is bonded with at least two other atoms, and hydrogen has only one electron in its last shell, so it can not make more than one bond.

Hence, here we have to assume that the central atom is boron.

Therefore, place boron in the center and hydrogens on either side.

And finally, draw the rough sketch

BH2- Lewis Structure (Step 1) — Rough sketch of BH₂^– Lewis structure | Image: Learnool

Lone pair

Here, we have a total of 3 electron pairs. And two B — H bonds are already marked. So we have to only mark the remaining one electron pair as a lone pair on the sketch.

Also remember that boron is a period 2 element, so it can not keep more than 8 electrons in its last shell. And hydrogen is a period 1 element, so it can not keep more than 2 electrons in its last shell.

Always start to mark the lone pairs from outside atoms. Here, the outside atoms are hydrogens. But no need to mark on hydrogen, because each hydrogen has already two electrons.

So for boron, there is one lone pair.

Mark the lone pair on the sketch as follows:

BH2- Lewis Structure (Step 2) — Lone pairs marked on BH₂^– Lewis structure | Image: Learnool

Formal charge

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

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

For boron atom, formal charge = 3 – 2 – ½ (4) = -1

For each hydrogen atom, formal charge = 1 – 0 – ½ (2) = 0

Here, the boron atom has a charge, so mark it on the sketch as follows:

BH2- Lewis Structure (Step 3) — Formal charges marked, and got the most stable Lewis structure of BH₂^– | Image: Learnool

Final structure

BH2- Lewis Structure (Final) — BH₂^– Lewis structure showing a negative (-1) charge | Image: Learnool

The final structure of BH₂^– includes a central boron atom connected to two hydrogen atoms through single covalent bonds. In this setup, the boron atom possesses a total of six electrons across two bonding pairs and one lone pair, representing a common exception where boron maintains an incomplete octet. Each hydrogen atom reaches its stable duet state through its single shared bond. This arrangement is the most stable because it optimizes the formal charge distribution; the boron atom carries a formal charge of -1, while both hydrogen atoms maintain a formal charge of zero. Consequently, this specific electronic distribution serves as the definitive and most accurate Lewis representation of the BH₂^– ion.

To complete the representation, draw square brackets around the entire Lewis structure and place a “-” or “-1” sign as a superscript outside the upper right bracket. This notation signifies that the negative charge is a property of the whole ion, resulting from the additional electron included in the total valence electron count.

Next: IO₃^– Lewis structure

External video

How to Draw the Lewis Dot Structure for BH2- – YouTube • Wayne Breslyn

External links

Deep

Learnool.com was founded by Deep Rana, who is a mechanical engineer by profession and a blogger by passion. He has a good conceptual knowledge on different educational topics and he provides the same on this website. He loves to learn something new everyday and believes that the best utilization of free time is developing a new skill.