How to calculate formal charges of hydroxide [OH]- 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 hydroxide [OH] ion and also the overall charge present on it.

So, continue reading!

Name of the molecule Hydroxide
Chemical formula[OH]
The formal charge on each H-atom0
The formal charge on the O-atom-1
The overall formal charge on [OH]-1

How to calculate the formal charges on OH 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 the hydroxide [OH] ion.

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

stable lewis structure of OH-

It consists of a total of 8 valence electrons. It consists of two atoms; one oxygen (O) atom which is bonded to one atom of Hydrogen (H) via a single covalent bond.

Three lone pairs of electrons are present at the O-atom, while no lone pair of electrons is present on the single-bonded H-atom.

It is the best possible Lewis structure of [OH] 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 [OH] Lewis structure.

For the oxygen atom

calculating formal charge on oxygen atom in OH-

  • Valence electrons of Oxygen = It is present in Group VI A = 6 valence electrons
  • Bonding electrons = 1 single bond = 2 electrons
  • Non-bonding electrons = 3 lone pairs =3(2) = 6 electrons
  • Formal charge on the Oxygen atom = 6 – 6 – 2/2 = 6 – 6 – 1 = 6 – 7 = -1

The formal charge on the O-atom in [OH] is -1.

For the hydrogen atom

calculating formal charge on hydrogen atom in OH-

  • 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 the H-atom in [OH]  is 0.

This calculation shows zero formal charges are present on the H-atom while a -1 formal charge is present on the oxygen atom, which is also the overall formal charge present on the [OH] molecular ion, as shown below. 

oh- lewis structure with formal charge

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

OH- formal charge

Also, check –

FAQ

How can you calculate OH formal charges?

The formal charges present on the bonded atoms in OH 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 O-atom in [OH]?

The oxygen (O) atom carries a -1 formal charge in [OH].

What is the formal charge on H-atom in [OH]?

Zero or no formal charges are present on the H-atom in [OH].

How many lone pairs and bond pairs of electrons are present on the oxygen atom in [OH]?

The oxygen atom contains three lone pairs and one bond pair of electrons in [OH] Lewis structure.

How many lone pairs and bond pairs of electrons are present on hydrogen atoms in [OH]?

The hydrogen atom contains no lone pairs, while one bond pair of electrons is present on it in [OH].

What is the overall formal charge on [OH]?

The overall formal charge on [OH]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 [OH], a -1 formal charge is present on the oxygen atom.
  • The hydrogen atom carries zero or no formal charges in [OH] Lewis structure.
  • The overall formal charge on [OH]is -1.
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