Tetrahydro Borate (BH4-): Lewis Structure, Molecular Geometry and Hybridization, Polarity

Tetrahydro borate ion (BH₄^–)is a class of inorganic or organic compounds. It is an anion. This article covers the Lewis structure, molecular geometry, hybridization, and MOT of BH4-.

Facts About Tetrahydro Borate (BH₄^–)

• BH4-is a molecular ion.
• It forms complex hydrides with metals.
• Its molecular weight is 14g/mol.

Valence Electrons

The outermost shell electrons of an atom are valence electrons. Valence electrons help in the formation of bonds. They also tell us about the chemical properties of the molecules.

Formal Charge

A formal charge is any charge on an atom in a molecule that shows all the bonding electrons have an equal share.

We can calculate the formal charge by the formula:

Formal charge = Valence shell electrons – total non-bonding electrons – total bonding electrons/2

The Lewis Structure of Tetrahydro Borate (BH₄^–)

You have to follow some general rules while drawing the Lewis Structure of a molecule. You can adjust the steps involved in making a structure as it may vary with the molecule. Let’s use those rules to draw the Lewis structure of BH₄^–. Take a pen and paper and draw with me the structure step by step.

• Find the total no. of valence electrons in boron and hydrogen atom
• Locate the center atom in the molecule.
• Find total electron pairs which make both bond and lone pairs of electrons
• Mark the charges present on the atoms
• Complete the octet rule
• Check stability by decreasing lone pairs

Step 1: Find the Total no. of Valence Electrons in Tetrahydro Borate (BH₄^–)

As you know, valence electrons are the electrons present in the last shell of an atom.

For Boron

You know that boron is an element of 13A. So there are 3 three valence electrons in Boron.

For Hydrogen

Hydrogen is an element of group 1A. Here we have a total of four hydrogen atoms and thus four valence electrons.

Also, a negative charge is present, which means one more electron.

Hence in BH4- molecule,

Total no. of valence electrons in BH4- = 3 + 1(4) + 1 = 8

Step 2: Find the Center Atom in BH₄^–

Usually, the least electronegative element is present at the centre. Here, boron is less electronegative than hydrogen. That’s why you will locate the boron in the centre of the molecule. The hydrogen atoms will surround the boron atom.

Step 3: Place Electrons between Atoms to Show a Chemical Bond

In the above sketch, you have to show the chemical bonds formed. So, you have to connect hydrogen atoms with the boron atom through single bonds.

Step 4: Complete the Octet Rule on Outside Atoms

In the Lewis structure of tetrahydro borate ion or BH₄^–, hydrogen atoms are the outer ones. Now, you have to complete the octet rule on the outside atoms.

If you look at the central atom boron, it must fulfil the octet rule. Since the boron atom has four single bonds around it, it indicates that it has eight valence electrons around it.

So this is the Lewis structure of BH₄^–. But you have to put brackets around the ion and show that it has a negative charge.

Importance of Lewis Structure

• A Lewis structure represents valence electrons in a molecule of a compound.
• Lewis Structure helps tells about the arrangement of atoms in a molecule.
• The structure not only lets us know about how the arrangement of bonds but also about the no of bonds formed
• This knowledge tells about the shape of the molecule and also explains the nature of the molecule as well.

Molecular Geometry and Shape of Tetrahydro Borate (BH₄^–)

Molecular geometry explains the three-dimensional arrangement of atoms. The geometry of a molecule tells us about the position of the bonds. It also describes the properties of the molecule.

In BH₄^–, sp3 hybridization occurs between boron orbitals. The sp3 hybridization accounts for the tetrahedral geometry of the molecule.

Bond Angle of BH₄^–

The bond angle in a tetrahedral molecule is usually 109˚. The four hybrid orbitals of a tetrahedral molecule make the bong angle approximately 109˚. In the structure of BH₄^–, there is an equal distribution of the electrons, so the H-B-H bond angle is 109˚.

The Polarity of BH₄^–

The overall charge on a molecule tells if the molecule is polar or non-polar. If we see the molecule of BH₄^–, all the dipole moments cancel out each other. It is because of the tetrahedral arrangement of the atoms.

Factors Affecting the Polarity

1. The electronegativity difference between bonded atoms

Since polarity means the pulling of electrons. You know that a more electronegative element attracts more electrons. As a result, electrons go towards the more negative element.

2. Space arrangement of bonds

The shared electron pair experiences an attractive force from other electron pairs. This causes the same atoms to have different polarities in different molecules.

Hybridization of Tetrahydro Borate (BH₄^–)

Hybridization is the mixing of atomic orbitals to make new hybridized orbitals. Depending on how orbitals are mixed or overlapping of orbitals with each other, six types of hybridization occur.

In the structure of BH₄^–, there are 4 bond pairs and 0 lone pairs of electrons. The central atom Boron undergoes sp3 hybridization. The sp3 hybridization makes the tetrahedral geometry.

Uses of Tetrahydro Borate (BH₄^–)

• The molecule BH₄^– forms complex hydrides with Li, Na, and Al.
• These complex hydrides are metal salts that are useful as drying agents.

Conclusion

BH4- or tetrahydro borate ion is sp3 hybridized molecule. So, the molecule is tetrahedral in shape. In this article, you have understood how the four H atoms and one B atom make a molecule with a complete octet rule.

Related Article:

Phosphite Ion (Po3-3); How to Draw Lewis Structure, Molecular Geometry, and Hybridization

Bromate Ion (BrO3-); How to Draw Lewis Structure, Molecular Geometry, Hybridization, And MOT Diagram

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