How to calculate formal charges of BH4- ion with lewis structure?

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In covalently bonded molecules, formal charge is the charge assigned to an atom based on the assumption that the bonded electrons are equally shared between concerning atoms, regardless of their electronegativity.

The overall formal charge present on a molecule is a measure of its stability.

The fewer the formal charges present on the bonded atoms in a molecule (close to zero), the greater the stability of its Lewis structure.

In this article, we will calculate the formal charges present on the bonded atoms in the tetrahydroborate [BH4] ion and also the overall charge present on it.

So, without any further delay, let us start reading!

Name of the molecular ion Tetrahydroborate
Chemical formula[BH4]
The formal charge on the B-atom-1
The formal charge on each H-atom0
The overall formal charge on [BH4]-1

How to calculate the formal charges on BH4 atoms?

The formal charges can be calculated using the formula given below:

formal charge formula

The formal charge of an atom = [valence electrons of an atom – non-bonding electrons – ½ (bonding electrons)]

  • The valence electrons (V.E) of an atom are the total number of electrons present in its valence shell. Valence electrons can be calculated by locating the position of the elemental atom in the Periodic Table.
  • Non-bonding electrons (N.E) are the number of lone pairs present on the atom. (1 lone pair means 2 nonbonding electrons).
  • Bonding electrons (B.E) are the total electrons shared with the atom via covalent chemical bonds. (1 single bond means 2 bonding electrons).

Now let us use this formula to calculate the formal charges in the most preferred Lewis structure of [BH4].

The most preferred Lewis representation of tetrahydroborate [BH4] is as shown below.

most stable BH4- lewis structure

It consists of a total of 8 valence electrons. A boron (B) atom is present at the center, which is bonded to four atoms of hydrogen (H), one on each side, via a single covalent bond.

No pair of electrons is present at the central B-atom or on any of the four single-bonded H-atoms in the BH4 Lewis structure.

It is the best possible Lewis structure of [BH4]because the formal charges are minimized in it, and thus, it is the most stable.

Let’s find out how we can determine the formal charges present on each atom in [BH4] Lewis structure.

For the central boron atom

calculating formal charge on Boron atom in BH4-

  • Valence electrons of boron = It is present in Group 3 A = 3 valence electrons
  • Bonding electrons = 4 single bonds = 4 (2) = 8 electrons
  • Non-bonding electrons = no lone pair = 0 electrons
  • Formal charge on the Boron atom = 3 – 0 – 8/2 = 3 – 0 – 4 = 3 – 4 = -1

The formal charge on the B-atom in [BH4] is -1.

For each hydrogen atom

calculating formal charge on hydrogen atom in BH4-

  • Valence electrons of hydrogen = It is present in Group I A = 1 valence electron
  • Bonding electrons = 1 single bond = 2 electrons
  • Non-bonding electrons = no lone pairs = 0 electrons
  • Formal charge on the Hydrogen atom = 1 – 0 – 2/2 = 1 – 0 – 1 = 1 – 1 = 0

The formal charge on each H-atom in [BH4]is 0.

The above calculation shows that zero formal charges are present on each of the four H-atoms while a -1 formal charge on the central boron atom, which is also the overall formal charge present on the tetrahydroborate [BH4]ion, as shown below. 

BH4- lewis structure with formal charge

The BH4 Lewis structure is finally enclosed in square brackets, and a -1 formal charge is placed at the top right corner.

BH4- formal charge

Also, check –

FAQ

How can you calculate BH4 formal charges?

The formal charges present on the bonded atoms in BH4 can be calculated using the formula given below:

V.E – N.E – B.E/2

Where –

⇒ V.E = valence electrons of an atom

⇒ N.E = non-bonding electrons, i.e., lone pairs

⇒ B.E = bonding electrons

What is the formal charge on central B-atom in [BH4]?

The central boron (B) atom carries -1 formal charge in [BH4].

What is the formal charge on H-atoms in [BH4]?

Zero or no formal charge is present on each of the four single-bonded H-atoms in [BH4] Lewis structure.

Do all of the hydrogen atoms carry the same formal charge in [BH4]?

Yes, all four hydrogen atoms are single covalently bonded to the central boron atom, and each carries zero formal charges in [BH4].

What is the overall formal charge on [BH4]?

The overall formal charge on [BH4] is –1.

Summary

  • The best possible Lewis structure of a molecule or molecular ion is the one in which the bonded atoms carry formal charges as close to zero as possible.
  • The formal charge formula is [ V.E – N.E – B.E/2].
  • In [BH4], a -1 formal charge is present on the central boron atom.
  • Each of the four single-bonded H-atoms carries zero or no formal charges in the BH4 Lewis structure.
  • The overall formal charge on BH4 is -1. In conclusion, it is a monovalent anion.  
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